Anatomy Ch 23: The Digestive System

gastritis

anything that breaches the gel-like mucosal barrier causes inflammation of the stomach wall, a condition called _______

response of the stomach to filling

the stomach stretches to accommodate incoming food, but internal stomach pressure remains constant until about 1.5 L of food have been ingested. thereafter, the pressure rises. the relatively unchanging pressure in a filling stomach is due to two factors: -receptive relaxation of smooth muscle in the stomach fundus and body occurs both in anticipation of and in response to food moving through the esophagus and into the stomach. the swallowing center of the brain stem coordinates this process, which is mediated by the vagus nerves -gastric accommodation is the intrinsic ability of visceral smooth muscle to exhibit the stress-relaxation response. in other words, the stomach can stretch without greatly increasing its tension and contracting expulsively. this capability is very important in hollow organs, like the stomach, that must serve as temporary reservoirs

lipids

triglycerides are the most abundant fats in the diet. the small intestine is the primary site of lipid digestion because the pancreas is the major source of fat-digesting enzymes, or lipases

tooth structure

-each tooth has two major regions: the crown and the root -the enamel-covered crown is the exposed part of the tooth above the gingiva, or gum, which surround the tooth like a tight collar -enamel, a brittle ceramic-like material thick as a dime, directly bears the force of chewing -enamel is heavily mineralized with calcium salts, and its densely packed hydroxyapatite (mineral) crystals are oriented in force-resisting columns perpendicular to the tooth's surface -this makes it the hardest substance in the body -the cells that produce enamel degenerate when the tooth erupts; consequently, decayed or cracked areas of enamel will not heal and must be artificially filled -the root is the portion of the tooth embedded in the jawbone -canine teeth, incisors, and premolars have one root, although the first upper premolars commonly have two -the first two upper molars have three roots, while the corresponding lower molars have two -the root pattern of the third molar varies, but a fused single root is most common -a constricted tooth region called the neck connects the crown and root -cement, a calcified connective tissue, covers the outer surface of the root and attaches the tooth to the thin periodontal ligament -this ligament anchors the tooth in the bony socket (alveolus) of the jaw, forming a fibrous joint called a gomphosis -where the gingiva borders on a tooth, it dips downward to form a shallow groove called the gingival sulcus -dentin, a protein-rich bonelike material, underlies the enamel cap and forms the bulk of a tooth -more resilient (flexible) than enamel, dentin acts as a shock absorber during biting and chewing -dentin surrounds a central pulp cavity containing a number of soft tissue structures (connective tissue blood vessels, and nerve fibers) collectively called pulp -pulp supplies nutrients to the tooth tissues and provides tooth sensation -where the pulp cavity extends into the root, it becomes the root canal -at the proximal end of each root canal is an apical foramen that allows blood vessels, nerves, and other structures to enter the pulp cavity -the teeth are served by the superior and inferior alveolar nerves, branches of the trigeminal nerve -the superior and inferior alveolar arteries, branches of the maxillary artery, supply blood -dentin contains radial striations called dentinal tubules -each tubule contains an elongated process of an odontoblast, the cell type that secretes and maintains the dentin -the odontoblasts line the pulp cavity just deep to the dentin -dentin forms throughout adult life and gradually encroaches on the pulp cavity -new dentin can also be laid down fairly rapidly to compensate for tooth damage or decay -enamel, dentin, and cement are all calcified and resemble bone (to differing extents), but they differ from bone because they are avascular -enamel differs from cement and dentin because it lacks collagen and is almost entirely mineral

peptic ulcers, gastric ulcers

-persistent damage to the underlying tissues can promote ________ _______, specifically called _______ _______ when they are erosions of the stomach wall -the most distressing symptom is gnawing epigastric pain that seems to bore through to your back -the pain typically occurs 1-3 hours after eating and is often relieved by eating again -the danger posed by ulcers is perforation of the stomach wall, leading to peritonitis and perhaps massive hemorrhage -for years, ulcers were blamed on factors that increased HCl production or reduced mucus secretion, including aspirin and non steroidal anti-inflammatory drugs (NSAIDS such as ibuprofen), smoking, spicy food, alcohol, coffee, and stress -although acidic conditions are necessary for ulcers to form, acidity in itself is not sufficient to cause them -ninety percent of recurrent ulcers are the work of a strain of acid-resistant, corkscrew-shaped helicobacter pylori bacteria which burrow like a drill bit through the mucus and destroy the protective mucosal layer -even more troubling are studies that link this bacterium to some stomach cancers -more than half of the population harbor H. pylori, but these pathological effects occur in only 10-20% of infected individuals -the antimicrobial activity of gastric mucin appears to protect most of us from H. pylori's invasive attacks -a breath test can easily detect the presence of H. pylori -a two-week-long course of antibiotics kills the bacteria, promotes healing of the ulcers, and prevents recurrence -for active ulcers, a blocker for H2 (histamine) receptors may also help because it inhibits HCl secretion by blocking histamine's effects -the relatively few peptic ulcers not caused by H. pylori generally result from long-term use of NSAIDs -in such noninfectious cases, blocking HCl secretion either directly (with pump inhibitors) or indirectly [with H2 (histamine) receptor blockers] is the therapy of choice

organs of the digestive system

fall into two main groups: those of the alimentary canal and accessory digestive organs

ileocecal valve control

most of the time, the ileocecal valve is closed however, two mechanisms cause it to relax and allow food residues to enter the cecum when ill motility increases: -the gastroileal reflex, a long neural reflex triggered by stomach activity, increases the force of segmentation in the ileum and relaxes the ileocecal valve -gastrin, a hormone released by the stomach, increases the motility of the ileum and relaxes the ileocecal valve -once the chyme has passed through, it exerts backward pressure that closes the valve's flaps, preventing regurgitation into the ileum -this reflex sweeps the contents of the previous meal completely out of the stomach and small intestine as the next meal is eaten

electrolyte absorption

-absorbed electrolytes come from both ingested foods and gastrointestinal secretions -most ions are actively absorbed along the entire length of the small intestine -but absorption of iron and calcium is largely limited to the duodenum -for most nutrients, the amount reaching the intestine is the amount absorbed, regardless of the nutritional state of the body -in contrast, absorption of iron and calcium is intimately related to the body's need for them at the same time

mechanisms of absorption

-absorption is the process of moving substances from the lumen of the gut into the body -epithelia are polarized and have an apical and basal side -because tight junctions join the epithelial cells (enterocytes) of the intestinal mucosa at their apical surfaces, substances usually cannot move between cells -instead, materials must pass through the enterocytes -materials enter an enterocyte through its apical membrane from the lumen of the gut and exit through the basolateral membrane into the interstitial fluid on the other side of the cell -once in the interstitial fluid, substances diffuse into the blood capillaries -from the capillary blood in the villus they are transported into the hepatic portal vein to the liver -the exception is some lipid digestion products, which enter the lacteal in the villus to be carried via lymphatic fluid to the blood -the structure of the plasma membrane means that nonpolar substances, which can dissolve in the lipid core of the membrane, can be absorbed passively -all other substances need a carrier mechanism -more nutrients are absorbed by active transport processes driven directly or indirectly by metabolic energy -there is much more flowing through the alimentary tube than food monomers -up to 10 L of food, drink, and GI secretions enter the alimentary canal daily, but only 1 L or less reaches the large intestine -virtually all of the foodstuffs, 80% of the electrolytes, and most of the water are absorbed in the small intestine -although absorption occurs all along the length of the small intestine, most of it is completed by the time chyme reaches the ileum -the major absorptive role of the ileum is to reclaim bile salts to be recycled back to the liver for resection -the absorptive capacity of the small intestine is truly remarkable and it is virtually impossible to exceed

sodium, chloride, and bicarbonate

-absorption of sodium ions in the small intestine is coupled to active absorption of glucose and amino acids -for the most part, anions passively follow the electrical potential established by sodium transport -in other words, Na+ is actively pumped out of the enterocyte by a Na+-K+ pump after entering those cells -usually, chloride ions passively follow Na+ -in the terminus of the small intestine, HCO3- is actively secreted into the lumen in exchange for Cl-

appendicitis

-acute inflammation of the appendix -results from a blockage (often by feces) that traps infectious bacteria in its lumen -unable to empty its contents, the appendix swells, squeezing off venous drainage, which may lead to ischemia and necrosis (low blood flow and tissue death) of the appendix -if the appendix ruptures, feces containing bacteria spray over the abdominal contents, causing peritonitis -the symptoms vary, but the first symptom is usually pain in the umbilical region -loss of appetite, nausea and vomiting, and pain relocalization to the lower right abdominal quadrant follow -is treated with surgical removal of the appendix (appendectomy) or, in select cases, with antibiotics -is most common during adolescence, when the entrance to the appendix is at its widest

digestion hydrolyzes food into nutrients that are absorbed across the gut epithelium

-after foodstuffs have spent even a short time in the stomach, they are unrecognizable, but mechanical breakdown has only changed their appearance -in contrast, digestion breaks down ingested foods into their chemical building blocks, which are very different molecules chemically -only these molecules are small enough to be absorbed across the wall of the small intestine

relationship of the digestive organs to the peritoneum

-all ventral body cavities contain slippery serous membranes -the peritoneum of the abdominopelvic cavity is the most extensive of these membranes -the visceral peritoneum covers the external surfaces of most digestive organs and is continuous with the parietal peritoneum that lines the body wall -between the two peritoneums is the peritoneal cavity, a slitlike potential space containing a slippery fluid secreted by the serous membranes -the serous fluid allows the mobile digestive organs to glide easily across one another and along the body wall as they carry out their activities

bacterial microbiota

-also called the bacterial flora -the ________ ________- of the large intestine consists of over a thousand different types of bacteria and accounts for a couple of pounds of our body weight -some of these bacteria colonize the colon via the anus, but others enter from the small intestine still "alive and kicking" after running the gauntlet of antimicrobial defenses (lysozyme, defensins, HCl, and protein-digesting enzymes)

alimentary canal

-also called the gastrointestinal (GI) tract or gut -is the continuous muscular tube that winds through the body from the mouth to the anus -it digests food-- breaks it down into smaller fragments-- and absorbs the digested fragments through its lining into the blood -the organs of the _______ ____- are the mouth, pharynx, esophagus, stomach, small intestine, and large intestine -the large intestine leads to the terminal opening, or anus -in a cadaver, the ________ _____ is approximately 9 m (about 30 ft) long, but in a living person, it is considerably shorter because of its muscle tone -food material in this tube is technically outside the body because the canal is open to the external environment at both ends

gluten-sensitive enteropathy

-also known as celiac disease -affects one in 100 people -this chronic genetic condition is caused by an immune reaction to gluten, a protein plentiful in all grains but corn and rice -breakdown products of gluten interact with molecules of the immune system in the GI tract, forming complexes -these complexes activate T cells, which then attack the intestinal lining, damaging intestinal villi and reducing the surface area of the brush border -bloating, diarrhea, pain, and malnutrition result -the usual treatment is to eliminate gluten-containing grains from the diet -in recent years, many gluten-free products have become available

motility of the small intestine

-there are two motility patterns in the small intestine -after a meal, segmentation is the principal form of motility. this motor pattern ensures the the chyme is thoroughly mixed with bile and pancreatic and intestinal juices -it also ensures that the absorbable products of digestion come into contact with the mucosa for absorption -between meals, the primary motor pattern is a form of peristalsis called the migrating motor complex -this peristalsis is largely a housekeeping function that sweeps debris toward the large intestine

clostridium difficile

-an anaerobic bacterium, is the most common cause of antibiotic-associated diarrhea, accounting for 14,000 deaths per year in the US -for some people, C. difficile is a normal, but small, fraction of the gut's bacteria -other people acquire C. difficile through the fecal-oral route (poor hand washing), particularly in hospital or long-term care settings -in either case, when other bacteria are wiped out by antibiotics, C. difficile flourishes in the gut and may cause pseudomembranous colitis (inflammation of the colon) that leads to bowel perforation and sepsis -because C. difficile infections are resistant to many antibiotics, they are notoriously difficult to treat and often recur -instead of using ever more powerful antibiotics, a new treatment strategy seeks to restore competitive bacteria to the gut's ecosystem by performing a fecal transplant -- transferring fecal bacteria from an uninfected donor to the patient cures C. difficile infections 90-100% of the time

composition of pancreatic juice

-approximately 1200 to 1500 ml of clear pancreatic juice is produced daily -it consists mainly of water, and contains enzymes and electrolytes (primarily bicarbonate ions) the high pH of pancreatic fluid helps neutralize acidic chyme entering the duodenum and provides the optimal environment for intestinal and pancreatic enzymes. the pancreatic enzymes include: -proteases (for proteins) -amylase (for starch) -lipases (for fats) -nucleases (for nucleic acids) -like pepsin of the stomach, pancreatic proteases are produced and released in inactive forms that are activated in the duodenum, where the do their work. this protects the pancreas from digesting itself -for example, within the duodenum, enteropeptidase, an enzyme bound to the plasma membrane of duodenal epithelial cells, activates trypsinogen to trypsin -trypsin, in turn, activates more trypsinogen and two other pancreatic proteases to their active forms, carboxypeptidase and chymotrypsin

water absorption

-approximately 9 L of water, mostly derived from GI tract secretions, enter the small intestine daily -water is the most abundant substance in chyme, and 95% of it is absorbed in the small intestine by osmosis -most of the rest is absorbed in the large intestine, leaving only about 0.1 L to soften the feces -the normal rate of water absorption is 300 to 400 ml per hour -water moves freely in both directions across the intestinal mucosa, but net osmosis occurs whenever a concentration gradient is established by the active transport of solutes (particularly Na+) into the mucosal cells -in this way, water uptake is effectively coupled to solute uptake and, in turn, affects the absorption of substances that normally pass by diffusion -as water moves into mucosal cells, these substances follow along their concentration gradients

mucous neck cells

-are found primarily in the "neck" but are also scattered deeper within the glands -they produce a thin, soluble mucus quite different from that secreted by the mucous cells of the surface epithelium -it is not yet understood what special function this acidic mucus performs

parietal cells

-are scattered among the chief cells mainly in the more apical (closer to the lumen) region of the glands -simultaneously secrete hydrochloric acid (HCl) and intrinsic factor -although they appear oval when viewed with a light microscope, parietal cells actually have three prongs that bear dense microvilli -this structure provides a huge surface area for secreting H+ AND Cl- into the stomach lumen -HCl makes the stomach contents extremely acidic, a condition necessary for activation and optimal activity of the protein-digesting enzyme pepsin -the acidity also helps digest food by denaturing proteins and breaking down cell walls of plant foods, and is harsh enough to kill many of the bacteria ingested with foods -intrinsic factor is a glycoprotein required for vitamin B12 absorption in the small intestine

accessory digestive organs

-are the teeth, tongue, gallbladder, and a number of large digestive glands-- the salivary glands, liver, and pancreas -the teeth and tongue are in the mouth, or oral cavity, while the digestive glands and gallbladder lie outside the GI tract and connect to it by ducts -produce a variety of secretions that help break down foodstuffs

mastication (chewing)

-as food enters the mouth, its mechanical breakdown begins with ____, or chewing -the cheeks and closed lips hold food between the teeth, the tongue mixes food with saliva to soften it, and the teeth cut and grind solid foods into smaller morsels -is partly voluntary and partly reflexive -we voluntarily put food into our mouths and contract the muscles that close our jaws -the pattern and rhythm of continued jaw movements are controlled mainly by stretch reflexes and in response to pressure inputs from receptors in the cheeks, gums, and tongue, but they can also be voluntary if desired

the stomach temporarily stores food and begins protein digestion

-below the esophagus, the GI tract expands to form the stomach, a temporary "storage tank" -besides serving as a holding area for ingested food, the stomach continues the demolition job begun in the oral cavity by further degrading food both physically and chemically -as a result food is converted into a slurry called chyme -it then delivers chyme into the small intestine

bile: composition and enterohepatic circulation

-bile is a yellow-green, alkaline solution containing bile salts, bile pigments, cholesterol, triglycerides, phospholipids, and a variety of electrolytes. of these, only bile salts and phospholipids aid the digestive process -bile salts, primarily salts of cholic and chenodeoxycholic acids are cholesterol derivatives -they play a crucial role in both the digestion and absorption of fats -many substances secreted in bile leave the body in feces, but bile salts are not among them -instead, a recycling mechanism called the enterohepatic circulation conserves bile salts -this helps minimize the amount of new bile salts that must be synthesized, which is important because they are recirculated 4 to 12 times per day -the chief bile pigment is bilirubin, a yellow waste product of the heme of hemoglobin formed during the breakdown of worn-out erythrocytes -the globin and iron parts of hemoglobin are saved and recycled -bilirubin, however, is absorbed from the blood by liver cells, excreted into bile, and is metabolized in the small intestine by resident bacteria -one of its breakdown products, stercobilin, gives feces a brown color -in the absence of bile, feces are gray-white and have fatty streaks because essentially no fats are digested or absorbed

tracheo-esophageal fistula

-there is an opening between the esophagus and the trachea -the esophagus often lacks a connection to the stomach -the baby chokes and becomes cyanotic during feedings because food enters the respiratory passageways -surgery can usually correct these defects

gallstones

-bile is the major vehicle for excreting cholesterol from the body, and bile salts keep the cholesterol dissolved within bile -too much cholesterol or too few bile salts allows the cholesterol to crystallize, forming __________ -can obstruct the flow of bile from the gallbladder, a condition called cholelithiasis -when the gallbladder or its duct contracts, the sharp crystals cause agonizing pain that radiates to the right thoracic region -are easy to diagnose because they show up well with ultrasound imaging -treatments include dissolving the crystals with drugs, pulverizing them with ultrasound vibrations (lithotripsy), vaporizing them with lasers, and the classical treatment, surgically removing the gallbladder (cholecystectomy) -when the gallbladder is removed, the bile duct enlarges to assume the bile-storing role -bile duct blockage prevents both bile salts and bile pigments from entering the intestine -as a result, yellow bile pigments accumulate in blood and eventually are deposited in the skin, causing it become yellow or jaundiced -jaundice caused by blocked ducts is called obstructive jaundice, but jaundice may also reflect liver disease (in which the liver is unable to carry out its normal metabolic duties

calcium

-calcium absorption is closely related to blood levels of ionic calcium -the active form of vitamin D promotes active calcium absorption -decreased blood levels of ionic calcium prompt parathyroid hormone (PTH) release from the parathyroid glands -besides facilitating the release of calcium ions from bone matrix and enhancing the reabsorption of calcium by the kidneys, PTH stimulates activation of vitamin D to calcitriol by the kidneys, which in turn accelerates calcium ion absorption in the small intestine

ankyloglossia

-children born with an extremely short lingual frenulum are sometimes referred to as "tongue-tied" -restricted tongue movement may interfere with breast-feeding -this congenital condition, called ____, is sometimes correctly surgically by snipping the frenulum

regulating chyme entry

-chyme entering the duodenum is usually hypertonic -for this reason, if large amounts of chyme rushed into the small intestine, the osmotic water loss from the blood into the intestinal lumen would result in dangerously low blood volume -additionally, the low pH of entering chyme must be adjusted upward and the chyme must be well mixed with bile and pancreatic juice for digestion to continue -feedback via the enter-gastric reflex and enterogastrones to the stomach pylorus carefully controls food movement into the small intestine to prevent the duodenum from being overwhelmed -the feedback mechanisms regulating chyme entry (the enterogastric reflex and enterogastrones) are the same as those that decrease gastric secretion

tooth and gum disease

-dental caries, or cavities, result from bacterial action that gradually demineralizes enamel and underlying dentin -decay begins when dental plaque (a film of sugar, bacteria, and other mouth debris) adheres to the teeth -bacterial metabolism of the trapped sugars produces acids, which dissolve the calcium salts of the teeth -once the salts are leached out, enzymes released by the bacteria readily digest the remaining organic matrix of the tooth -frequent brushing helps prevent caries by removing plaque -more serious than tooth decay is the effect of unremoved plaque on the gum -as dental plaque accumulates, it calcifies, forming calculus, or tartar -these stony-hard deposits disrupt the seal between gingiva and teeth, deepening the sulcus and putting the gums at risk for infection by pathogenic anaerobic bacteria -in the early stages of such an infection, called gingivitis, the gums are red, sore, swollen, and may bleed -gingivitis is reversible if the calculus is removed, but if it is neglected the bacteria eventually form pockets of infection which become inflamed -neutrophils and other immune cells attack not only the intruders but also body tissues, carving deep pockets around the teeth, destroying the periodontal ligament, and activating osteoclasts which dissolve the bone -this serious condition, periodontal disease or periodontitis, affects up to 95% of all people over age 35 and accounts for 80-90% of tooth loss in adults -tooth loss from periodontitis is not inevitable. various treatments can alleviate the bacterial infestations and encourage the surrounding tissues to reattach to the teeth and bone -periodontal disease may jeopardize more than just teeth -some contend that it increases the risk of heart disease and stroke in at least two ways: 1) the chronic inflammation promotes atherosclerotic plaque and 2) bacteria entering the blood from infected gums stimulate the formation of clots that clog coronary and cerebral arteries -risk factors for periodontal disease include smoking, diabetes mellitus, and oral (tongue or lip) piercing

mechanism of digestion: enzymatic hydrolysis

-digestion is a catabolic process that breaks down large food molecules to monomers (chemical building blocks) -digestion is accomplished by enzymes secreted into the lumen of the alimentary canal by intrinsic and accessory glands -enzymatic breakdown of any food molecule is hydrolysis because it involves adding a water molecule to each molecular bond to be broken (lysed) -most digestion is done in the small intestine -pancreatic enzymes break large chemicals (usually polymers) into smaller pieces that are, in turn, broken down into individual components by the intestinal (brush border) enzymes -alkaline pancreatic juice neutralizes the acidic chyme that enters the small intestine from the stomach -this provides the proper environment for operation of the enzymes -both pancreatic juice (the main source of lipases) and bile are necessary for fat breakdown

aging and the digestive system

-during old age, GI tract activity declines -fewer digestive juices are produced, absorption is less efficient, and peristalsis slows -the result is less frequent bowel movements and, often, constipation -taste and smell are less acute, and periodontal disease often develops -many elderly people live alone or on reduced income -these factors, along with increasing physical disability, tend to make eating less appealing, and many of our elderly citizens are poorly nourished -diverticulosis, fecal incontinence, and cancer of the GI tract are fairly common problems of the aged -stomach and colon cancers rarely have early signs, and often metastasize before a person seeks medical attention -should metastasis occur, secondary cancer of the liver is almost guaranteed because of the "detour" the splanchnic venous blood takes through the liver via the hepatic portal circulation -however, when detected early, most GI tract cancers are treatable -the best advice is to have regular dental and medical checkups -most oral cancers are detected during routine dental examinations, 50% of all rectal cancers can be felt digitally, and nearly 80% of colon cancers can be seen and removed during a colonoscopy -screening for colon cancer is recommended after age 50

chief cells

-occur mainly in the basal regions of the gastric glands -the cuboidal _____ _____ produce pepsinogen, the inactive form of the pepsin -when these cells are stimulated, the first pepsinogen molecules they relate are activated by HCl encountered in the apical region of the gland -but once pepsin is present, it also catalyzes the conversion of pepsinogen to pepsin -the activation process involves removing a small peptide fragment from pepsinogen, causing it to change shape and expose its active site -this positive feedback process is limited only by the amount of pepsinogen present -chief cells also secrete lipase (fat-digesting enzymes) that account for about 15% of overall GI lipolysis

vomiting

-emesis -is an unpleasant experience that empties the stomach by a different route -many factors signal the stomach to "launch lunch" but the most common are extreme stretching of the stomach or intestine or irritants such as bacterial toxins, excessive alcohol, spicy foods, and certain drugs -bloodborne molecules and sensory impulses stream from the irritated sites to the emetic center of the medulla where they initiate a number of motor responses -before vomiting, an individual typically feels nauseated, is pale, and salivates excessively -a deep inspiration directly precedes vomiting -the diaphragm and abdominal wall muscles contract, increasing abdominal pressure -the gastroesophageal sphincter relaxes, and the soft palate rises to close off the nasal passages -as a result, the stomach (and perhaps duodenal) contents are forced upward through the esophagus and pharynx and out the mouth -excessive vomiting can cause dehydration and severely disrupt the body's electrolyte and acid-base balance -because large amounts of HCl are lost in vomitus, the blood becomes alkaline as the stomach attempts to replace its lost acid

digestive system after birth

-feeding is a newborn baby's most important activity, and several reflexes enhance the infant's ability to obtain food -the rooting reflex helps the baby find the nipple, and the sucking reflex helps the baby hold onto the nipple and swallow -newborn babies tend to double their birth weight within six months, and their caloric intake and food processing ability are extraordinary -for example a 6-week-old infant weighing about 4 kg (less than 9 lbs) drinks about 600 ml of milk daily - a 65-kg adult (143 lb) would have to drink 10 L of milk to ingest a corresponding volume of fluid -however, the stomach of a new-born infant is very small, so feeding must be frequent (every 3-4 hrs) -peristalsis is inefficient, and vomiting is not unusual -as the teeth break through the gums, the infant progresses to solid foods and is usually eating an adult diet by the age of 2 years -as a rule, the digestive system operates throughout childhood and adulthood with relatively few problems -however, contaminated food or extremely spicy or irritating food sometimes cause gastroenteritis , inflammation of the GI tract -ulcers and gallbladder problems-- inflammation or cholecystitis and gallstones-- are problems of middle age

the palate

-forming the roof of the mouth, has two distinct parts; the hard palate anteriorly and the soft palate posteriorly -the hard palate is underlain by the palatine bones and the palatine processes of the maxillae, and it forms a rigid surface against which the tongue forces food during chewing -the mucosa on either side of its raphe, a midline ridge, is slightly corrugated, which helps create friction -the soft palate is a mobile fold formed mostly of skeletal muscle that rises reflexively to close off the nasopharynx when we swallow -laterally, the soft palate is anchored to the tongue by the palatoglossal arches and to the wall of the oropharynx by the more posterior palatopharyngeal arches -these two paired folds form the boundaries of the faucet, the arched area of the oropharynx that contains the palatine tonsils -projecting downward from the free edge of the soft palate is the fingerlike uvula

large intestine

-frames the small intestine on three sides and extends from the ileocecal valve to the anus -its diameter is greater than that of the small intestine, but it is much shorter -its major digestive functions are to absorb most of the remaining water from indigestible food residues, store the residues temporarily, and then eliminate them from the body as semisolid feces, also called stool -it also absorbs metabolites produced by resident bacteria as they ferment carbohydrates not absorbed in the small intestine

histology of the alimentary canal

-from the esophagus to the anal canal, the walls of the alimentary canal have the same four basic layers or tunics-- mucosa, submucosa, muscularis externa, and serosa -each layer contains a predominant tissue type that plays a specific role in food breakdown

the pharynx

-from the mouth, food passes posteriorly into the oropharynx and then the laryngopharynx, both common passageways for food, fluids, and air (the nasopharynx has no digestive role) -the histology of the pharyngeal wall resembles that of the oral cavity -the mucosa contains a friction-resistant stratified squamous epithelium well supplied with mucus-producing glands -the external muscle layer consists of two skeletal muscle layers -the cells of the inner layer run longitudinally -those of the outer layer, the pharyngeal constrictor muscles, encircle the wall like three stacked fists -contractions of these muscles propel food into the esophagus below

types of gland cells

-glands of the stomach fundus and body, where most digestion occurs, are substantially larger and produce the majority of the stomach secretions -the glands in these regions contain a variety of secretory cells, including mucous neck, parietal, chief, and enteroendocrine cells

hepatitis C

-has emerged as the most important infectious liver disease in the US because it produces persistent or chronic liver infections -more than 4 million Americans are infected and over 10,000 die annually due to sequels of HVC infection -however, the life-threatening C form of hepatitis can now be successfully treated by a 12-week combination drug therapy

hiatal hernia

-heartburn is also common in those with a hiatal hernia, a structural abnormality (most often due to abnormal relaxation or weakening of the gastroesophageal sphincter) in which the superior part of the stomach protrudes slightly above the diaphragm -since the diaphragm no longer reinforces the sphincter, gastric juice may enter the esophagus, particularly when lying down -if the episodes are frequent and prolonged, esophagitis (inflammation of the esophagus) and esophageal ulcers may result -an even more threatening sequel is esophageal cancer -treatment varies, but GERD is usually addressed with lifestyle and dietary modifications, along with antacids and certain prescription drugs

regulation of bile and pancreatic secretion

-hormones and neural stimuli regulate both the secretion of bile and pancreatic juice and their release into the small intestine -the hormones include two enterogastrones-- cholecystokinin and secretin -bile salts themselves are the major stimulus for enhanced bile secretion -after a fatty meal. when the enterohepatic circulation is returning large amounts of bile salts to the liver, its output of bile rises dramatically -secretion also stimulates liver cells to secrete bile -when no digestion is occurring, the hepatopancreatic sphincter is closed and the released bile backs up the cystic duct into the gallbladder, where it is stored until needed -although the liver makes bile continuously, bile does not usually enter the small intestine until the gallbladder contracts

after a meal

-if we examine the small intestine with X-ray fluoroscopy after it is "loaded" with a meal, it looks like the intestinal contents are being massaged-- alternately contracting and relaxing rings of smooth muscle simply move the chyme backward and forward a few centimeters at a time -as with stomach peristalsis, intrinsic pacemaker cells (in the myenteric plexus) initiate these segmenting movements -the intensity of segmentation is altered by long and short reflexes, which parasympathetic activity enhances and sympathetic activity decreases, and by hormones. the more intense the contractions, the greater the mixing -some very weak, short-distance peristalsis does occur even in the full small intestine -the pacemakers in the duodenum depolarize more frequently than those in the ileum -as a result, segmentation also moves intestinal contents slowly and steadily toward the ileocecal valve at a rate that allows ample time to complete digestion and absorption -unlike the strength of contraction, the basic contractile rhythm of each intestinal region is not affected by reflexes or hormones

malabsorption

-impaired nutrient absorption, has many and varied causes -it can result from anything that interferes with the delivery of bile or pancreatic juice to the small intestine -factors that damage the intestinal mucosa (severe bacterial infections and some antibiotics) or reduce its absorptive surface area are also common causes

lactose intolerance

-in some people, intestinal lactase is present at birth but then becomes deficient due to genetic factors -when people with __________ __________ consume lactose, the undigested disaccharides create osmotic gradients that prevent water from being absorbed in the intestines and also pull water from the interstitial space into the intestines -the result is diarrhea -bacterial metabolism of the undigested solutes produces large amounts of gas that result in bloating, flatulence, and cramping pain -for the most part, the solution to this problem-- add lactase enzyme "drops" to your milk or take a lactase tablet before consuming milk products

carbohydrates

-in the average diet, most (up to 60%) digestible carbohydrates are in the form of starch, with smaller amounts of disaccharides and monosaccharides -only three monosaccharides are common in our diet: glucose, fructose, and galactose -the more complex carbohydrates that our digestible system is able to break down to monosaccharides are the disaccharides sucrose (table sugar), lactose (milk sugar) and maltose (grain sugar) and the polysaccharides glycogen and starch -digestion of starch begins in the mouth -salivary amalyse, present in saliva, splits starch into oligosaccharides, smaller fragments of two to eight linked glucose molecules -starch digestion continues until salivary amylase is inactivated by stomach acid and broken apart by the stomach's protein-digesting enzymes -generally speaking, the larger the meal, the longer salivary amylase continues to work in the stomach because foodstuffs in its relatively immobile fundus are poorly mixed with gastric juices -humans lack enzymes capable of breaking down most other polysaccharides, such as cellulose -as a result, indigestible polysaccharides do not nourish us, but they do help move food along the GI tract by providing fiber

splanchnic circulation

-includes those arteries that branch off the abdominal aorta to serve the digestive organs and the hepatic portal circulation -the arterial supply-- the branches of the celiac trunk that serve the spleen, liver, and stomach, and the mesenteric arteries that serve the small and large intestines-- normally receives one-quarter of the cardiac output -this percentage increases after a meal -the hepatic portal circulation collects nutrient-rich venous blood draining from the digestive viscera and delivers it to the liver

hepatitis

-inflammation of the liver -has many causes including the toxic effects of alcohol, drugs, and wild mushrooms -six viruses that cause ___ have been identified and named HVA to HVF -two of these (HVA and HVE) are transmitted enterically (acquired through eating contaminated food) and the infections they cause tend to be self-limiting -those transmitted via blood-- most importantly, HVB and HVC-- are linked to chronic hepatitis, liver cirrhosis, and cancer

iron

-ionic iron, essential for hemoglobin production, is actively transported into the mucosal cells, where it binds to the protein ferritin -the intracellular iron-ferritin complexes then serve as local storehouses for iron -when body reserves of iron are adequate, only 10-20% is allowed to pass into the portal blood, and most of the stored iron is lost as the enterocyte later slough off -however, when iron reserves are depleted (as during acute or chronic hemorrhage), iron uptake from the intestine and its release to the blood accelerate -in the blood, iron binds to transferrin, a plasma protein that transports it in the circulation -menstrual bleeding is a major route of iron loss in females, and premenopausal women require about 50% more iron in their diets -the intestinal enterocytes of women have about four times as many iron transport proteins as do those of men, and little iron is lost from the body other than that lost in menses

mesentery

-is a double layer of peritoneum-- a sheet of two serous membranes fused back to back-- that extends to the digestive organs from the body wall have the following functions: -they provide routes for blood vessels, lymphatics, and nerves to reach the digestive tract -they hold organs in place -they store fat -in most places, the mesentery is dorsal and attaches to the posterior abdominal wall, but there are ventral mesenteries too, such as the one that extends from the liver to the anterior abdominal wall -some digestive organ mesenteries have specific names (such as the omenta) or are called "ligaments" (even though these peritoneal folds are nothing like the fibrous ligaments that connect bones) -not all alimentary canal organs are suspended by a mesentery -for example, during development, some regions of the small intestine adhere to the dorsal abdominal wall -in doing so, they lose their mesentery and come to lie posterior to the peritoneum -these organs, which include most of the pancreas and duodenum (the first part of the small intestine), are called retroperitoneal organs -by contrast, digestive organs (like the stomach) that keep their mesentery and remain in the peritoneal cavity are called intraperitoneal or peritoneal organs

irritable bowel syndrome (IBS)

-is a functional GI disorder not explained by anatomical or biochemical abnormalities -affected individuals have recurring (or persistent) abdominal pain that is relieved by defecation -additionally, they may have changes in the consistency and frequency of their stools, and varying complaints of bloating, flatulence, nausea, and depression -stress is a common precipitating factor, and stress management is an important aspect of treatment

the esophagus

-is a muscular tube about 25 cm (10 inches) long and is collapsed when not involved in food propulsion -as food moves through the laryngopharynx, the epiglottis closes off the larynx and incoming food is routed posteriorly into the esophagus -takes a fairly straight course through the mediastinum of the thorax -it pierces the diaphragm at the esophageal hiatus to enter the abdomen -it joins the stomach at the cardial orifice within the abdominal cavity -the cardial orifice is surrounded by the gastroesophageal or cardiac sphincter, which is a physiological sphincter -that is, it acts as a sphincter, but the only structural evidence of this sphincter is a slight thickening of the circular smooth muscle at that point -the muscular diaphragm, which surrounds this sphincter, helps keep it closed when food is not being swallowed -mucous cells on both sides of the sphincter help protect the esophagus from reflux of stomach acid -unlike the mouth and pharynx, the esophagus wall has all four of the basic alimentary canal layers described earlier some features of interest: -the esophageal mucosa contains a nonkeratinized stratified squamous epithelium. at the esophagus-stomach junction, that abrasion-resistant epithelium changes abruptly to the simple columnar epithelium of the stomach, which is specialized for secretion -the submucosa contains mucus-secreting esophageal glands. as a bolus moves through the esophagus, it compresses these glands, causing them to secrete mucus that "greases" the esophageal walls and aids food passage -the muscularis externa is skeletal muscle in its superior third, a mixture of skeletal and smooth muscle in its middle third, and entirely smooth muscle in its inferior third -instead of a serosa, the esophagus has a fibrous adventitia composed entirely of connective tissue, which blends with surrounding structures along its route

gallbladder

-is a thin-walled muscular sac that may appear green when filled with bile -the size of a kiwi fruit, it snuggles in a shallow fossa on the inferior surface of the liver from which its rounded fundus protrudes -stores bile that is not immediately needed for digestion and concentrates it by absorbing some of its water and ions -when empty, its mucosa is thrown into honeycomb like folds that, like the rugae of the stomach, allow the organ to expand as it fills -its muscular wall contracts to expel bile into the cystic duct -from there bile flows into the bile duct -like most of the liver, it is covered by visceral peritoneum

the mouth

-is also called the oral cavity or buccal cavity -its boundaries are the lips anteriorly, cheeks laterally, palate superiorly, and tongue inferiorly -its anterior opening is the oral orifice -posteriorly, the oral cavity is continuous with the oropharynx -the walls are lined with a thick stratified squamous epithelium which withstands considerable friction -the epithelium on the gums, hard palate, and dorsum of the tongue is slightly keratinized for extra protection against abrasion during eating

pancreas

-is important to the digestive process because it produces enzymes that break down all categories of foodstuffs -is a soft, tadpole-shaped gland that extends across the abdomen from its tail (next to the spleen) to its head, which is encircled by the C-shaped duodenum -most of the pancreas is retroperitoneal and lies deep to the greater curvature of the stomach -contains exocrine and endocrine parts the exocrine part of the pancreas produces pancreatic juice and consists of the following: -acini. acini are clusters of secretory acing cells that produce the enzyme-rich component of pancreatic juice. acinar cells are full of rough endoplasmic reticulum and exhibit deeply staining zymogen granules. these granules contain inactive digestive enzymes (proenzymes) -ducts. a system of ducts transports the secretions of the acinar cells. in addition, the epithelial cells of the smallest ducts secrete the water that makes up the bulk of the pancreatic juice and the bicarbonate that makes this secretion alkaline (about pH 8) the endocrine part of the pancreas is a scattering of mini endocrine glands called pancreatic islets. these islets release insulin and glucagon, hormones that play an important role in carbohydrate metabolism

peritonitis

-is inflammation of the peritoneum -it can arise from a piercing abdominal wound, a perforating ulcer that leaks stomach juices into the peritoneal cavity, or poor sterile technique during abdominal surgery -however, most commonly it results from a burst appendix that sprays bacteria-containing feces all over the peritoneum -in ________, the peritoneal coverings tend to stick together around the infection site -this localizes the infection, providing time for macrophages to prevent the inflammation from spreading -if it becomes widespread within the peritoneal cavity, it is dangerous and often lethal -treatment includes removing as much infectious debris as possible and administering megadoses of antibiotics

gross anatomy of the small intestine

-is the body's major digestive organ -within its twisted passageway, digestion is completed (with the help of bile and pancreatic enzymes) and virtually all absorption occurs -extends from the pyloric sphincter to the ileocecal valve (sphincter) where it joins the large intestine -it is the longest part of the alimentary canal, but is only about half the diameter of the large intestine -although 6-7 m long in a cadaver, it is only 7-13 ft long during life because of muscle tone -has three subdivisions: the duodenum, which is mostly retroperitoneal, and the jejunum and ileum, both intraperitoneal organs -the relatively immovable duodenum which curves around the head of the pancreas is about 10 inches long. although it is the shortest intestinal subdivision, the duodenum has the most features of interest, including the major duodenal papilla -the jejunum about 8 ft long, extends from the duodenum to the ileum -the ileum joins the large intestine at the ileocecal valve -the jejunum and ileum hang in sausage like coils in the central and lower part of the abdominal cavity, suspended from the posterior abdominal wall by a fan-shaped mesentery -the large intestine encircles these more distal parts of the small intestine -the arterial supply of the small intestine is primarily from the superior mesenteric artery -the veins parallel the arteries and typically drain into the superior mesenteric vein -from there, the nutrient-rich venous blood from the small intestine drains into the hepatic portal vein, which carries it to the liver -nerve fibers serving the small intestine include parasympathetics from the vagus and sympathetics from the thoracic sphlanchnic nerves, both relayed through the superior mesenteric (and celiac) plexus

cephalic (reflex) phase

-occurs before food enters the stomach -only a few minutes long, this phase is triggered by the aroma, taste, sight, or thought of food -these triggers act via the vagus nerve to stimulate gastric glands, getting the stomach ready for its digestive chore

cirrhosis

-is the last stage of progressive chronic inflammation of the liver -typically results from severe chronic hepatitis due to chronic alcoholism, NAFLD, or viral hepatitis -while damaged hepatocytes can regenerate, the liver's connective (scar) tissue regenerates faster -liver activity is depressed and the liver becomes fibrous with scar tissue -the scar tissue obstructs blood flow throughout the hepatic portal system, causing portal hypertension -liver transplants are the only clinically proven effective treatment for patients with end-stage liver disease -the one- and five- year survival rate of such transplants is approximately 90% and 75% respectively -the regenerative capacity of a healthy liver is exceptional. it can regenerate to its former size in 6-12 months even after surgical removal or loss of 80% of its mass -this means that part of a living donor's liver can be removed for transplant without long-term harm to the donor

submucosa

-just external to the mucosa, is areolar connective tissue containing a rich supply of blood and lymphatic vessels, lymphoid follicles, and nerve fibers that supply the surrounding tissues of the GI tract wall -its abundant elastic fibers enable the stomach, for example, to regain its normal shape after temporarily storing a large meal

the teeth

-lie in sockets (alveoli) in the gum-covered margins of the mandible and maxilla -we masticate, or chew, by opening and closing our jaws and moving them side to side while using our tongue to move the food between our teeth -in the process, the teeth tear and grind the food, physically breaking it down into smaller fragments -ordinarily, by age 21, two sets of teeth, the primary and permanent dentitions, have formed -the primary dentition consists of the deciduous teeth, also called milk or baby teeth -the first teeth to appear, at about age 6 months, are the lower central incisors -additional pairs of teeth erupt at one- to two- month intervals until about 24 months, when all 20 milk teeth have emerged -although primary teeth will eventually be replaced by permanent teeth, these first teeth nevertheless deserve careful attention. decaying primary teeth can be painful and may lead to serious infection -primary teeth also serve as important "place holders" for developing permanent teeth -primary teeth can be kept healthy by brushing and by limiting exposure to sugary liquids, especially from prolonged bottle feeding -as the deep-lying permanent teeth enlarge and develop, the roots of the milk teeth are resorbed from below, causing them to loosen and fall out between ages 6 and 12 -generally, all the permanent teeth but the third molars have erupted by the end of adolescence -the third molars, also called wisdom teeth, emerge between ages 17 and 25 -there are usually 32 permanent teeth in a full set, but sometimes the wisdom teeth never erupt or are completely absent -when a tooth remains trapped in the jawbone, it is said to be impacted -impacted teeth can cause a good deal of pressure and pain and must be removed surgically -wisdom teeth are most commonly involved

gastric contractile activity

-like the esophagus, the stomach exhibits peristalsis -after a meal, peristalsis begins near the gastroesophageal sphincter, where it produces gentle rippling movements of the thin stomach wall -but as the contractions approach the pylorus, where the stomach musculature is thicker, they become much more powerful -consequently, the contents of the fundus and body (food storage area) remain relatively undisturbed, while foodstuffs in and around the pyloric antrum receive a lively pummeling and mixing -the pyloric part of the stomach, which holds about 30 ml of chyme, acts as a dynamic filter that allow only liquids and small particles to pass through the barely open pyloric valve -normally, each peristaltic wave reaching the pyloric muscle squirts 3 ml or less of chyme into the small intestine -because the contraction also closes the valve, the rest is propelled backward into the stomach, where it is mixed further. this back-and-forth pumping action (retropulsion) effectively breaks up solids -although the strength of the stomach's peristaltic waves can be modified, their rate is constant-- always around three per minute -this contractile rhythm is set by enteric pacemaker cells, muscle-like non contractile cells formerly called interstitial cells of Cajal -located in the myenteric plexus, the pacemaker cells depolarize and depolarize spontaneously three times each minute, establishing the so-called cyclic slow waves of the stomach, or its basic electrical rhythm (BER) -since gap junctions couple the pacemakers electrically to the rest of the smooth muscle sheet, their "beat" is transmitted efficiently and quickly to the entire muscularis -the pacemakers set the maximum frequency of contraction, but they do not initiate the contractions or regulate their force -instead, they generate sub threshold depolarization waves, which are then "ignited" (enhance by further depolarization and brought to threshold) by neural and hormonal factors -the same factors that increase gastric secretions also enhance the strength of stomach contractions -distension of the stomach wall by food activates stretch receptors and gastrin-secreting cells, both of which ultimately stimulate gastric smooth muscles and increased gastric motility -for this reason, the more food there is in the stomach, the more vigorous the stomach mixing and emptying movements will be-- within certain limits

sources of enzymes for digestion

-most of the substances required for digestion-- bile, digestive enzymes, and bicarbonate ions-- are imported from the liver and pancreas -for this reason, anything that impairs liver or pancreatic function or delivery of their juices to the small intestine severely hinders our ability to digest food and absorb nutrients -brush border enzymes perform the final digestion of food into the simple components that can be absorbed by intestinal cells -brush border enzymes are not secreted. instead, they remain bound to the plasma membranes of the enterocytes

the tongue

-occupies the floor of the mouth -is composed of interlacing bundles of skeletal muscle fibers -during chewing, the tongue grips the food and constantly repositions it between the teeth -the tongue also mixes food with saliva, forming it into a compact mass called a bolus and then initiates swallowing by pushing the bolus posteriorly into the pharynx -the versatile tongue also helps us form consonants when we speak -has both intrinsic and extrinsic skeletal muscle fibers -the intrinsic muscles are confined in the tongue and are not attached to bone -their muscle fibers, which run in several different planes, allow the tongue to change its shape (but not its position), becoming thicker, thinner, longer, or shorter as needed for speech and swallowing -the extrinsic muscles extend to the tongue from their points of origin on bones of the skull or the soft palate -the extrinsic muscles alter the tongue's position -they protrude it, retract it, and move it from side to side -the tongue has a median septum of connective tissue, and each half contains identical muscle groups -a fold of mucosa called the lingual frenulum secures the tongue to the floor of the mouth and limits its posterior movements -the superior tongue surface bears papillae, peglike projections of the underlying mucosa -the conical filiform papillae roughen the tongue surface, helping us lick semisolid foods and providing friction for manipulating foods. these papillae, the smallest and most numerous type, align in parallel rows on the tongue dorsum. they contain keratin, which stiffens them and gives the tongue its whitish appearance -the mushroom-shaped fungiform papillae are scattered widely over the tongue surface. each has a vascular core that gives it a reddish hue -eight to twelve large vallate papillae are located in a V-shaped row at the back of the tongue. they resemble the fungiform papillae but have an additional surrounding furrow -pleatlike foliate papillae are located on the lateral aspects of the posterior tongue -the fungiform, vallate, and foliate papillae house taste buds -immediately posterior to the vallate papillae is the terminal sulcus, a groove that distinguishes the portion of the tongue that lies in the oral cavity (its body) from its posterior portion in the oropharynx (its root) -the mucosa covering the root of the tongue lacks papillae, but it is still bumpy because of the nodular lingual tonsil, which lies just deep to its mucosa

gastric phase

-once food reaches the stomach, local neural and hormonal mechanisms initiate the _____ phase. this phase lasts three to four hours and provides about two-thirds of the gastric juice released -stimulation. the most important secretory stimuli are distension, peptides, and low acidity 2a) stomach digestion activates stretch receptors and initiates both short and long reflexes. in the long reflexes, impulses travel to the medulla and then back to the stomach via vagal fibers 2b) chemical stimuli provided by partially digested proteins, caffeine, and rising pH directly activate gastrin-secreting enteroendocrine cells called G cells in the stomach antrum. during this phase, gastrin plays a major role in stimulating parietal cells to secrete HCl. it prods parietal cells to spew out HCl by 1) by acting directly on receptors on these cells and 2) by stimulating enteroendocrine cells to release histamine -when protein foods are in the stomach, the pH of the gastric contents generally rises because proteins act as buffers to tie up H+ -the rise in pH stimulates gastrin secretion and subsequently HCl release, which in turn provides the acidic conditions needed to digest proteins -the more protein in the meal, the greater the amount of gastrin and HCl released -as proteins are digested, the gastric contents gradually becomes more acidic, which again inhibits the gastrin-secreting cells -this negative feedback mechanism helps maintain optimal pH and working conditions for gastric enzymes -inhibition. 2c) highly acidic (pH below 2) gastric contents inhibit gastrin secretion-- a situation that commonly occurs between meals -stress, fear, anxiety, or anything that triggers the fight-or-flight response inhibits gastric secretion because the sympathetic division overrides parasympathetic (vagal) controls of digestion

metabolic functions of gut bacteria

-our gut bacteria helps us by recovering energy from otherwise indigestible foods and synthesizing some vitamins -fermentation. gut bacteria ferment some indigestible carbohydrates and the mucin in gut mucus. the resulting short-chain fatty acids can be absorbed and used for fuel by the body's cells -unfortunately, fermentation also produces a mixture of gases. some of these gases, such as dimethyl sulfide, are quite odorous. about 500 ml of gas (flatus) is produced each day, much more when we eat foods (such as beans) rich in indigestible carbohydrates -vitamin synthesis. B complex vitamins and some of the vitamin K the liver needs in order to produce several clotting proteins are synthesized by gut bacteria

non-alcoholic fatty liver disease (NAFLD)

-outpacing even hepatitis C and alcohol-associated liver damage, ________ has become the most common liver disease in North America -it affects about 30% of the general population, but 70% of the obese -obesity and increased insulin resistance are associated with abnormal lipid metabolism and liver inflammation, which cause ________ -there are usually no symptoms, but it predisposes the patient to develop full-blown cirrhosis or even liver cancer

potassium

-potassium ions move across the intestinal mucosa passively by facilitated diffusion (or leaky tight junctions) -as water is absorbed from the lumen, rising potassium levels in chyme create a concentration gradient for its absorption -anything that interferes with water absorption (resulting in diarrhea) not only reduces potassium absorption but also "pulls" K+ from the interstitial space into the intestinal lumen

cystic fibrosis

-primarily affects the lungs, but it also impairs the activity of the pancreas -in this genetic disease, the mucous glands produce abnormally thick mucus, which blocks ducts and passageways of organs -blockage of the pancreatic duct prevents pancreatic juice from reaching the small intestine -this impairs digestion, and most fats and fat-soluble vitamins are not digested or absorbed -consequently, the stools are bulky and fat laden -the pancreatic problems can be handled by administering pancreatic enzymes with meals

composition of saliva

-saliva is largely water-- 97 to 99.5%-- and therefore is hypo-osmotic -its osmolarity depends on the specific glands that are active and the stimulus for salivation -as a rule, saliva is slightly acidic (pH to 6.75 to 7.00), but its pH may vary its solutes include: -electrolytes (Na+, K+, Cl-, PO4^3-, and HCO3-) -the digestive enzymes salivary amylase and lingual lipase (lingual lipase makes only a minor contribution to overall fat digestion) -the proteins mucin, lysozyme, and IgA -metabolic wastes (urea and uric acid) -when dissolved in water, the glycoprotein mucin forms thick mucus that lubricates the oral cavity and hydrates foodstuffs protects against microorganisms because it contains: -IgA antibodies -lysozyme, a bactericidal enzyme that inhibits bacterial growth in the mouth and may help prevent tooth decay -defensins. besides acting as a local antibiotic, defensins function as cytokines to call defensive cells (lymphocytes, neutrophils, etc.) into the mouth for battle

enteric nervous system participates in both short and long reflex arcs

-short reflexes are mediated entirely by enteric nervous system plexuses in response to stimuli within the GI tract. control of the patterns of segmentation and peristalsis is largely automatic, involving pacemaker cells and reflex arcs between enteric neurons in the same or different organs -long reflexes involve CNS integration centers and extrinsic autonomic nerves. the enteric nervous system sends info to the CNS via visceral sensory fibers - it receives sympathetic and parasympathetic motor fibers from the autonomic nervous system - these enter the intestinal wall to synapse with neurons in the intrinsic plexuses. long reflexes can be initiated by stimuli arising inside or outside of the GI tract -in these reflexes, the enteric nervous system acts as a way station for the autonomic nervous system, allowing extrinsic controls to influence digestive activity -generally speaking, parasympathetic inputs enhance digestive activity and sympathetic impulses inhibit them

regulation of gastric motility and emptying

-stomach contractions not only accommodate its filling and cause its emptying, but they also compress, knead, and mix the food with gastric juice to produce chyme -the processes of mechanical breakdown and propulsion are inseparable in the stomach due to a unique type of peristalsis

the muscularis externa

-surrounds the submucosa -also simply called the muscularis -this layer is responsible for segmentation and peristalsis -it typically has an inner circular layer and an outer longitudinal layer of smooth muscle cells -in several places along the tract, the circular layer thickens, forming sphincters that act as valves to control food passage from one organ to the next and prevent back flow

the teeth (continued)

-teeth are classified according to their shape and function as incisors, canines, premolars, and molars -the chisel-shaped incisors are adapted for cutting or nipping off pieces of food -the conical or fanlike canines (cuspids or eye-teeth) tear and pierce -the premolars (bicuspids) and molars have broad crowns with rounded cusps (tips) best suited for grinding or crushing -the molars, with four or five cusps, are the best grinders -during chewing, the upper and lower molars repeatedly lock together, an action that generates tremendous crushing forces -the dental formula is a shorthand way of indicating the numbers and relative positions of the different types of teeth -this formula is written as a ratio, uppers over lowers, for one-half of the mouth -since the other side is a mirror image, we obtain total dentition by multiplying the dental formula by 2 -the primary dentition consists of two incisors (I), one canine (C), and two molars (M) on each side of each jaw, and its dental formula is written as 2I, 1C, 2M (upper jaw)/ 2I, 1C, 2M (lower jaw) x 2 (20 teeth)

anatomy of duct systems

-the bile duct, delivering bile from the liver, and the main pancreatic duct, carrying pancreatic juice from the pancreas, unite in the wall of the duodenum, the first section of the small intestine -they fuse together at a bulblike structure called the hepatopancreatic ampulla -the ampulla opens into the duodenum via the volcano-shaped major duodenal papilla -a smooth muscle valve called the heptopancreatic sphincter controls the entry of bile and pancreatic juice - a smaller accessory pancreatic duct empties directly into the duodenum just proximal to the main duct

relationship of the large intestine to the peritoneum

-the colon is retroperitoneal, except for its transverse and sigmoid parts -these parts are intraperitoneal and anchored to the posterior abdominal wall by mesentery sheets called mesocolons -the rectum (and sometimes the cecum) is also retroperitoneal

cleft palate and cleft lip

-the digestive system is susceptible to many congenital defects that interfere with feeding -the most common are _______ ______, in which the palatine bones or palatine processes of the maxillae (or both) fail to fuse, and _______ _______, which often occur together -of the two, cleft palate is far more serious because the child is unable to suck properly

enteric nervous system

-the digestive system, however, must create the optimal environment for its functioning within the lumen of the GI tract, an area that is actually outside the body -essentially all digestive tract regulatory mechanisms control luminal conditions so that food breakdown and absorption can occur there as effectively as possible -to accomplish this, the GI tract has its own ________ ______ _______ (sometimes called the gut brain) which consists of over 100 million neurons -is in the in-house nerve supply of the alimentary canal -it is staffed by enteric neurons that communicate widely with one another to regulate digestive system activity -these semiautonomous enteric neurons constitute the bulk of the two major intrinsic nerve plexuses (ganglia interconnected by unmyelinated fiber tracts) found in the walls of the alimentary canal: the submucosal and myenteric nerve plexuses -these plexuses interconnect like chicken wire all along the GI tract and regulate digestive activity throughout its length -the submucosal nerve plexus occupies the submucosa and the large myenteric nerve plexus lies between the circular and longitudinal muscle layers of the muscularis externa -enteric neurons of these plexuses provide the major nerve supply to the GI tract wall and control GI tract motility (motion)

developmental aspects of the digestive system

-the epithelial lining of the developing alimentary canal, or primitive gut, forms from endoderm -the rest of the wall arises from mesoderm -the anteriormost endoderm (that of the foregut) touches a depressed area of the surface ectoderm called the stomodeum -the two membranes fuse, forming the oral membrane, which soon breaks through to form the opening of the mouth -similarly, the end of the hindgut fuses with an ectodermal depression, the proctodeum, to form the cloacal membrane, which then breaks through to form the anus -by week 5, the alimentary canal is a continuous "tube" extending from mouth to anus and opens to the external environment at each end -shortly after that, the glandular organs (salivary glands, liver with gallbladder, and pancreas) buds out from the mucosa at various points -these glands retain their connections, which become ducts leading into the digestive tract -during fetal life, the developing infant receives all of its nutrients through the placenta -nonetheless, the fetal GI tract is "trained" in utero for future food digestion as the fetus naturally swallows some of the surrounding amniotic fluid -this fluid contains several chemicals that stimulate GI maturation, including gastrin and epidermal growth factor

heartburn

-the first symptom of gastroesophageal reflux disease (GERD), is the burning, radiating substernal pain that occurs when stomach acid regurgitates into the esophagus -symptoms are so similar to those of a heart attack that many first-time sufferers of heartburn are rushed to the ER -is most likely when a person has eaten or drunk to excess, and in conditions that force abdominal contents superiorly, such as extreme obesity, pregnancy, and running, which splashes stomach contents upward with each step

histology of the small intestine wall

-the four layers typical of the GI tract are also seen in the small intestine, but the mucosa and submucosa are modified to reflect the intestine's functions in the digestive pathway -between the villi, which are specialized for absorption, the small intestine mucosa is studded with tubular glands called intestinal crypts. the crypts decrease in number along the length of the small intestine five major types of cells are found in the mucosal epithelium of the villi and crypts: -enterocytes form the bulk of the epithelium. they are simple columnar absorptive cells bound by tight junctions and richly endowed with microvilli. these cells bear the primary responsibility for absorbing nutrients and electrolytes in the villi. in the crypts, enterocytes are primarily secretory cells that secrete intestinal juice, a watery mixture that contains mucus and serves as a carrier fluid for absorbing nutrients from chyme. -goblet cells are mucus-secreting cells found in the epithelia of the villi and crypts -enteroendocrine cells are the source of the enterogastrones--secretin and cholecystokinin. they are mostly found scattered in the crypts but some are also found in the villi -paneth cells, found deep in the crypts, are specialized secretory cells that fortify the small intestine's defenses by releasing antimicrobial agents such as defenses and lysozyme. these secretions destroy certain bacteria and help to determine which bacteria colonize the intestinal lumen -stem cells continuously divide in the depths of the crypts. their daughter cells differentiate to become all of the other cell types. most of these daughter cells differentiate as they gradually migrate up the villi. paneth cells, on the other hand, migrate to the very bottom of the crypts. enterocytes at the tips of the villi undergo apoptosis and are shed, renewing the villus epithelium every three to five days -treatments for cancer, such as radiation therapy and chemotherapy, preferentially target rapidly dividing cells. they kill cancer cells, but also nearly obliterate the rapidly dividing GI tract epithelium. many patients suffer nausea, vomiting, and diarrhea, after each treatment -mucosa-associated lymphoid tissue (MALT) includes both individual lymphoid follicles and aggregate lymphoid nodules, the latter called peyer's patches -peyer's patches are primarily located in the lamina propria but occasionally protrude into the submucosa below -their increasing abundance toward the distal end of the small intestine reflects the fact that this region contains huge numbers of bacteria that must be prevented from entering the bloodstream. -the lamina propria of the mucosa contains large numbers of immunoglobulin A (IgA)-secreting plasma cells that help protect against intestinal pathogens -the submucosa is typical areolar connective tissue. elaborate mucus-secreting duodenal glands in the submucosa of the duodenum produce an alkaline (bicarbonate-rich) mucus that helps neutralize acidic chyme moving in from the stomach -when this protective mucus barrier is inadequate, the intestinal wall erode and duodenal ulcers result -the muscularis is typical and bilayered. except for the bulk of the duodenum, which is retroperitoneal and has an adventitia, visceral peritoneum (serosa) covers the external intestinal surface

keeping pathogenic bacteria in check

-the immune system and the gut bacteria live in a dynamic equilibrium -the immune system destroys any bacteria that threaten to breach the mucosal barrier -the gut bacteria, on the other hand, instruct the immune system not to overreact to their presence in the lumen -potentially harmful bacteria in our large intestine are kept in check in two ways -first, beneficial bacteria outcompete and actively suppress harmful bacteria, and as a result normally vastly outnumber them -second, our immune system prevents bacteria from entering the body through the gut epithelium -an elegant system keeps the bacteria from breaching the mucosal barrier -dendritic cells sample the microbial antigens in the lumen -they then migrate to the nearby lymphoid follicles within the gut mucosa (MALT) and trigger an IgA antibody-mediated response restricted to the gut lumen -this prevents the bacteria from straying into tissues deep to the mucosa where they might elicit a much more widespread systemic response -while the immune system keeps the gut bacteria in check, the gut bacteria also profoundly shape our immune system responses -for example, the type of bacteria present influences the balance between subtypes of T cells, which in turn affects the balance between pro- and anti- inflammatory responses -the coexistence of the bacterial microbiota with our immune system does sometimes fail. when that happens, the painful and debilitating condition known as inflammatory bowel disease may result

the mucosa

-the innermost layer is the ______, or mucous membrane, a moist epithelial membrane that lines the alimentary canal lumen from mouth to anus its major functions are to: -secrete mucus, digestive enzymes, and hormones -absorb the end products of digestion into the blood -protect against infectious disease -the mucosa in a particular region of the GI tract may perform one or all three of these functions -more complex than most the mucosae in the body, the typical digestive mucosa consists of three sublayers: 1) a lining epithelium 2) a lamina propria and 3) a muscularis mucosae -except for that of the mouth, esophagus, and anus where it is stratified squamous, the epithelium of the mucosa is a simple columnar epithelium rich in mucus-secreting cells -the slippery mucus it produces protects certain digestive organs from being digested by enzymes working within their cavities and eases food passage along the tract -in the stomach and small intestine, the mucosa also contains both enzyme-synthesizing and hormone-secreting cells -in such sites, the mucosa is a diffuse endocrine organ as well as part of the digestive organ -the lamina propria, which underlies the epithelium, is loose areolar connective tissue -its capillaries nourish the epithelium and absorb digested nutrients -its isolated lymphoid follicles, part of MALT (the mucosa-associated lymphoid tissue) help defend us against bacteria and other pathogens, which have rather free access to our digestive tract -particularly large collections of lymphoid follicles occur within the pharynx (as the tonsils) and in the appendix -external to the lamina propria is the muscularis mucosae, a scant layer of smooth muscle cells that produces local movements of the mucosa that can enhance absorption and secretion

intestinal juice

-the intestinal glands normally secrete 1 to 2 L of intestinal juice daily -the major stimulus for its production comes from hypertonic or acidic chyme -normally, intestinal juice is slightly alkaline and isotonic with blood plasma -intestinal juice is largely water but also contains some mucus, which is secreted both by the duodenal glands and by goblet cells of the mucosa

gross anatomy of the large intestine

-the large intestine exhibits three features not seen elsewhere-- teniae coli, haustra, and epiploic appendages -except for its terminal end, the longitudinal muscle layer of its muscularis is mostly reduced to three bands of smooth muscle called teniae coli -their tone puckers the wall of the large intestine into rocketlike sacs called haustra -another obvious feature of the large intestine is its epiploic appendages, which are small fat-filled pouches of visceral peritoneum that hang from the surface of the large intestine

subdivisions of the large intestine

-the large intestine has the following subdivisions: cecum, appendix, colon, rectum, and anal canal -the saclike cecum, which lies below the ileocecal valve in the right iliac fossa, is the first part of the large intestine -attached to the posteromedial surface of the cecum is the blind, wormlike appendix -the appendix contains masses of lymphoid tissue, and as part of MALT, it plays an important role in body immunity -additionally, it serves as a storehouse of bacteria and recolonizes the gut when needed -however, the appendix has an important structural shortcoming-- its twisted structure makes it susceptible to blockage -the colon has several distinct regions. proximally, as the ascending colon, it travels up the right side of the abdominal cavity to the level of the right kidney. here it makes a right-angle turn-- the right colic (hepatic) flexure-- and travels across the abdominal cavity as the transverse colon -directly, anterior to the spleen, it bends acutely at the left colic (splenic) flexure and descends down the left side of the posterior abdominal wall as the descending colon. inferiorly, it enters the pelvis, where it becomes the S-shaped sigmoid colon -in the pelvis, at the level of the third sacral vertebra, the sigmoid colon joins the rectum, which runs posteroinferiorly just in front of the sacrum. despite its name, the rectum has three lateral curves or bends, represented internally as three transverse folds called rectal valves. these valves stop feces from being passed along with gas (flatus) -the anal canal, the last segment of the large intestine, lies in the perineum, entirely external to the abdominopelivc cavity -about 3 cm long, it begins where the rectum penetrates the levator ani muscle of the pelvic floor and opens to the body exterior to the anus -the anal canal has two sphincters, an involuntary internal anal sphincter composed of smooth muscle (part of the muscularis) and a voluntary external anal sphincter composed of skeletal muscle -the sphincters, which act like purse strings, open and close the anus, are ordinarily closed except during defecation -the rectum and anal canal lack teniae coli and haustra -however, the rectum's muscularis muscle layers are complete and well developed, consistent with its role in generating strong contractions to expel feces

the lips and cheeks

-the lips (labia) and the cheeks, which help keep food between the teeth when we chew, are composed of a core of skeletal muscle covered externally by skin -the orbicularis oris muscle forms the fleshy lips; the cheeks are formed largely by the buccinators -the recess bounded externally by the lips and cheeks and internally by the gums and teeth is the oral vestibule -the area that lies within the teeth and gums is the oral cavity proper -the labial frenulum is a median fold that joins the internal aspect of each lip to the gum

microscopic anatomy of the liver

-the liver is composed of sesame seed-sized structural and functional units called liver lobules -each lobule is a roughly hexagonal (six-sided) structure consisting of plates of liver cells, or hepatocytes, organized like bricks in a garden wall -the hepatocyte plates radiate outward from a central vein running in the longitudinal axis of the lobule at each of the six corners of a lobule is a portal triad (portal tract region) so named because it contains three basic structures: -a branch of the hepatic artery (supplying oxygen-rich arterial blood to the liver) -a branch of the hepatic portal vein (carrying venous blood laden with nutrients from the digestive viscera) -a bile duct -between the hepatocyte plates are enlarged, heavily fenestrated liver sinusoids -blood from both the hepatic portal vein and the hepatic artery proper percolates from the triad regions through these sinusoids and empties into the central vein -from the central veins blood eventually enters the hepatic veins, which drain the liver, and empties into the inferior vena cava -forming part of the sinusoid walls are star-shaped stellate macrophages, also called hepatic macrophages. they remove debris such as bacteria and worn-out blood cells from the blood as it flows past the versatile hepatocytes have large amounts of both rough and smooth ER, Golgi apparatus, peroxisomes, and mitochondria. equipped in this way, the hepatocytes can: -secrete some 900 ml of bile daily -process blood borne nutrients in various ways -store fat-soluble vitamins -play important roles in detoxification, such as ridding the blood of ammonia by converting it to urea -secreted bile flows through tiny canals, called bile canaliculi that run between adjacent hepatocytes toward the bile duct branches in the portal triads -although most illustrations show the canaliculi as discrete tubular structures, their walls are actually formed by the apical membranes of adjoining hepatocytes -blood and bile flow in opposite directions in the liver lobule -bile entering the bile ducts eventually leaves the liver via the common hepatic duct to travel toward the duodenum

the liver secretes bile; the pancreas secretes digestive enzymes

-the liver, gallbladder, and pancreas are accessory organs associated with the small intestine -the liver has many metabolic and regulatory roles -its digestive function: producing bile for export to the duodenum (first part of the small intestine) -bile is a fat emulsifier that breaks fats into tiny particles to make them more readily digestible -although the liver also processes nutrient-laden venous blood delivered to it from the digestive organs, this is a metabolic rather than a digestive role -the gallbladder is chiefly a storage organ for bile -the pancreas supplies most of the enzymes that digest chyme as well as bicarbonate that neutralizes stomach acid

peristalsis

-the major means of propulsion -involves alternating waves of contraction and relaxation of muscles in the organ walls -its main effect is to squeeze food along the tract, but some mixing occurs as well -peristaltic waves are so powerful that, once swallowed, food and fluids will reach your stomach even if you stand on your head

control of salivation

-the minor salivary glands secrete saliva continuously in amounts just sufficient to keep the mouth moist -but when food enters the mouth, the major glands are activated and large amounts of saliva pour out -the average output of saliva is about 1500 ml per day, but can be much higher when salivary glands are appropriately stimulated -salivation is primarily controlled by the parasympathetic division of the autonomic nervous system -when we ingest food, chemoreceptors and mechanoreceptors in the mouth send signals to the salivatory nuclei in the brain stem (pons and medulla) -as a result, the parasympathetic nervous system activity increases -impulses sent via motor fibers in the facial (VII) and glossopharyngeal (IX) nerves dramatically increase the output of watery (serous), enzyme-rich saliva -the chemoreceptors are activated most strongly by acidic substances -the mechanoreceptors are activated by virtually any mechanical stimulus in the mouth--even chewing rubber bands -sometimes just the sight or smell of food is enough to get the juices flowing -irritation of the lower GI tract by bacterial toxins, spicy foods, or hyperacidity also increases salivation -this response may help wash away or neutralize the irritants -in contrast to parasympathetic controls, the sympathetic division (specifically fibers in T1-T3) causes release of a thick, mucin-rich saliva -strong activation of the sympathetic division constricts blood vessels serving the salivary glands and almost completely inhibits saliva release, causing a dry mouth -dehydration also inhibits salivation because low blood volume reduces filtration pressure at capillary beds

digestive processes of the mouth

-the mouth and its accessory digestive organs are involved in four of the six digestive processes described earlier -the mouth 1) ingests, 2) begins mechanical breakdown by chewing, 3) initiates propulsion by swallowing, and 4) starts the digestion of polysaccharides -absorption does not occur in the mouth except for a few drugs that are absorbed through the oral mucosa -chewing and swallowing are the processes that promote mechanical breakdown and propulsion, respectively

nucleic acids

-the nuclei of the cells of ingested foods contain DNA and RNA -pancreatic nucleases in pancreatic juice hydrolyze the nucleic acids to their nucleotide monomers -intestinal brush border enzymes (nucleosidases and phosphatases) then break the nucleotides apart to release their nitrogenous bases, pentose sugars, and phosphate ions -special carriers in the epithelium of the villi actively transport the breakdown products of nucleic acid digestion across the epithelium. these then enter the blood

serosa

-the outermost layer of the intraperitoneal organs, is the visceral peritoneum -in most alimentary canal organs, it is formed of areolar connective tissue covered with mesothelium, a single layer of squamous epithelial cells -in the esophagus, which is located in the thoracic instead of the abdominopelvic cavity, the ______ is replaced by an adventitia, ordinary dense connective tissue that binds the esophagus to surrounding structures -retroperitoneal organs have both an adventitia (on the side facing the dorsal body wall) and a serosa (on the side facing the peritoneal cavity)

digestive processes: swallowing

-the pharynx and esophagus merely serve as conduits to pass food from the mouth to the stomach -their single digestive system function is food propulsion, accomplished by deglutition or swallowing -to send food on its way to the mouth, it is first compacted by the tongue into a bolus and is then swallowed. this complicated process involves the coordinated activity of over 22 separate muscle groups two major phases involved in deglutition: -the buccal phase occurs in the mouth and is voluntary. it ends when a food bolus or a "bit of saliva" leaves the mouth and stimulates tactile receptors in the posterior pharynx, initiating the next phase -the pharyngeal-esophageal phase is involuntary and is controlled by the swallowing center in the brain stem (medulla and lower pons) -various cranial nerves, most importantly in the vagus nerves, transmit motor impulses from the swallowing center to the muscles of the pharynx and esophagus -once food enters the pharynx, respiration is momentarily inhibited and all routes except the desired one into the digestive tract are blocked off -solid foods pass from the oropharynx to the stomach in about 8 seconds and fluids, aided by gravity, pass in 1 to 2 seconds

defecation

-the rectum is usually empty, but when mass movements force feces into it, stretching of the rectal wall initiates the defecation reflex -this parasympathetic spinal reflex causes the sigmoid colon and the rectum to contract, and the internal anal sphincter to relax -as feces are forced into the anal canal, messages reach the brain allowing us to decide whether the external (voluntary) anal sphincter should open or remain constricted to stop passage of feces temporarily -if defecation is delayed, the reflex contractions end within a few seconds and the rectal walls relax -the next mass movement initiates the defecation reflex again until the person chooses to defecate or the urge becomes irresistible -during defecation, the muscles of the rectum contract to expel the feces -we aid this process voluntarily by closing the glottis and contracting our diaphragm and abdominal wall muscles to increase the intra-abdominal pressure ( a procedure called the valsalva maneuver) -we also contract the levator ani muscle which lifts the anal canal superiorly -this lifting action leaves the feces below the anus-- and outside the body -involuntary or automatic defecation (fecal incontinence) occurs in infants because they have not yet gained control of their external anal sphincter -it also occurs in those with spinal cord transections

gross anatomy of the liver

-the ruddy, blood-rich liver is the largest gland in the body, weighing about 1.4 kg (3 lb) in the average adult -shaped like a wedge, it occupies most of the right hypochondriac and epigastric regions, extending farther to the right of the body midline than to the left -located under the diaphragm, the liver lies almost entirely within the rib cage, which provides some protection -has four primary lobes. the largest, the right lobe, is visible on all liver surfaces and separated from the smaller left lobe by a deep fissure -the posterior most caudate lobe and the quadrate lobe, which lies inferior to the left lobe, are visible in an inferior view of the liver -a mesentery, the falciform ligament, separates the right and left lobes anteriorly and suspends the liver from the diaphragm and anterior abdominal wall -running along the inferior edge of the falciform ligament is the round ligament, or ligament teres, a fibrous remnant of the fetal umbilical vein -except for the superiormost liver area (the bare area), which touches the diaphragm, the entire liver is enclosed by the visceral peritoneum -the lesser omentum anchors the liver to the lesser curvature of the stomach -the hepatic artery proper and the hepatic portal vein, which enter the liver at the porta hepatis, and the common hepatic duct, which runs inferiorly from the liver, all travel through the lesser omentum to reach their destinations -the gallbladder rests in a recess on the inferior surface of the right liver lobe -the traditional scheme of defining liver lobes is based on superficial features of the liver -hepatic surgeons use a different system that divides the liver into eight segments based on its internal anatomy relative to its vascular and biliary supply -this system delineates sections that can be removed while encountering the fewest major vascular structures and the lowest risk -bile leaves the liver lobes through the right and left hepatic ducts -these fuse to form the large common hepatic duct, which travels downward toward the duodenum -along its course, that duct fuses with the cystic duct draining the gallbladder to form the bile duct

vitamin absorption

-the small intestine absorbs dietary vitamins, and the large intestine absorbs some of the K and B vitamins made by its gut bacterial "guests" -fat-soluble vitamins (A,D, E, and K) dissolve in dietary fats, become incorporated into the micelles, and move across the villus epithelium passively (by diffusion) -it follows that gulping pills containing fat-soluble vitamins without simultaneously eating some fat-containing food results in little or no absorption of these vitamins -most water-soluble vitamins (B vitamins and vitamin C) are absorbed via specific active or passive transporters -the exception is vitamin B12, which is a very large, charged molecule -intrinsic factor, produced by the stomach, binds to vitamin B12 -the vitamin B12- intrinsic factor complex then binds to specific mucosal receptor sites in the terminal ileum, which trigger its active uptake by endocytosis

modifications of the small intestine for absorption

-the small intestine is highly adapted for absorbing nutrients -its length alone provides a large surface area, and its wall has three structural modifications-- circular folds, villi, and microvilli--that amplify its absorptive surface enormously -the circular folds are deep, permanent folds of the mucosa and submucosa. these folds force chyme to spiral through the lumen, slowing its movement and allowing time for full nutrient absorption -villi are fingerlike projections of the mucosa that give it a velvety texture, much like the soft nap of a towel -the villi are large and leaflike in the duodenum (the intestinal site of most active absorption) and gradually narrow and shorten along the length of the small intestine -in the core of each villus is a dense capillary bed and a wide lymphatic capillary called a lacteal. digested foodstuffs are absorbed through the enterocytes into both the capillary blood and the lacteal -microvilli are long densely packed cytoplasmic extensions of the absorptive cells of the mucosa that give the mucosal surface a fuzzy appearance called the brush border -the plasma membranes of the microvilli bear enzymes referred to as brush border enzymes, which complete the digestion of carbohydrates and proteins in the small intestine

gross anatomy of the stomach

-the stomach lies in the upper left quadrant of the peritoneal cavity, nearly hidden by the liver and diaphragm -the adult stomach varies from 15 to 25 cm ( 6 to 10 inches) long, but its diameter and volume depend on how much food it contains -an empty stomach has a volume of about 50 ml and a cross-sectional diameter only slightly larger than the large intestine, but when it is really distended it can hold about 4 L (1 gal) of food and may extend nearly to the pelvis -when empty, the stomach collapses inward, throwing its mucosa (and submucosa) into large, longitudinal folds called rugae -the small carpal part, or cardia, surrounds the cardial orifice through which food enters the stomach from the esophagus -the fundus is the stomach's dome-shaped part, tucked beneath the diaphragm, that bulges superolaterally to the cardia -the body, or the mid portion of the stomach, is continuous inferiorly with the funnel-shaped pyloric part -the wider and more superior area of the pyloric part, the pyloric antrum, narrows to form the pyloric canal, which terminates at the pylorus -the pylorus is continuous with the duodenum through the pyloric sphincter or valve, which controls stomach emptying -the convex lateral surface of the stomach is its greater curvature, and its concave medial surface is the lesser curvature -extending from these two curvatures are two mesenteries, called omenta, that help tether the stomach to other digestive organs and the body wall -the lesser omentum runs from the liver to the lesser curvature of the stomach, where it becomes continuous with the visceral peritoneum covering the stomach -the greater omentum drapes inferiorly from the greater curvature of the stomach to cover the coils of the small intestine -it then runs dorsally and superiorly, wrapping the spleen and the transverse portion of the large intestine before blending with the mesocolon, a dorsal mesentery that secures the large intestine to the parietal peritoneum of the posterior abdominal wall -the greater omentum is riddled with fat deposits that give it the appearance of a lacy apron -it also contains large collections of lymph nodes -the immune cells and macrophages in these nodes "police" the peritoneal cavity and intraperitoneal organs -the stomach is served by the autonomic nervous system -sympathetic fibers from thoracic splanchnic nerves are relayed through the celiac ganglion -parasympathetic fibers are supplied by the vagus nerve -the arterial supply of the stomach is provided by branches (gastric and splenic) of the celiac trunk -the corresponding veins are part of the hepatic portal system and ultimately drain into the hepatic portal vein

the mucosal barrier

-the stomach mucosa is exposed to some of the harshest conditions in the entire digestive tract -gastric juice is corrosively acidic (the H+ concentration in the stomach can be 100,000 times that found in blood), and its protein-digesting enzymes can digest the stomach itself however, the stomach protects itself by producing the ________ _____. three factors create this barrier: -a thick coating of bicarbonate-rich mucus builds up on the stomach wall -the epithelial cells of the mucosa are joined together by tight junctions that prevent gastric juice from leaking into underlying tissue layers -damaged epithelial mucosal cells are shed and quickly replaced by division of undifferentiated stem cells that reside where the gastric pits join the gastric glands -the stomach surface epithelium of mucous cells is completely renewed every three to six days, but the more sheltered glandular cells deep within the gastric glands have a much longer life span

regulation of gastric emptying

-the stomach usually empties completely within four hours after a meal -however, the larger the meal (the greater the stomach distension) and the more liquid its contents, the faster the stomach empties -fluids pass quickly through the stomach -solids linger, remaining until they are well mixed with gastric juice and converted to the liquid state -the rate of gastric emptying also depends as much--and perhaps more--on the contents of the duodenum as on what is in tandem -as chyme enters the duodenum, receptors in its wall respond to chemical signals and to stretch -this initiates the enterogastric reflex and the hormonal (enterogastrone) mechanisms that inhibit gastric secretion as we described earlier -these mechanisms also prevent further duodenal filling by reducing the force of pyloric contractions -a carbohydrate rich meal moves through the duodenum rapidly, but fats form an oily layer at the top of the chyme and are digested more slowly by enzymes acting in the intestine -for this reason, when chyme entering the duodenum is fatty, reflexes slow stomach emptying, and food may remain in the stomach six hours or more

microscopic anatomy of the stomach

-the stomach wall contains the four layers typical of most of the alimentary canal, but its muscularis and mucosa are modified for the special roles of the stomach -besides the usual circular and longitudinal layers of smooth muscle, the muscularis externa has an incomplete innermost layer of smooth muscle fibrils that runs obliquely -together, the smooth muscles of the stomach allow it not only to mix, churn, and move food along the tract, but also to pummel the food, physically breaking it down into smaller fragments and ramming it into the small intestine -the lining epithelium of the stomach mucosa is a simple columnar epithelium composed entirely of mucous cells -they produce a cloudy, protective two-layer coat of alkaline mucus in which the surface layer consists of viscous, insoluble mucus that traps a layer of bicarbonate-rich fluid beneath it -this otherwise smooth lining is dotted with millions of deep gastric pits, which lead into tubular gastric glands that produce the stomach secretion called gastric juice -the cells forming the walls of the gastric pits are primarily mucous cells, but those composing the gastric glands vary in different stomach regions -for example, the cells in the glands of the cardia and pylorus primarily secrete mucus, whereas cells of the pyloric antrum produce mucus and several hormones including most of the stimulatory hormone called gastrin

intestinal phase

-this phase of gastric secretion begins with a brief stimulatory component followed by inhibition. -stimulation. the initial stimulatory part of the intestinal phase is set into motion as partially digested food fills the first part (duodenum) of the small intestine. this stimulates intestinal mucosal cells to release intestinal (enteric) gastrin, a hormone that encourages the gastric glands to continue their secretory activity. this stimulatory effect is brief because it is overridden by inhibitory stimuli as the intestine fills. -inhibition. four main factors in the duodenum cause it to put the "brakes" on gastric secretion. distension of the duodenum or the presence of acidic, fatty, or hypertonic chyme all trigger both neuronal and hormonal signals to tell the stomach "enough already". these same four factors also decrease gastric emptying. these brakes on gastric activity protect the small intestine from excessive acidity. they also prevent a massive influx of chyme from overwhelming the digestive and absorptive capacities of the duodenum by matching the amount of entering chyme to the processing abilities of the small intestine. inhibition is achieved in two ways: 1) enterogastric reflex: the duodenum inhibits acid secretion in the stomach by short reflexes through the enteric nervous system and by long reflexes involving sympathetic and vagus nerves 2) enterogastrones: the enterogastrone hormones are released by a scattering of enteroendocrine cells in the duodenal mucosal epithelium. the two most important enterogastrones are secretin and cholecystokinin (CCK). the enterogastrones inhibit gastric secretion and also play other roles

between meals

-true peristalsis occurs only after most nutrients have been absorbed -at this point, segmenting movements wane and the duodenal mucosa begins to release the hormone motilin -as motilin blood levels rise, peristaltic waves are initiates in the proximal duodenum every 90 to 120 minutes and sweep slowly along the intestine, moving 50-70 cm (about 2 ft) before dying out -each successive wave begins a bit more distally, a pattern of peristaltic activity called the migrating motor complex (MMC) -a complete "trip" from duodenum to ileum takes about two hours -the process then repeats itself, sweeping the last remnants of the meal plus bacteria, sloughed-off mucosal cells, and other debris into the large intestine -this "housekeeping" function prevents bacteria in the large intestine from entering the small intestine -as food again enters the stomach with the next meal, segmentation replaces peristalsis

enteroendocrine cells

-typically located deep in the gastric glands, release a variety of chemical messengers directly into the interstitial fluid of the lamina propria -some of these, for example, histamine and serotonin act locally as paracrines -others, such as somatostatin, act both as paracrines locally and as hormones that diffuse into the blood capillaries to influence several digestive system target organs -gastrin, a hormone, plays essential roles in regulating stomach secretion and motility

regulation of gastric secretion

-under normal conditions the gastric mucosa pours out as much as 3 L of gastric juice-- an acid brew so potent it can dissolve nails-- every day. both neural and hormonal mechanisms control gastric secretion -neural controls consist of both long (vagus nerve-mediated) and short (local enteric) nerve reflexes. in each case, acetylcholine (ACh) is released, stimulating the output of gastric juice. when the stomach is stimulated by the vagus nerves (which are parasympathetic), secretory activity of virtually all of its glands increases. in contrast, activation of sympathetic nerves depresses secretory activity -hormonal control of gastric secretion is largely the province of gastrin. it stimulates secretion of HCl by the stomach, and of hormones (mostly gastrin antagonists) by the small intestine -control of HCl-secreting parietal cells is multifaceted. it is stimulated by three chemicals: ACh, gastrin, and histamine -when only one of the three chemicals binds to parietal cell receptors, HCl secretion is scanty, but when all three bind, HCl pours forth -antihistamines that bind to and block the H2 (histamine) receptors of parietal cells are used to treat gastric ulcers caused by hyperacidity . histamine is a key player -stimuli acting at three distinct sites-- the head, stomach, and small intestine-- provoke or inhibit gastric secretions -accordingly, the three phases of gastric secretion are called the cephalic, gastric, and intestinal phases. one or more phases may occur at the same time

diarrhea

-watery stools -result from any condition that rushes food residue through the large intestine before that organ has had sufficient time to absorb the remaining water -causes include irritation of the colon by bacteria, or less commonly, prolonged physical jostling of the digestive viscera (occurs in marathon runners) -prolonged diarrhea may result in dehydration and electrolyte imbalance (acidosis and loss of potassium) -conversely, when food remains in the colon for extended periods, too much water is absorbed and the stool becomes hard and difficult to pass -this condition, called constipation, may result from insufficient fiber or fluid in the diet, improper bowel habits, lack of exercise, or laxative abuse

digestive processes in the large intestine

-what is finally delivered to the large intestine contains few nutrients, but it still has 12 to 24 hrs more to spend there -except for a small amount of digestion of that residue by the gut bacteria, no further food breakdown occurs in the large intestine -the large intestine harvests vitamins made by the gut bacteria and reclaims most of the remaining water and some of the electrolytes (particularly sodium and chloride) -however, nutrient absorption is not its major function -as mentioned, the primary concerns of the large intestine are propulsive activities that force fecal material toward the anus and eliminate it from the body (defecation) -the large intestine is important for our comfort, but it is not essential for life -if the colon is removed, the terminal ileum can be brought out to the abdominal wall in a procedure called an ileostomy -from there, food residues are eliminated into a sac attached to the abdominal wall

motility of the large intestine

-when food enters the colon through the ileocecal valve, the colon becomes motile, but its contractions are sluggish or short-lived compared to those of the small intestine -the movements most seen in the colon are haustral contractions, slow segmenting movements that last about one minute and occur every 30 min or so -these contractions, which occur mainly in the ascending and transverse colon, reflect local controls of smooth muscle within the walls of the individual haustra -as a haustrum fills with food residue, the distension stimulates its muscle to contract. these movements mix the residue, which aids in water absorption -mass movements (mass peristalsis) are long, slow-moving but powerful contractile waves that move over large areas of the colon three or four times daily and force the contents toward the rectum. typically, they occur during or just after eating. -the presence of food in the stomach activates the gastroileal reflex in the small intestine and the propulsive gastrocolic reflex in the colon -haustral contractions in the descending and sigmoid colon promote the final drying out of the feces -this part of the colon also stores feces until mass movements propel the feces into the rectum -fiber in the diet strengthens colon contractions and softens the feces, allowing the colon to act like a well-oiled machine -the semisolid feces delivered to the rectum contain undigested food residues, mucus, sloughed-off epithelial cells, millions of bacteria, and just enough water to allow their smooth passage -of the 500 ml or so of food residue entering the cecum daily, approximately 150 ml becomes feces

mechanism of HCl secretion

-when parietal cells are appropriately stimulated, H+ is actively pumped into the stomach lumen by H+-K+ ATPases (proton pumps) -as acid is pumped into the stomach, base (HCO3-) is exported into the blood. this flow of base is called the alkaline tide

diverticulitis

-when the diet lacks fiber and the volume of residues in the colon is small, the colon narrows and its contractions become more powerful, increasing the pressure on its walls -this promotes formation of diverticula, small herniations of the mucosa through the colon walls -this condition, called diverticulosis, most commonly occurs in the sigmoid colon, and affects over half of people over age 70 -in 4-10% of cases, it progresses to diverticulitis, in which the diverticula become inflamed and may rupture, leaking into the peritoneal cavity, which can be life threatening

lipid digestion and absorption in the small intestine

1) emulsification. bile salts in the duodenum break large fat globules into smaller fat droplets, increasing the surface area available to lipase enzymes 2) digestion. pancreatic lipases hydrolyze triglycerides, yielding monoglycerides and free fatty acids. 3) micelle formation. free fatty acids and monoglycerides assemble with bile salts, forming micelles. micelles ferry their contents to enterocytes 4) diffusion. fatty acids and monoglycerides diffuse from micelles into enterocytes 5) chylomicron formation. fatty acids and monoglycerides are recombined and packaged with other fatty substances and proteins to form chylomicrons. 6) chylomicron transport. chylomicrons are extruded from enterocytes by exocytosis, enter lacteals, and are carried away from the intestine in lymph

digesting and absorbing carbohydrates in the small intestine

1) pancreatic amylase breaks down starch and glycogen into oligosaccharides and disaccharides. starchy foods and other digestible carbohydrates that escape being broken down by salivary amylase are acted on by pancreatic amylase in the small intestine. about 10 min after entering the small intestine, starch is entirely converted to various oligosaccharides, mostly maltose 2) brush border enzymes break oligo- and disaccharides into monosaccharides. intestinal brush border enzymes further digest these products to monosaccharides. the most important brush border enzymes are dextrinase and glucoamylase, which act on oligosaccharides composed of more than three simple sugars, and maltase, sucrase, and lactase, which hydrolyze maltose, sucrose, and lactose respectively into their constituent monosaccharides. because the intestine can absorb only monosaccharides, all dietary carbohydrates must be digested to monosaccharides to be absorbed 3) monosaccharides are cotransported across the apical membrane of the enterocyte. glucose and galactose, liberated by the breakdown of starch and disaccharides, are shuttled by secondary active transport with Na+ into the enterocytes. fructose, on the other hand, enters the cells by facilitated diffusion. the proteins that transport monosaccharides into the cells are located very close to the disaccharidase enzymes on the brush border. they combine with the monosaccharides as soon as the disaccharides are broken down 4) monosaccharides exit across the basolateral membrane by facilitated diffusion. all types of monosaccharides move out of the enterocytes by facilitated diffusion and pass into the capillaries via intercellular clefts

the process of digesting and absorbing proteins in the small intestine

1) pancreatic proteases break down proteins and protein fragments into smaller pieces and some individual amino acids. protein fragments entering the small intestine are greeted by a host of proteolytic enzymes. trypsin and chymotrypsin cleave the proteins into smaller peptides. carboxypeptidases split off one amino acid at a time from the end of the polypeptide chain that bears the carboxyl group 2) brush border enzymes break oligo- and dipeptides into amino acids. a variety of brush border peptidases liberate individual amino acids from either end of a peptide chain (carboxypeptidases and amino peptidases), while dipeptidases break pairs of amino acids apart. carboxypeptidases and amino peptidases can independently dismantle a protein, but the teamwork between these enzymes and between trypsin and chymotrypsin, which attack the more internal parts of the protein, speeds up the process tremendously 3) amino acids are cotransported across the apical membrane of the enterocyte. several types of carriers transport the different amino acids resulting from protein digestion. most of these carriers, like those for glucose and galactose, are coupled to the active transport of sodium. short chains of two or three amino acids (dipeptides and tripeptides) are also actively absorbed using H+- dependent cotransport. they are digested to their amino acids within the enterocytes 4) amino acids exit across the basolateral membrane via facilitated diffusion. they then enter capillaries via intercellular clefts

basic concepts of regulating digestive activity

1. Digestive activity is provoked by a range of mechanical and chemical stimuli. receptors involved in controlling GI tract activity are located in the walls of the tract's organs. these receptors respond to several stimuli, most importantly stretching of the organ by food in the lumen, changes in osmolarity (solute concentration) and pH of the contents, and the presence of substrates and end products of digestion 2. Effectors of digestive activity are smooth muscle and glands. when stimulated, receptors in the GI tract initiate reflexes that stimulate smooth muscle of GI tract walls to mix lumen contents and move them along the tract. reflexes can also activate or inhibit glands that secrete digestive juices into the lumen or hormones into the blood 3. Neurons (intrinsic and extrinsic) and hormones control digestive activity. the nervous system controls digestive activity via both intrinsic controls (involving short reflexes entirely within the enteric nervous system) and extrinsic controls (involving long reflexes) -the stomach and small intestine also contain hormone-producing cells -when stimulated, these cells release their products to the interstitial fluid in the extracellular space -blood and interstitial fluid distribute these hormones to their target cells in the same or different digestive tract organs, where they affect secretion or contraction

digestive processes in the stomach

except for ingestion and defecation, the stomach is involved in the "menu" of digestive activities: -propulsion. like the esophagus, the stomach exhibits peristalsis -mechanical breakdown. the churning action provided by the stomach's smooth muscle during peristalsis causes mechanical breakdown of stomach contents -digestion. protein digestion begins in the stomach and is the main type of enzymatic breakdown that occurs there. HCl produced by stomach glands denatures dietary proteins in preparation for enzymatic digestion. (the unfolded amino acid chain is more accessible to the enzymes). the most important protein-digesting enzyme produced by the gastric mucosa is pepsin. in infants, however, the stomach glands also secrete rennin, an enzyme that acts on milk protein (casein), converting it to a curdy substance that looks like soured milk. fat digestion occurs primarily in the small intestine, but gastric and lingual lipase acting in the acidic ph of the stomach also contribute -absorption. not much is absorbed in the stomach, but two common lipid-soluble substances-- alcohol and aspirin-- pass easily through the stomach mucosa into the blood -despite the obvious benefits of preparing food to enter the intestine, the only stomach function essential to life is secretion of intrinsic factor. intrinsic factor is required for intestinal absorption of B12, needed to produce mature erythrocytes. in its absence, pernicious anemia results. however, if vitamin B12 is administered by injection, individuals can survive with minimal digestive problems even after oral gastrectomy (stomach removal)

major processes that occur during digestive system activity

involves six essential activities -ingestion: taking food into the digestive tract (eating) -propulsion: moves food through the alimentary canal, includes swallowing, which is initiated voluntarily, and peristalsis, an involuntary process -mechanical breakdown: increases the surface area of ingested food, physically preparing it for digestion by enzymes -mechanical processes include chewing, mixing food with saliva by the tongue, churning food in the stomach, and segmentation (rhythmic local constrictions of the small intestine). segmentation mixes food with digestive juices and makes absorption more efficient by repeatedly moving different parts of the food mass over the intestinal wall -digestion: involves a series of steps in which enzymes secreted into the lumen (cavity) of the alimentary canal break down complex food molecules to their chemical building blocks. it is a catabolic process -absorption: is the passage of digested end products (plus vitamins, minerals, and water) from the lumen of the GI tract through the mucosal cells by active or passive transport into the blood or lymph -defecation: eliminates indigestible substances form the body via the anus in the form of feces

proteins

proteins digested in the GI tract include: -dietary proteins (typically about 125 g per day) -enzyme proteins secreted into the GI tract by its various glands (15-25 g) -protein derived from sloughed and disintegrating mucosal cells -healthy individuals digest much of this protein all the way to its amino acid monomers -protein digestion begins in the stomach when pepsinogen secreted by the chief cells is activated to pepsin -pepsin functions optimally in the acidic pH range found in the stomach: 1.5-2.5 -it preferentially cleaves bonds involving the amino acids tyrosine and phenylalanine, breaking the proteins down into polypeptides and free amino acids -pepsin, which hydrolyzes 10-15% of ingested protein, is inactivated by the high pH in the duodenum, so its proteolytic activity is restricted to the stomach

salivary glands

saliva: -cleanses the mouth -dissolves food chemicals so they can be tasted -moistens food and helps compact it into a bolus -contains the enzyme amylase that begins the digestion of starchy foods -most saliva is produced by the major or extrinsic salivary glands that lie outside the oral cavity and empty their secretions into it -minor or intrinsic salivary glands (buccal glands and others) scattered throughout the oral cavity mucosa augment the output slightly -the major salivary glands are paired compound alveolar or tubuloalveolar glands that develop from the oral mucosa and remain connected to it by ducts -the large, roughly triangular parotid gland lies anterior to the ear between the masseter muscle and the skin. its prominent duct parallels the zygomatic arch, pierces the buccinator muscle, and opens into the vestibule next to the second upper molar -branches of the facial nerve run through the parotid gland on their way to the muscles of facial expression. for this reason, surgery on this gland can result in facial paralysis -normal salivary gland function is vital for oral health -if too little saliva is made, an uncomfortably dry mouth results, a condition called xerostomia -this lack of moisture may lead to difficulty with chewing and swallowing, and oral infections -xerostomia can be caused by certain medications ,diabetes, HIV/AIDS, and sjogren's syndrome (an autoimmune disease that affects moisture-producing glands throughout the body) -about the size of a walnut, the submandibular gland lies along the medial aspect of the mandibular body. its duct runs beneath the mucosa of the oral cavity floor and opens at the base of the lingual frenulum -the small, almond-shaped sublingual gland lies anterior to the submandibular gland under the tongue and opens via 10-20 ducts into the floor of the. mouth -the salivary glands are composed of two types of secretory cells: serous and mucous -serous cells produce a watery secretion containing enzymes, ions, and a tiny bit of mucin, whereas mucous cells produce mucus, a stringy, viscous solution -the parotid and submandibular glands contain mostly serous cells -buccal glands have approximately equal numbers of serous and mucous cells -the sublingual glands contain mostly mucous cells

microscopic anatomy of the large intestine

the wall of the large intestine differs in several ways from that of the small intestine: -there are no circular folds, villi, or brush border because most food is absorbed before reaching the large intestine -its mucosa is thicker, its abundant crypts are deeper, and the crypts contain tremendous numbers of goblet cells. mucus produced by goblet cells eases the passage of feces and protects the intestinal wall from irritation by acids and gases released by resident bacteria -like the small intestine, the large intestine mucosa is simple columnar epithelium except in the anal canal -the mucosa of the anal canal, a stratified squamous epithelium, merges with the true skin surrounding the anus and is quite different from the mucosa in the rest of the colon, reflecting the greater abrasion that this region receives -superiorly, it hangs in long ridges or folds called anal columns -anal sinuses, recesses between the anal columns, exude mucus when compressed by feces, which aids in emptying the anal canal -together, the inferior margins of the anal sinuses form a wavy line -superior to this line, visceral sensory fibers innervate the mucosa, and so it is relatively insensitive to pain -the area inferior to the line, however, is very sensitive to pain because it is innervated by somatic sensory fibers -two superficial venous plexuses are associated with the anal canal, one with the anal columns, and the other with the anus itself. -if these (hemorrhoidal) veins become dilated and inflamed, itchy varicosities called hemorrhoids result