Ch 24: The Digestive System

Anatomy of Liver (Anterior)

-Liver is wrapped in a tough fibrous capsule and is covered by a layer of visceral peritoneum -*Falciform ligament* marks the division between the organ's left and right lobes -Thickening in posterior margin of falciform ligament is the *round ligament*--this fibrous band marks the path of fetal umbilical vein

Chief Cells

-Most abundant near base of gastric gland -These cells secrete *pepsinogen* (an inactive proenzyme) -Acid in gastric lumen converts pepsinogen to *pepsin* (protein digesting enzyme) =Pepsin functions most effectively at strongly acidic pH of 1.5-2 -Newborn infants produce *rennin* and *gastric lipase* which are enzymes important for digestion of milk

Histology of Pancreas

-Partitions of connective tissue divide the interior of pancreas into distinct lobules -Blood vessels and tributaries of the pancreatic ducts are situated within these CT septa -*Compound tubuloalveolar gland* -In each lobule, the ducts branch repeatedly before ending in blind pockets called *pancreatic acini* =Each pancreatic acini is lined with simple cuboidal epithelium -*Pancreatic islets*: endocrine tissues of pancreas, are scattered among the acini--1% of pancreas

Liver's regulatory activities affect

-*Carbohydrate metabolism*: liver stabilizes blood glucose levels at about 90 mg/dL =If blood glucose levels decrease, hepatocytes break down glycogen reserves and release glucose into the bloodstream =They also synthesize glucose from other carbohydrates or from available amino acids =The synthesis of glucose from other compounds is called *gluconeogenesis* =If blood glucose levels rise, hepatocytes remove glucose from bloodstream--they either store it as glycogen or use it to synthesize lipids that can be stored in the liver or other tissues =Circulating hormones such as insulin and glucagon regulate metabolic activities -*Lipid metabolism*: liver regulates circulating levels of triglycerides, fatty acids, and cholesterol =When those levels decline, the liver breaks down its lipid reserves and releases the breakdown products into the bloodstream =When levels are high, the lipids are removed for storage--however regulation takes place after lipid levels have risen within the general circulation -*Amino acid metabolism*: liver removes excess amino acids from the bloodstream =These amino acids can be used to synthesize proteins or can be converted to lipids or glucose for energy storage -*Waste product removal*: when converting amino acids to lipids or carbs, or when breaking down amino acids to get energy, the liver strips off the amino groups--*deamination*--which forms ammonia (toxic waste product) =Liver neutralizes ammonia by converting it to *urea* (fairly harmless compound excreted by kidneys) =Liver also removes other waste products: circulating toxins, and drugs from blood for inactivation, storage, or excretion -*Vitamin storage*: fat soluble vitamins (A, D, E, and K) and vitamin B12 are absorbed from the blood and stored in the liver =These reserves are used when your diet contains inadequate amounts of those vitamins -*Mineral storage*: liver converts iron reserves to ferritin and stores this protein-iron complex -*Drug inactivation*: liver removes and breaks down circulating drugs, limiting the duration of their effects

Swallowing

-*Deglutition*: complex process that can be initiated voluntarily but proceeds automatically once it begins -*Swallowing reflex* begins when tactile receptors on palatal arches and uvula are stimulated by passage of the bolus -Information is relayed to the *swallowing center* of MO over the trigeminal and glossopharyngeal cranial nerves -Motor commands from this center signal the pharyngeal musculature, producing a coordinated and stereotyped pattern of muscle contraction -Takes less than second for pharyngeal muscles to propel the bolus into esophagus--during this time the respiratory centers are inhibited and breathing stops -Swallowing takes place at regular intervals as saliva collects at back of mouth -3 phases: *buccal, pharyngeal, and esophageal phases* -*Primary peristaltic waves* are peristaltic movements coordinated by afferent and efferent fibers -*Secondary peristaltic waves* may be required to push bolus if it is dry or poorly lubricated

Digestive System consists of

-*Digestive tract*: a muscular tube, also called the gastrointestinal (GI) tract or alimentary canal -*Major organs*: oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine -*Accessory organs*: teeth, tongue, and various glandular organs

Major Hormones of the Duodenum

-*Gastrin*: secreted by G cells in the duodenum when they are exposed to large quantities of incompletely digested proteins =The functions include promoting increased stomach motility and stimulating the production of gastric acids and enzymes =Also produced by the stomach -*Secretin*: is released when chyme arrives in the duodenum =Secretin's primary effect is an increase in the secretion of bile (by the liver) and buffers (by the pancreas), which in turn act to increase the pH of the chyme =Secondary effects: secretin reduces gastric motility and secretory rates -*Gastric Inhibitory Peptide (GIP)*: is secreted when fats and carbohydrates enter the small intestine =The inhibition of gastric activity is accompanied by the stimulation of insulin release at the pancreatic islets =Secondary effects: stimulating duodenal gland activity, stimulating lipid synthesis in adipose tissue, and increasing glucose use by skeletal muscles -*Cholecystokinin (CCK)*: is secreted when chyme arrives in the duodenum , especially when chyme contains lipids and partially digested proteins =In the pancreas, CCK accelerates the production and secretion of all types of digestive enzymes =It causes a relaxation of the hepatopancreatic sphincter and contraction of the gallbladder, resulting in ejection of bile and pancreatic juice into the duodenum--net effects of CCk are to increase the secretion of pancreatic enzymes and to push pancreatic secretions and bile into the duodenum =The presence of CCK in high concentrations has 2 additional effects: it inhibits gastric activity, and it appears to have CNS effects that reduce the sensation of hunger -*Vasoactive Intestinal Peptide (VIP)*: stimulates the secretion of intestinal glands, dilates regional capillaries, and inhibits acid production in the stomach =By dilating capillaries in active areas of intestinal tract, VIP provides an efficient mechanism for removing absorbed nutrients -*Enterocrinin*: released when chyme enters the duodenum--it stimulates mucin production by the submucosal glands

Major Organs of the Digestive Tract

-*Oral Cavity*: ingestion, mechanical processing with accessory organs (teeth and tongue), moistening, mixing with salivary secretions -*Pharynx*: muscular propulsion of materials into the esophagus -*Esophagus*: transport of materials to the stomach -*Stomach*: chemical breakdown of materials by acid and enzymes; mechanical processing through muscular contractions -*Small Intestine*: enzymatic digestion and absorption of water, organic substrates, vitamins, and ions -*Large Intestine*: dehydration and compaction of indigestible materials in preparation for elimination

Pancreatic Enzymes

-*Pancreatic alpha-amylase* (a carbohydrase): an enzyme that breaks down certain starches =Almost identical to salivary amylase -*Pancreatic lipase*: breaks down certain complex lipids, releasing products such as fatty acids that can be easily absorbed -*Nucleases*: break down RNA or DNA -*Proteolytic enzymes*: break apart certain proteins =Include *proteases* (break apart large protein complexes) and *peptidases* (break small peptide chains into individual amino acids) =Account for about 70% of total pancreatic enzyme production =These enzymes are secreted as inactive proenzymes--activated only after reaching small intestine =Release of proenzyme rather than active enzyme protects secretory cells from destructive effects of their own products =Proenzymes secreted by pancreas: *trypsinogen, chymotrypsinogen, procarboxypeptidase, proelastase* -Inside duodenum, enteropeptidase in brush border and lumen triggers conversion of trypsinogen to *trypsin* (active protease) =Trypsin activates other proenzymes, producing *chymotrypsin, carboxypeptidase, and elastase* =Each enzyme attacks peptide bonds linking specific amino acids and ignores others--together they break down proteins into a mixture of dipeptides and tripeptides and amino acids

Salivary and Pancreatic Enzymes

-*Salivary amylase and pancreatic alpha-amylase* both function at a pH of 6.7-7.5 -Carbohydrate digestion begins in mouth during mastication, through action of salivary amylase from parotid and submandibular salivary glands -Salivary amylase breaks down starches (complex carbs) into a mixture of mostly disaccharides and trisaccharides -Salivary amylase continues to digest starches and glycogen in food for one to two hours before stomach acids render the enzyme inactive -In duodenum, pancreatic alpha-amylase breaks down remaining complex carbs

Accessory Organs of the Digestive System

-*Teeth*: mechanical processing by chewing (mastication) -*Tongue*: assists mechanical processing with teeth, sensory analysis -*Salivary Glands*: secretion of lubricating fluid containing enzymes that break down carbohydrates -*Liver*: secretion of bile (important for lipid digestion), storage of nutrients, many other vital functions -*Gallbladder*: storage and concentration of bile -*Pancreas*: exocrine cells secrete buffers and digestive enzymes; endocrine cells secrete hormones

Mesenteries

-Portions of digestive tract are suspended within the peritoneal cavity by sheets of membrane that connect the parietal peritoneum with visceral peritoneum -*Mesenteries*: are double sheets of peritoneal membrane -Areolar tissue between the mesothelial surfaces provides a route to and from the digestive tract for blood vessels, nerves, and lymphatic vessels -Mesenteries stabilize the portions of the attached organs -Also they prevent the intestines from becoming entangled during digestive movements or sudden changes in body position

Intestinal Secretions

-1.8 liters of watery *intestinal juice* enters the intestinal lumen each day -Intestinal juice moistens chyme, helps buffer acids, and keeps both the digestive enzymes and products of digestion in solution =Most of this fluid arrives by osmosis and rest is secreted by intestinal glands, stimulated by activation of touch receptors and stretch receptors in intestinal walls -Duodenal glands help protect the duodenal epithelium from gastric acids and enzymes -These glands increase their secretion in response to (1) local reflexes, (2) the release of hormone *enterocrinin* by enteroendocrine cells of the duodenum, and (3) parasympathetic stimulation through the vagus nerves -Sympathetic stimulation inhibits the duodenal glands, leaving the duodenal lining unprepared for arrival of chyme

Central Gastric Reflexes

-2 central reflexes (*gastroenteric reflex and gastroileal reflex*) are triggered by stimulation of stretch receptors in stomach wall as it fills -These reflexes accelerate movement along small intestine while the enterogastric reflex controls the rate of chyme entry into duodenum -Rate of movement of chyme into small intestine is highest when stomach is greatly distended and meal contains little protein =Large meal containing small amounts of protein, large amounts of carbs, wine, or coffee will leave stomach quickly--because alcohol and caffeine stimulate gastric secretion and motility -*Vomiting reflex* occurs in response to irritation of fauces, pharynx, esophagus, stomach, or small intestine =These sensations are relayed to vomiting center of MO which coordinates motor responses =Pyloric sphincter relaxes and contents of duodenum are discharged back into stomach by strong peristaltic waves that travel toward stomach =Vomiting or *emesis* occurs as gastroesophageal sphincter relaxes and stomach regurgitates its contents through esophagus and pharynx and out through mouth

Vitamin Absorption

-2 major groups of vitamins: fat-soluble vitamins and water-soluble vitamins -*Fat-soluble vitamins*: include vitamins A, D, E, and K; their structure allows them to dissolve in lipids =Enter duodenum in fat droplets mixed with triglycerides =Remain in association with lipids as they form emulsion droplets and micelles =Then absorbed from micelles along with fatty acids and monoglycerides -*Water-soluble vitamins*: include the B vitamins (milk and meats) and vitamin C (citrus fruits) =All but one are easily absorbed by diffusion across digestive epithelium--vitamin B12 cannot be absorbed by intestinal mucosa in normal amounts =Instead it must be bound to *intrinsic factor* (glycoprotein secreted by parietal cells of stomach)--the combo is then absorbed through active transport

Dental Succession

-2 sets of teeth form during development -First to form are *deciduous teeth* which are the temporary teeth of the *primary dentition* -Deciduous teeth are also called primary teeth or baby teeth--most kids have 20 teeth -They are later replaced by *secondary dentition* or permanent dentition of larger adult jaws -*Eruption*: emergence of secondary teeth -Impacted teeth: those that develop in locations that do not allow them to erupt

Salivary Glands

-3 pairs of salivary glands secrete into oral cavity -Each pair has a distinctive cellular organization and produces *saliva* (mixture of glandular secretions) with slightly different properties 1. Large *parotid salivary glands* 2. *Sublingual salivary glands* 3. *Submandibular salivary glands*

Small Intestine

-90% of nutrient absorption takes place here, rest is in large intestine -Has 3 segments: duodenum, jejunum, and ileum -*Duodenum*: segment closest to stomach; it is a mixing bowl; it receives chyme from stomach and digestive secretions from pancreas and liver; curves in a C that encloses pancreas -*Jejunum*: reenters the peritoneal cavity, supported by sheet of mesentery; bulk of chemical digestion and nutrient absorption occurs here -*Ileum*: the final segment of the small intestine; longest; ends at *ileocecal valve*; this sphincter controls the flow of material from the ileum into the cecum of the large intestine -Small intestine fills much of peritoneal cavity -Its position is stabilized by mesentery proper -Primary blood vessels involved are branches of the superior mesenteric artery and superior mesenteric vein

Lamina Propria

-A layer of areolar tissue that contains blood vessels, sensory nerve endings, lymphatic vessels, smooth muscle cells, and scattered lymphatic tissue -In the oral cavity, pharynx, esophagus, stomach, and duodenum the lamina propria also contains secretory cells of mucous glands -In most areas of the digestive tract, the lamina propria contains a narrow sheet of smooth muscle and elastic fibers--*muscularis mucosae* -The smooth muscle cells in the muscularis mucosae are arranged in 2 concentric layers -The inner layer encircles the lumen (circular muscle) and the outer layer contains muscle cells arranged parallel to the long axis of the tract (longitudinal layer) -Contractions in these layers alter the shape of the lumen and move the epithelial pleats and folds

Submucosa

-A layer of dense irregular connective tissue that binds the mucosa to the muscularis externa -The submucosa has numerous blood vessels and lymphatic vessels -In some regions it also contains exocrine glands that secrete buffers and enzymes into the lumen of the digestive tract -Along its outer margin, the submucosa contains a network of intrinsic nerve fibers and scattered neurons--*submucosal plexus* =It contains sensory neurons, parasympathetic ganglionic neurons, and sympathetic postganglionic fibers that innervate the mucosa and submucosa

Mesocolon

-A mesentery associated with a portion of the large intestine -*Transverse mesocolon*: supports the transverse colon -*Sigmoid mesocolon*: supports the sigmoid colon -These 2 are all that remain of the original embryonic mesocolon

Intestinal Movements

-After chyme has arrived in duodenum, weak peristaltic contractions move it slowly toward the jejunum -The contractions are myenteric reflexes that are not under CNS control--motor neurons in the submucosal and myenteric plexuses control these short reflexes -Stimulation of parasympathetic system increases sensitivity of weak myenteric reflexes and speeds up both local peristalsis and segmentation -The gastroenteric and gastroileal reflexes speed up movement along the small intestine--opposite effect of enterogastric reflex -Hormones released by digestive tract can enhance or suppress reflexes

Regional Specializations

-Primary function of duodenum: receive chyme from stomach and neutralize its acids before they can damage the absorptive surfaces of small intestine -Jejunum: most nutrient absorption takes place before ingested materials reach ileum =Contains circular folds and villi for absorption -Ileum: lack circular folds, lamina propria contain aggregated lymphoid nodules, Peyer's patches =The lymphocytes in the nodules protect the small intestine from bacteria and that normally inhabit the large intestine

Rectum

-An expandable organ for the temporary storage of feces -Movement of fecal material into the rectum triggers the urge to defecate -Last portion of rectum, *anal canal*, contains small longitudinal folds called *anal columns* -The *anus* is the exit of the anal canal--there the epidermis becomes keratinized -Circular muscle layer of the muscularis externa in this region forms the *internal anal sphincter* =Smooth muscle cells of this sphincter are not under voluntary control -*External anal sphincter* guards the anus, consists of a ring of skeletal muscle fibers that encircles the distal portion of the anal canal--this sphincter consists of skeletal muscle and is under voluntary control -Lamina propria and submucosa of anal canal contain a network of veins

Control of Salivary Secretions

-Autonomic nervous system controls salivary secretions -Each salivary gland has parasympathetic and sympathetic innervation -Parasympathetic outflow originates in the *salivatory nuclei* of MO and synapses in the submandibular and otic ganglia -Any object in your mouth can trigger a salivary reflex =It stimulates receptors monitored by the trigeminal nerve (N V) or taste buds innervated by cranial nerve VII, IX, or X -Parasympathetic stimulation speeds up secretion by all the salivary glands--produce large amounts of saliva -Salivatory nuclei are influenced by other brain stem nuclei as well as activities of higher centers--chewing with empty mouth, smelling food, thinking about food causes rise in salivary secretion rates -Irritating stimuli in esophagus, stomach, or intestines also speed up production of saliva--increased saliva production in response to unpleasant stimuli helps reduce stimulus by dilution by rinsing or buffering strong acids or bases

Organic Wastes

-Bacteria convert bilirubin to urobilinogens and stercobilinogens--some urobilinogens are absorbed into bloodstream and excreted into urine -Urobilinogens and stercobilinogens remaining in colon are coverted to *urobilins* and *stercobilins* by exposure to oxygen--which gives feces their color -Bacterial action breaks down peptides that remain in the feces--this action generates (1) ammonia, in the form of soluble ammonium ions; (2) indole and skatole, 2 nitrogen-containing compounds that are primarily responsible for odor of feces; (3) hydrogen sulfide, a gas with a rotten egg odor -Liver removes these toxins and converts them to nontoxic compounds that can be released into blood and excreted at kidneys -Intestinal enzymes do not alter indigestible carbohydrates--these materials arrive at colon intact =These complex polysaccharides provide reliable nutrient source for bacteria in colon =Metabolic activities of these bacteria create small amounts of *flatus* (intestinal gas)

Large Intestine

-Begins at ileum and ends at anus -Stores digestive wastes and reabsorbs water -Resident bacteria in large intestine are important source of vitamins, especially vitamin K, biotin, and vitamin B5 -Divided into 3 parts: (1) pouchlike *cecum*, (2) *colon*, (3) rectum -Receives blood from branches of the superior mesenteric and inferior mesenteric arteries -Superior mesenteric and inferior mesenteric veins collect venous blood from large intestine

Intestinal Phase

-Begins when chyme first enters small intestine -Function is to control rate of gastric emptying to ensure that secretory, digestive, and absorptive functions of small intestine can proceed with reasonable efficiency -*Neural responses*: chyme leaving the stomach decreases the distension in the stomach, reducing the stimulation of stretch receptors =Distension of duodenum by chyme stimulates stretch receptors and chemoreceptors taht trigger the *enterogastric reflex* (inhibits both gastrin production and gastric contractions and stimulates the contraction of pyloric sphincter, which prevents further discharge of chyme)--at same time, local reflexes at duodenum stimulate mucus production which helps protect the duodenal lining from the arriving acid and enzymes -*Hormonal responses*: arrival of chyme in duodenum triggers hormonal responses: =Arrival of lipids and carbs stimulate secretion of cholecystokinin (CCK) and gastric inhibitory peptide (GIP) =Decrease in pH below 4.5 stimulates secretion of secretin =Partially digested proteins in duodenum stimulate G cells that secrete gastrin, which circulates to stomach and speeds gastric processing

Cephalic Phase

-Begins when you see, smell, taste, or think of food -This phase is directed by the CNS -Prepares the stomach to receive food -The neural output proceeds by way of the parasympathetic division of the autonomic nervous system -The vagus nerves innervate the submucosal plexus of the stomach -Postganglionic parasympathetic fibers innervate mucous cells, chief cells, parietal cells, and G cells of the stomach -In response to stimulation, production of gastric juices speeds up -Phase only lasts minutes

Gastric Phase

-Begins with arrival of food in stomach and builds on the stimulation provided during the cephalic phase -Continue for 3 to 4 hours while acid and enzymes process the ingested materials -The stimuli that initiate gastric phase are (1) distension of stomach, (2) increase in pH of gastric contents, (3) presence of undigested materials in the stomach, especially proteins and peptides -Consists of local response, neural response, and hormonal response -*Local response*: distension of gastric wall stimulates release of histamine in lamina propria, which binds to receptors on the parietal cells and stimulates acid secretion -*Neural response*: stimulation of stretch receptors and chemoreceptors triggers short reflexes coordinated in the submucosal and myenteric plexuses, which activates the stomach's secretory cells--stimulation of myenteric plexus produces powerful contractions called *mixing waves* in muscularis externa -*Hormonal response*: neural stimulation and presence of peptides and amino acids in chyme stimulate the secretion of hormone gastrin, primarily by G cells--gastrin travels in bloodstream to parietal and chief cells, whose increased secretions reduce pH of gastric juice; gastrin also stimulates gastric motility

Falciform Ligament

-Between the liver and the anterior abdominal wall -Helps stabilize the position of the liver relative to the diaphragm and abdominal wall

Lesser Omentum

-Between the stomach and the liver -Stabilizes the position of the stomach and provides an access route for blood vessels and other structures entering or leaving the liver

Brush Border Enzymes

-Brush border enzymes of intestinal microvilli break disaccharides and trisaccharides into monosaccharides prior to absorption -*Maltase* splits bonds between 2 glucose molecules of disaccharide *maltose* -*Sucrase* breaks down *sucrose* into glucose and fructose -*Lactase* hydrolyzes the disaccharide *lactose* into glucose and galactose--no more lactase = *lactose intolerant*

Oral Cavity

-Buccal cavity -Functions: (1) *sensory analysis* of food before swallowing; (2) *mechanical processing* through actions of the teeth, tongue, and palatal surfaces; (3) *lubrication* by mixing with mucus and saliva; and (4) limited *digestion* of carbohydrates and lipids -Lined by *oral mucosa* which has stratified squamous epithelium =A layer of keratinized cells covers regions exposed to severe abrasion, such as superior surface of tongue and opposing surface of hard palate =Epithelial lining of cheeks, lips, and inferior surface of tongue is thin and nonkeratinized -Nutrients are not absorbed in the oral cavity, but the mucosa inferior to the tongue is thin enough and vascular enough to permit rapid absorption of lipid-soluble drugs -Mucosae of the *cheeks* are supported by pads of fat and buccinator muscles -The mucosa of each cheek is continuous with that of the lips or *labia* -The *vestibule* is the space between the cheeks (or lips) and the teeth -*Gingivae* or gums are ridges of oral mucosa that surround the base of each tooth on the alveolar processes of the maxillary bones and mandible =In most regions, gingivae are firmly bound to the periostea of underlying bones =Frenulum of lower lip is a fold of mucosa that extends from gingiva to lower lip, attaching the lip to gum -Posterior margin of the soft palate supports the *uvula* (dangling process that helps prevent food from entering the pharynx too soon) -*Palatoglossal arch* extends between the soft palate and the base of the tongue -A curving line that connects the palatoglossal arches and uvula forms the boundaries of the *fauces* (the arched opening between the soft palate and the base of the tongue) -The fauces serve as passageway between the oral cavity and oropharynx -*Palatopharyngeal arch*: extends from soft palate to pharyngeal wall

Histology of Liver

-CT divides each lobe of liver into *liver lobules* (the basic functional units of the liver) -Adjacent lobules are separated by an *interlobular septum* -Hepatocytes in liver lobule form a series of irregular plates arranged like the spokes of a wheel =The plates are only one cell thick =Exposed hepatocyte surfaces are covered with short microvilli -Within a lobule, sinusoids between adjacent plates empty into the *central vein* -The liver sinusoids lack a basement membrane, so large openings between the endothelial cells allow solutes to pass out of the bloodstream and into spaces surrounding hepatocytes -Lining of sinusoids contains typical endothelial cells and large number of *Kupffer cells*--these phagocytic cells are part of the monocyte-macrophage system =They engulf pathogens, cell debris, and damaged blood cells =They also store iron, some lipids, and heavy metals that are absorbed by the digestive tract -Blood enters the liver sinusoids from small branches of the hepatic portal vein and hepatic artery proper -A typical liver lobule has a hexagonal shape--there are six *portal areas* or *portal triads*, one at each corner of the lobule =Portal area contains 3 structures: (1) branch of the hepatic portal vein, (2) branch of the hepatic artery proper, (3) small branch of the bile duct -Branches from the arteries and veins deliver blood to the sinusoids of adjacent liver lobules -As blood flows through the sinusoids, hepatocytes absorb solutes from plasma and secrete materials such as plasma proteins -Blood then leaves the sinusoids and enters the central vein of the lobule -The central veins ultimately merge to form the hepatic veins, which empty into the inferior vena cava

Water Absorption

-Cells cannot secrete or absorb water--all movement of water across lining of digestive tract involves passive water flow down osmotic gradients -Water is reabsorbed in the small intestine and colon

Coordination of Secretion and Absorption

-Combination of neural and hormonal mechanisms coordinates the activities of the digestive glands -These regulatory mechanisms are centered on the duodenum, where acids must be neutralized and appropriate enzymes added -Neural mechanisms involving the CNS prepare the digestive tract for activity (through parasympathetic innervation) or inhibit is activity (through sympathetic innervation) -Neural mechanisms also coordinate the movement of materials along the length of the digestive tract (through enterogastric, gastroenteric, and gastroileal reflexes) -Motor neurons synpasing in the digestive tract release a variety of neurotransmitters--many of these are released in the CNS

Neural Mechanisms

-Control digestive tract movements -*Short reflexes* are triggered by chemoreceptors or stretch receptors in the walls of the tract; the controlling neurons are located in the myenteric plexus--the reflexes are called *myenteric reflexes* -*Long reflexes* involving interneurons and motor neurons in the CNS provide a higher level of control over digestive and glandular activities, generally controlling large scale peristalsis that moves materials from one region to another =May involve parasympathetic motor fibers in the glossopharyngeal (N IX), vagus (N X), or pelvic nerves that synapse in the myenteric plexus

Long Reflex

-Coordinated by sacral parasympathetic system -Stimulates mass movements that push feces toward the rectum from descending colon and sigmoid colon

Sublingual Salivary Glands

-Covered by mucous membrane of floor of mouth -These glands produce a mucous secretion that acts as a buffer and lubricant -Numerous *sublingual ducts* open along either side of the lingual frenulum

Histology of Large Intestine

-Diameter of colon is 3x that of small intestine, but wall is much thinner -Major characteristics: lack of villi, abundance of mucous cells, presence of distinctive intestinal glands -Glands in large intestine are deeper than small intestine and are dominated by mucous cells -Mucosa does not produce enzymes--any digestion results from enzymes introduced by small intestine or bacterial action -Mucus provides lubrication as the fecal material becomes drier and more compact -Mucus is secreted as local stimuli such as friction or exposure to harsh chemicals trigger short reflexes involving local nerve plexuses -Large lymphoid nodules are scattered throughout the lamina propria and submucosa -The muscularis externa's longitudinal layer has been reduced to the muscular bands of teniae coli -Mixing and propulsive contractions resemble that of small intestine

Hormonal Mechanisms

-Digestive hormones can enhance or diminish the sensitivity of the smooth muscle cells to neural commands -These hormones produced by enteroendocrine cells in the tract travel through the bloodstream to reach their target organs -Tract produces at least 18 hormones that affect almost every aspect of digestion, sand some of them also affect the activities of other systems -These hormones are peptides produced by *enteroendocrine cells*--endocrine cells in the epithelium of the tract

Segmentation

-When most areas of the small intestine and some portions of the large intestine undergo cycles of contraction that churn and fragment the bolus, mixing the contents with intestinal secretions -Does not follow a set pattern -Does not push materials along the tract in any one direction

Processing and Absorbing of Nutrients

-Digestive system breaks down physical structure of ingested material and disassembles the component molecules into smaller fragments--this eliminates any antigenic properties to not trigger immune response -Cells absorb molecules released into bloodstream and either (1) break them down to provide energy for synthesis of ATP or (2) use these molecules to synthesize carbs, proteins, and lipids -*Hydrolysis*: when digestive enzymes break bonds between components of carbs (with simple sugars), proteins (with amino acids), and lipids (with fatty acids) -*Carbohydrases* break bonds between simple sugars -*Proteases* split linkages between amino acids -*Lipases* separate fatty acids from glycerides

Gastric Glands

-Gastric pits communicate with gastric glands -They extend deep into underlying lamina propria -Dominated by 2 types of secretory cells: *parietal cells and chief cells* -They secrete about 1500 mL of *gastric juice* a day

Movement of Large Intestine

-Gastroileal and gastroenteric reflexes move materials into cecum while you eat -Movement from cecum to transverse colon is slow to allow water absorption to covert thick material into sludgy paste -Peristaltic waves move material along length of colon -Segmentation movements (haustral churning) mix the contents of adjacent haustra -Powerful peristaltic contractions (*mass movements*) occur few times a day--moving material from transverse colon through rest of large intestine -Stimulus is distension of stomach and duodenum -Commands are relayed over intestinal nerve plexuses -Contractions force feces into rectum and produce the conscious urge to defecate -Rectal chamber is usually empty, except when powerful peristaltic contraction forces feces out of sigmoid colon--distension of rectal wall then triggers *defecation reflex* (involves 2 positive feedback loops which are triggered by stimulation of stretch receptors in rectum)

Stomach

-Has 4 functions: (1) storage of ingested food; (2) mechanical breakdown of ingested food; (3) disruption of chemical bonds in food through the action of acid and enzymes; and (4) production of *intrinsic factor* (a glycoprotein needed for absorption of vitamin B12 in small intestine) =Ingested substances combine with secretions of glands of stomach, producing a viscous, highly acidic, soupy mixture of partially digested food called *chyme*

Colon

-Has a large diameter and thinner wall than the small intestine -The wall of the colon forms a series of pouches, or *haustra*--the creases between the haustra affect the mucosal lining as well, producing a series of internal folds =Haustra permit the colon to expand and elongate -3 separate longitudinal bands of smooth muscle called *teniae coli* run along the outer surfaces of the colon just deep to the serosa =These bands correspond to the outer layer of the muscularis externa in other portions of the GI tract =Muscle tone within the teniae coli creates the haustra -The serosa of the colon contains numerous teardrop-shaped sacs of fat called *omental appendices*

Esophagus

-Hollow muscular tube that conveys solid food and liquids to the stomach -Begins posterior to cricoid cartilage and descends toward thoracic cavity posterior to trachea, continuing inferiorly along posterior wall of mediastinum -Enters the abdominopelvic cavity through *esophageal hiatus* (an opening in the diaphragm) -Esophagus empties into the stomach -Innervated by parasympathetic and sympathetic fibers from esophageal plexus -Comparable zone at inferior end of esophagus normally remains in state of active contraction--this state prevents backflow (reflux) of materials from stomach into esophagus -Neither region has well defined sphincter muscle--but still use terms *upper and lower esophageal sphincter*

Gallbladder

-Hollow, pear-shaped organ that stores and concentrates bile prior to its excretion into the small intestine -This muscular sac is located in a fossa, or recess, in the posterior surface of the liver's right lobe -Divided into 3 regions: (1) *fundus*, (2) *body*, (3) *neck* -The cystic duct extends from the gallbladder to the point where it unties with the common hepatic duct to form the common bile duct -At the duodenum, the common bile duct meets the pancreatic duct before emptying into a chamber called the *duodenal ampulla* (receives buffers and enzymes from the pancreas and bile from the liver and gallbladder) =The duodenal ampulla opens into the duodenum at the *duodenal papilla* (small mound) -Muscular *hepatopancreatic sphincter* encircles the lumen of the common bile duct and generally the pancreatic duct and duodenal ampulla as well

Anatomy of Liver (Posterior)

-Impression left by the inferior vena cava marks the division between the right lobe and small *caudate lobe* -*Quadrate lobe* lies inferior to caudate lobe, sandwiched between the left lobe and gallbladder -Afferent blood vessels and other structures reach the liver by traveling within the connective tissue of lesser omentum--they converge at a region called *porta hepatis*

Mucosa

-Inner lining of the digestive tract is the mucosa membrane -Consists of an epithelium, moistened by glandular secretions, and a lamina propria of areolar tissue -The mucosal epithelium is either simple or stratified, depending on its location and the stresses placed on it =Mechanical stresses are most severe in the oral cavity, pharynx, and esophagus--these structures are lined by a stratified squamous epithelium =The stomach, small intestine, and almost entire length of large intestine (where absorption occurs) have a simple columnar epithelium that contains mucous cells =Scattered among the columnar cells are *enteroendocrine cells* (secrete hormones that coordinate the activities of the digestive tract and the accessory glands) -The lining of the of the digestive tract appears as longitudinal folds which disappear as the tract fills =The lining has permanent transverse folds called *circular folds* or *plicae circulares* =The folding increases the surface area available for absorption -The secretions of gland cells in the mucosa and submucosa are carried to the epithelial surfaces by ducts

Absorption of Monosaccharide

-Intestinal epithelium absorbs monosaccharides by *facilitated diffusion and cotransport* mechanisms--both mechanisms involve a carrier protein -3 differences: 1. Facilitated diffusion moves only one molecule or ion through plasma membrane, whereas contransport moves more than one molecule or ion through the membrane at the same time 2. Facilitated diffusion does not require ATP 3. Facilitated diffusion does not take palce if there is an opposing concentration gradient for the particular molecule or ion -The cotransport system that takes up glucose also brings sodium ions into the cell =Deliver valuable nutrients to cytoplasm, but also bring in sodium ions that must be ejected by sodium-potassium exchange pump -The simple sugars that are transported into the cell at its apical surface diffuse through the cytoplasm, they then reach the interstitial fluid by facilitated diffusion across the basolateral surfaces, these monosaccharides diffuse into capillaries of villus for eventual transport to liver in hepatic portal vein

Histology of Small Intestine

-Intestinal lining has series of transverse folds called *circular folds* or *plicae circulares*--permanent features and do not disappear when small intestine fills =They greatly increase surface area available for absorption -*Intestinal villi*: fingerlike projections of mucosa =Covered by simple columnar epithelium that is carpeted with microvilli =The cells are said to have a *brush border* because the microvilli project from the epithelium like bristles on a brush =Mucosa contains circular folds that supports forest of villi, and epithelial cells bearing microvilli cover each villus--this arrangement increases total area for absorption by factor of more than 600 =Lamina propria of each villus contains an extensive network of capillaries that originate in a vascular network within the submucosa =These capillaries carry absorbed nutrients to the hepatic portal circulation for delivery to the liver =The liver then adjusts the nutrient concentrations of blood before the blood reaches the general systemic circulation =Each villus also contains a lymphatic capillary called a *lacteal*--they transport materials that cannot enter blood capillaries like *chylomicrons* that reach the venous circulation through thoracic duct which delivers lymph in to left subclavian vein =Intestinal villi move back and forth exposing epithelial surfaces to liquefied intestinal contents--movement is brought upon by contractions of muscularis mucosae and smooth muscle cells =This movement makes absorption more efficient by quickly eliminating local differences in nutrient concentration--they also squeeze lacteals helping to move lymph out of villi

Intestinal Hormones

-Intestinal tract secretes a variety of peptide hormones with similar chemical structures -Many of these hormones have multiple effects in several regions of digestive tract, and in accessory glandular organs too -Other intestinal hormones are produced in small quantities: *motilin* (stimulates intestinal contractions), *villikinin* (promotes movement of villi and associated lymph flow, and *somatostatin* (inhibits gastric secretion)

Liver

-Largest visceral organ -One of our most versatile organs and center for metabolic regulation in the body -This large, firm, reddish-brown organ performs essential metabolic and synthetic functions -Nearly 1/3 of blood supply to liver is arterial blood from hepatic artery proper--the rest is venous blood from hepatic portal vein, which begins in capillaries of esophagus, stomach, small intestine, and most of large intestine -Liver cells, *hepatocytes*, adjust circulating levels of nutrients through selective absorption and secretion -The blood leaving the liver returns to the systemic circuit through the hepatic veins -These veins open into inferior vena cava

Submandibular Salivary Glands

-Lie along the inner surfaces of the mandible within a depression called *mandibular groove* -Cells of the submandibular glands secrete a mixture of buffers, glycoproteins called *mucins*, and salivary amylase -*Submandibular ducts* open into the mouth on either side of the lingual frenulum immediately posterior to the teeth

Parotid Salivary Glands

-Lie inferior to zygomatic arch deep to skin covering the lateral and posterior surface of the mandible -Each gland has an irregular shape -It extends from mastoid process of temporal bone across outer surface of masseter muscle -Parotid salivary glands produce serous secretion containing large amounts of *salivary amylase*--which breaks down starches (complex carbohydrates) -Secretions of each parotid gland are drained by *parotid duct* which empties into the vestibule at the second upper molar

Pancreas

-Lies posterior to stomach -Has a *head, body, and tail* -Retroperitoneal and is firmly bound to the posterior wall of the abdominal cavity -Surface has a lumpy, lobular texture -Thin, transparent capsule of connective tissue wraps the entire organ -Arterial blood reaches the pancreas by way of branches of splenic, superior mesenteric, and common hepatic arteries -Pancreatic arteries and pacreaticoduodenal arteries are major branches from these vessels -Splenic vein and its branches drain the pancreas -Pancreas is primarily an exocrine organ, it produces digestive enzymes and buffers -*Pancreatic duct* delivers secretions to duodenum =Extends within the attached mesentery to reach the duodenum, where it meets the *common bile duct* from the liver and gallbladder =The two ducts empty into the *duodenal ampulla* (chamber located roughly halfway along the length of duodenum) -*Accessory pancreatic duct*: branches from pancreatic duct =Empties into duodenum independently, outside duodenal ampulla

Lipid Digestion and Absorption

-Lipid digestion involves *lipid lipase* from glands on tongue and *pancreatic lipase* from pancreas -Triglycerides are most important and abundant dietary lipids--consist 3 fatty acids attached to single molecule of glycerol; lingual and pancreatic lipases break off 2 leaeving monoglycerides -Lipases are water soluble--so can only attack exposed surfaces of lipid drops -Lingual lipase begins breaking down triglycerides in the mouth and continues for variable time within stomach -Bile salts improve chemical digestion by emulsifying the lipid drops into tiny emulsion droplets, thereby helping with access for pancreatic lipase -Pancreatic lipase then breaks apart triglycerides to form a mixture of fatty acids and monoglycerides--as they are released, they interact with bile salts in surrounding chyme to form small lipid-bile salt complexes called *micelles* -When micelle contacts intestinal epithelium, the lipids diffuse across plasma membrane and enter the cytoplasm -The intestinal cells synthesize new triglycerides from the monoglycerides and fatty acids--these along with absorbed steroids, phospholipids, and fat soluble vitamins are coated with proteins turning into *chylomicrons* -Intestinal cells then secrete chylomicrons into interstitial fluid by exocytosis--the protein coating keeps them suspended in interstitial fluid, but are too large to diffuse into capillaries -Most of them diffuse into intestinal lacteals, from there they proceed along lymphatic vessels and through thoracic duct and enter bloodstream at left subclavian vein -Most bile salts within micelles are reabsorbed by sodium-linked cotransport

Physiology of the Liver

-Liver carries out more than 200 functions -The functions fall into 3 general categories: (1) *metabolic regulation*, (2) *hematological regulation*, (3) *bile production*

Metabolic Regulation

-Liver is primary organ involved in regulating the composition of circulating blood -All blood leaving the absorptive surfaces of the digestive tract enters the hepatic portal system and flows into the liver -Liver cells extract nutrients or toxins from the blood before it reaches the systemic circulation through the hepatic veins -Liver removes and stores excess nutrients -It corrects nutrient deficiencies by mobilizing stored reserves or performing synthetic activities

Hematological Regulation

-Liver receives about 25% of cardiac output -It is the largest blood reservoir in your body -As blood passes through, the liver performs the following functions: =*Phagocytosis and antigen presentation*: Kupffer cells in the liver sinusoids engulf old or damaged red blood cells, cellular debris, and pathogens, removing them from bloodstream; Kupffer cells are antigen-presenting cells (APCs) that can stimulate an immune response =*Synthesis of plasma proteins*: hepatocytes synthesize and release most of the plasma proteins, these proteins include albumins, various types of transport proteins, clotting proteins, and complement proteins =*Removal of circulating hormones*: liver is primary site for absorption and recycling of E, NE, insulin, thyroid hormones, and steroid hormones, such as sex hormones and corticosteroids; liver also absorbs cholecalciferol (vitamin D3) from blood, then it converts into an intermediary product that is released back into bloodstream, which kidneys then absorb and use it to generate calcitriol (hormone important to Ca++ metabolism) =*Removal of antibodies*: liver absorbs and breaks down antibodies, releasing amino acids for recycling =*Removal or storage of toxins*: liver absorbs lipid-soluble toxins in diet and stores them in lipid deposits where they don't disrupt cellular functions, liver then removes other toxins from bloodstream and either breaks them down or excretes them in bile =*Synthesis and secretion of bile*: liver synthesizes bile and excretes it into the lumen of duodenum; hormonal and neural mechanisms regulate bile secretion; bile consists mostly of water with minor amounts of ions, bilirubin, cholesterol, and *bile salts*, water and ions help dilute and buffer acids in chyme as it enters small intestine, bile salts are synthesized from cholesterol in liver

Bile Duct System

-Liver secretes a fluid called *bile* into a network of narrow channels between the opposing membranes of adjacent liver cells -These passageways called *bile canaliculi* extend outward away from central vein -Eventually they connect with fine *bile ductules* which carry bile to bile ducts in the nearest portal area -*Right and left hepatic ducts* collect bile from all bile ducts of liver lobes--these ducts unite to form the *common hepatic duct* which leaves the liver -The bile in the common hepatic duct either flows into the *common bile duct* which empties into the duodenal ampulla or enters the *cystic duct* which leads to the gallbladder -The *common bile duct* is formed by the union of the *cystic duct* and the common hepatic duct =Passes within the lesser omentum toward the stomach, turns, and penetrates the wall of the duodenum to meet the pancreatic duct at the duodenal ampulla

Local Factors

-Local factors interact with neural and hormonal mechanisms to regulate the activities of the digestive system -Initial regulation of digestive function occurs at the local level -Local environmental factors such as the pH, volume, or chemical composition of the intestinal contents can have a direct effect on digestive cavity in that segment of the GI tract -Some of these local factors have a direct effect on local digestive activities -They also stimulate the release of chemicals -Prostaglandins, histamine, and other chemicals released into interstitial fluid may affect adjacent cells within a small segment of the tract -Summary: local factors are the primary stimulus for digestion; they coordinate responses to changes in the pH of the contents of the lumen, physical distortion of the wall of the GI tract, or the presence of chemicals (either specific nutrients or chemical messengers released by cells of the mucosa)

Physiology of the Gallbladder

-Major function of the gallbladder is *bile storage*, but it is only released into the duodenum under stimulation of intestinal hormone CCK =Without CCK, the hepatopancreatic sphincter remains closed, so bile exiting the liver in the common hepatic duct cannot flow through the common bile duct and into the duodenum--instead it enters the cystic duct and is stored within the expandable gallbladder -Whenever chyme enters the duodenum, CCK is released relaxing the hepatopancreatic sphincter and stimulating contractions of the gallbladder that push bile into the small intestine =The amount of CCK secreted increases markedly when the chyme contains large amounts of lipids -The gallbladder also functions in *bile modification* -When full the gallbladder contains 40-70 mL of bile, the composition of the bile gradually changes as it remains in the gallbladder =Much of the water is absorbed and bile salts and other components of bile become increasingly concentrated =If bile becomes too concentrated, crystals of insoluble minerals and salts begin to form--*gallstones* =Small gallstones are not a problem, but in *cholecystitis* gallstones are so large that they can damage the wall of the gallbladder or block the cystic duct or common bile duct--this makes bile more dilute and its entry into small intestine is not as closely tied to arrival of food in duodenum

Tongue

-Manipulates food inside the mouth and brings in foods -Primary functions of tongue are (1) mechanical processing by compression, abrasion, and distortion; (2) manipulation to assist in chewing and to prepare food for swallowing; (3) sensory analysis by touch, temperature, and taste receptors; and (4) secretion of mucins and enzyme lingual lipase -Divided into anterior *body* (oral portion) and posterior *root* (pharyngeal portion) -Superior surface or *dorsum* of body contains a forest of fine projections--*lingual papillae* -Thickened epithelium covering each papilla assists the tongue in moving materials -V-shaped line of vallate papillae roughly marks the boundary between the body and the root of the tongue, which is located in oropharynx -Epithelium covering inferior surface of tongue is thinner and more delicate than of dorsum -Along inferior midline is the *lingual frenulum* (thin fold of mucous membrane that connects the body of the tongue to the mucosa covering the floor of the oral cavity -Ducts from 2 pairs of salivary glands open on either side of the lingual frenulum, which serves to prevent extreme movements of the tongue -Tongue's epithelium is flushed by secretions of small glands that extend into underlying lamina propria =These secretions contain water, mucins, and enzyme *lingual lipase* =This enzyme works over a broad pH range (3-6) enabling it to start lipid digestion immediately =Because lingual lipase tolarates an acid environment, it can continue to break down lipids for a considerable time after food reaches stomach -Tongue contains 2 groups of skeletal muscles: large *extrinsic tongue muscles* (perform all gross movements of tongue) and smaller *intrinsic tongue muscles* (change shape of tongue and assist the extrinsic muscles during precise movements =Both are under control of hypoglossal cranial nerves (N XII)

Cecum

-Material arriving from ileum first enters an expanded pouch called *cecum* -Ileum attaches to medial surface of cecum and opens into the cecum at the *ileocecal valve* -The cecum collects and stores materials from the ileum and begins the process of compaction -The slender, hollow *appendix* is attached to the posteromedial surface of the cecum -A small mesentery called the *mesoappendix* connects the appendix to the ileum and cecum -Lymphoid nodules dominate the mucosa and submucosa of the appendix -Primary function is as an organ of the lymphatic system -Inflammation of appendix is known as *appendicitis*

Functions of Bile

-Most lipids are not water soluble -Mechanical processing in stomach creates large drops containing a variety of lipids -Pancreatic lipase is not lipid soluble, so enzymes can interact with lipids only at surface of lipid droplet--larger the droplet, the more lipids are inside, isolated and protected from these enzymes -Bile salts break the droplets apart in process called *emulsification* which dramatically increases surface area accessible to enzymes =Emulsification creates tiny emulsion droplets with a superficial coating of bile salts--formation of tiny droplets increases the surface area available for enzymatic attack; layer of bile salts also facilitates interaction between lipids and lipid-digesting enzymes from pancreas -After lipid digestion has been completed, bile salts promote absorption of lipids by intestinal epithelium--more than 90% of bile salts are reabsorbed, primarily in ileum as lipid digestion is completed =Reabsorbed bile salts enter hepatic portal circulation and liver collects and recycles them =Cycling of bile salts from liver to small intestine and back is called *enterohepatic circulation of bile*

Histology of Esophagus

-Mucosa: contains nonkeratinized, stratified squamous epithelium -Mucosa and submucosa are packed into large folds that extend the length of the esophagus =These folds allow for expansion during passage of large bolus =Muscle tone in walls keeps lumen closed except when you swallow -Muscularis mucosae consists of an irregular layer of smooth muscle -Submucosa: contains scattered *esophageal glands* =They produce mucus that reduces friction between bolus and esophageal lining -Muscularis externa: has usual inner circular and outer longitudinal layers =Skeletal muscle fibers in superior third of esophagus =Middle third contains mixture of skeletal and smooth muscle tissue =Inferior third contains smooth muscle tissue -Adventitia: anchors to esophagus, esophagus is retroperitoneal

Intestinal Glands

-Mucous cells between the columnar epithelial cells eject mucins onto the intestinal surfaces -At bases of the villi are entrances to the *intestinal glands* -These glandular pockets extend deep into the underlying lamina propria -Near the base of each intestinal gland, stem cell divisions produce new generations of epithelial cells, which are continuously displaced toward the intestinal surface--in a few days the new cells reach the tip of a villus and are shed into the intestinal lumen; the disintegration of the shed cells adds enzymes to the lumen -*Brush border enzymes* are integral membrane proteins on the surfaces of intestinal microvilli =These enzymes break down materials that come in contact with the brush border =The epithelial cells then absorb the breakdown products, once the epithelial cells are shed they disintegrate within the lumen, releasing both intracellular and brush border enzymes =*Enteropeptidase* is a brush border enzyme that enters the lumen this way and does not directly participate in digestion--instead it activates a key pancreatic proenzyme, trypsinogen -Intestinal glands also contain enteroendocrine cells that produce several intestinal hormones, including gastrin, cholecystokinin, and secretin -Submucosa contains *duodenal glands* (submucosal glands)--produce copious amounts of mucus when chyme arrives from stomach =The mucus protects the epithelium from the acidity of chyme and also contains bicarbonate ions that help raise the pH of the chyme =As chyme travels the length of the duodenum, its pH increases from 1-2 to 7-8 =Duodenal glands also secrete hormone *urogastrone* which inhibits gastric acid production and stimulates the division of epithelial stem cells along the digestive tract

Movement of Digestive Materials

-Muscular layers of the digestive tract consist of *visceral smooth muscle tissue* -The smooth muscle along the digestive tract has rhythmic cycles of activity due to *pacesetter cells* =These cells undergo spontaneous depolarization, triggering a wave of contraction that spreads throughout the entire muscular sheet =Located in the muscularis mucosae and muscularis externa, which surround the lumen of the GI tract -Coordinated contractions of the muscularis externa play a vital role in moving materials along the tract, through *peristalsis*, and in mechanical processing, through *segmentation*

Peristalsis

-Muscularis externa propels materials from one portion of the digestive tract to another by contractions known as peristalsis -Consists of waves of muscular contractions that move a *bolus* (soft rounded ball of digestive contents) along the length of the digestive tract -During a peristaltic movement, the circular muscles contract behind the bolus while circular muscles ahead of the bolus relax =Longitudinal muscles ahead of the bolus then contract, shortening adjacent segments =A wave of contraction in the circular muscles then forces the bolus forward

Pancreas has 2 distinct functions

-One endocrine and one exocrine -Endocrine cells of pancreatic islets secrete insulin and glucagon into bloodstream to control blood sugar -Exocrine cells include the acinar cells and epithelial cells that line the duct system =Together these exocrine cells secrete *pancreatic juice* (alkaline mixture of digestive enzymes, water, and ions) into small intestine -Acinar cells secrete pancreatic enzymes, which do most of digestive work in small intestine =Pancreatic enzymes break down ingested materials into small molecules suitable for absorption -Water and ions secreted by cells lining the pancreatic ducts help dilute and neutralize acid in chyme

Vitamins

-Organic molecules that are important as cofactors or coenzymes in many metabolic pathways -Normal bacterial residents of colon generate 3 vitamins that supplement our diets: 1. *Vitamin K*: fat soluble vitamin in liver requires for synthesizing 4 clotting factors, including prothrombin--intestinal bacteria produce about half of vitamin K requirements 2. *Biotin*: water soluble vitamin important in various reactions--like glucose metabolism 3. *Vitamin B5 (pantothenic acid)*: water soluble vitamin required in maufacture of steroid hormones and some neurotransmitters -Vitamin K deficiencies lead to impaired blood clotting--result from not enough lipids in diet which impairs absorption of all fat-soluble vitamins, or problems affecting lipid processing and absorption such as inadequate bile production or chronic diarrhea

Physiology of Pancreas

-Pancreas secretes about 1000 mL of pancreatic juice a day -Hormones from the duodenum control these secretory activities -When chyme arrives in duodenum, *secretin* is released which triggers pancreatic secretion of watery buffer solution with pH of 7.5-8.8 -Secretion also contains bicarbonate and phosphate buffers that help raise the pH of the chyme -*Cholecystokinin* stimulates the production and secretion of pancreatic enzymes -Stimulation by vagus nerves increases secretion of pancreatic enzymes -Pancreas starts to synthesize enzymes before food even reaches stomach (during cephalic phase)--head start is important because enzyme synthesis takes much longer than production of buffers

Pyloric Glands

-Produce primarily a mucous secretion rather than enzymes or acid -Enteroendocrine cells are scattered among and produce at least 7 hormones, most notably gastrin -*Gastrin* is produced by *G cells*, which are most abudant in the gastric pits of pyloric antrum =Gastrin stimulates secretion by both parietal and chief cells, as well as contractions of gastric wall that mix and stir the gastric contents -Pyloric glands also contain *D cells* which release *somatostatin* (hormone that inhibits the release of gastrin) =D cells continuously release their secretions into interstitial fluid adjacent to G cells =Neural and hormonal stimuli can override this inhibition of gastrin production when stomach is preparing for digestion or is already engaged in digestion -*Ghrelin*: hormone produced by *P/D1 cells* lining the fundic region of the stomach, increase before meals to initiate hunger =Ghrelin levels decrease shortly after eating to curb appetite =It is antagonistic to *leptin* (hormone derived from fat tissue that induces satiety) -*Obestatin*: decreases appetite and inhibit thirst

Regulation of Gastric Activity

-Production of acid and enzymes by gastric mucosa can be (1) controlled by CNS; (2) regulated by short reflexes of enteric nervous system, coordinated in the wall of the stomach; (3) regulated by hormones of the digestive tract -Gastric control proceeds in 3 overlapping phases: *cephalic phase, gastric phase, and intestinal phase*

Protein Digestion and Absorption

-Proteins must first be disrupted through mechanical processing and chemical processing in order to expose peptide bonds to enzymatic attack -*Pepsin*: proteolytic enzyme secreted in inactive form by chief cells of stomach, works effectively at pH of 1.5-2, breaks peptide bonds within polypeptide chain -When chyme enters duodenum, enteropeptidase from small intestine triggers conversion of trypsinogen to trypsin -Buffers increase pH to 7-8 -Pancreatic proteases can now begin working -Trypsin, chymotrypsin, and elastase are like pepsin that that they break specific peptide bonds within polypeptide -Carboxypeptidase also acts in small intestine--chops off last amino acid of polypeptide chain, thus producing free amino acids -*Dipeptidases*: enzymes that break short peptide chains into individual amino acids which are absorbed through both facilitated diffusion and cotransport mechanisms

Absorption in the Large Intestine

-Reabsorption of water is an important function of the large intestine -Also absorbs number of other substances that remain in the feces or were secreted into the digestive tract along its length -Most of bile salts entering are promptly reabsorbed in cecum and transported in blood to liver for secretion into bile

Saliva

-Salivary glands produce 1-1.5 liters of saliva each day -99.4% water and 0.6% electrolytes, buffers, glycoproteins, antibodies, enzymes, and waste products -Glycoproteins called *mucins* give saliva its lubricating action -Saliva continuously flushes oral surfaces, keeping them clean -Buffers in saliva keep pH of mouth near 7.0 and also prevent buildup of acids produced by bacteria -Antibodies (IgA) and lysozyme which help control populations of oral bacteria -Saliva produced when you eat has many functions: =Lubricating the mouth =Moistening and lubricating food in the mouth =Dissolving chemicals that can stimulate the taste buds and provide sensory information about the food =Beginning the digestion of complex carbohydrates before food is swallowed--*salivary amylase* and also lingual lipase

Parietal Cells

-Secrete *intrinsic factor* -This glycoprotein helps absorb *vitamin B12* (essential for erythropoiesis) across the intestinal lining -Secrete *hydrochloric acid (HCl)* =Do not produce HCl in cytoplasm, instead H+ and Cl- are transported independently by different mechanisms -Secretions of parietal cells can keep stomach contents at pH 1.5-2 -Highly acidic environment does not digest chyme, but has 4 functions: 1. Acidity of gastric juices kills most of microorganisms ingested with food 2. Acidity denatures proteins and inactivates most of enzymes in food 3. Acidity helps break down plant cell walls and connective tissues in meat 4. Acidic environment is essential for activation and function of *pepsin* (protein digesting enzyme secreted by chief cells)

Serosa/Adventitia

-Serous membrane that covers the muscularis externa along most portions of the digestive tract inside the peritoneal cavity -There is no serosa covering the muscularis externa of the oral cavity, pharynx, esophagus, and rectum -Instead, a dense network of collagen fibers firmly attaches the digestive tract to adjacent structures--fibrous sheath called *adventitia*

Histology of Stomach

-Simple columnar epithelium lines all portions of the stomach -Epithelium is a *secretory sheet* which produces a carpet of mucus that covers the interior surface of the stomach -The alkaline mucous layer protects epithelial cells against the acid and enzymes in the gastric lumen -Shallow depression called *gastric pits* open onto the gastric surface =Mucous cells at base of each gastric pit actively divide, replacing superficial cells that are shed into chyme =Lifespan of gastric epithelial cell is 3-7 days--killed by alcohol exposure or other chemicals

Muscularis Externa

-Smooth muscle cells dominate this region =They are arranged in an inner circular layer and outer longitudinal layer =These layers play an essential role in mechanical processing and in moving materials along the digestive tract -The movements of the digestive tract are coordinated primarily by sensory neurons, interneurons, and motor neurons of the enteric nervous system (ENS) =The ENS is primarily innervated by the parasympathetic division of the ANS =Sympathetic postganglionic fibers also synapse here -Many of these fibers continue onward to innervate the mucosa and the *myenteric plexus* (a network of parasympathetic ganglia, sensory neurons, interneurons, and sympathetic postganglionic fibers) =It lies sandwiched between the circular and longitudinal muscle layers -Parasympathetic stimulation increases muscle tone and activity -Sympathetic stimulation decreases muscle tone and activity

Ion Absorption

-Sodium (Na+) ions are most abundant cations in food =Enter intestinal cells by diffusion, cotransport with another nutrient, or by active transport =These ions are then pumped into interstitial fluid across base of cell =Rate of Na+ uptake from lumen is generally proportional to concentration of Na+ in intestinal contents =Rate of sodium ion absorption by digestive tract is increased by aldosterone -Calcium (Ca++) ion absorption involves active transport at epithelial surface =Calcitriol speeds up rate of transport

Digestion and Absorption in the Stomach

-Stomach carries out preliminary digestion of proteins by pepsin =It allows digestion of carbs and lipids by salivary amylase and lingual lipase =These enzymes continue to work until pH throughout contents of stomach falls below 4.5 -As stomach contents become more fluid and pH approaches 2.0, pepsin activity increases =Protein disassembly begins =Protein digestion is not completed in stomach, however pepsin has enough time to break down complex proteins into smaller peptide and polypeptide chains before chyme enters duodenum -Digestion takes place in stomach, but absorption does not =This is because (1) epithelial cells are covered by blanket of alkaline mucus and not directly exposed to chyme; (2) epithelial cells lack that specialized transport mechanisms of cells that line the small intestine; (3) gastric lining is somewhat impermeable to water; (4) digestion has not been completed by time chyme leaves stomach--most carbs, lipids, and proteins are only partially broken down *Some drugs can be absorbed in the stomach

Anatomy of the Stomach

-Stomach has shape of expanded J -Short *lesser curvature* forms the medial surface of the organ -Long *greater curvature* forms the lateral surface -Divided into 4 regions: cardia, fundus, body, pylorus -Stomach's volume increases as you eat, and decreases as chyme enters small intestine -When stomach is relaxed (empty), mucosa has prominent folds called *rugae*--these temporary features let gastric lumen expand =As stomach fills, rugae flatten out until they almost disappear -Muscularis mucosae and muscularis externa of stomach contain extra layers of smooth muscle cells in addition to circular and longitudinal layers =Muscularis mucosae contain an outer, circular layer of muscle cells =Muscularis externa has an inner *oblique layer* of smooth muscle =The extra layers of smooth muscle strengthen the stomach wall and assist in mixing and churning essential to formation of chyme

Intestinal Absorption

-Takes 5 hours for materials to pass from duodenum to the end of the ileum -Organ's absorptive effectiveness is enhanced by the fact that so much of the mucosa is movable -The microvilli can be moved by their supporting microfilaments, the individual villi by smooth muscle cells, groups of villi by the muscularis mucosae, and the circular folds by the muscularis mucosae and the muscularis externa -These movements stir and mix the intestinal contents, changing the environment around each epithelial cell from moment to moment

Greater Omentum

-The dorsal mesentery of the stomach becomes greatly enlarged and forms an enormous pouch that extends inferiorly between the body wall and the anterior surface of the small intestine -Hangs like an apron from the lateral and inferior borders of the stomach -Adipose tissue in the greater omentum conforms to the shapes of the surrounding organs, providing padding and protection across the anterior and lateral surfaces of the abdomen -When an individual gains weight this adipose tissue contributes to the beer belly--lipids here are important energy reserve -Greater omentum insulates to reduce heat loss across the anterior abdominal wall

Mesentery Proper

-Thick mesenterial sheet -Provides stability, but permits some independent movement

The abdominopelvic cavity contains the peritoneal cavity, which is lined by a serous membrane

-This membrane consists of a superficial mesothelium covering a layer of areolar tissue -Serous membrane is divided into visceral peritoneum and the parietal peritoneum -*Visceral peritoneum*: covers organs that project into the peritoneal cavity -*Parietal peritoneum*: lines the inner surfaces of the body wall -The serous membrane lining the peritoneal cavity continuously produces peritoneal fluid, which lubricates the surfaces--because thin layer of peritoneal fluid separates the parietal and visceral surfaces, they can slide without friction and resulting irritation =The membrane secretes and reabsorbs about 7 liters of fluid each day, but the volume within the peritoneal cavity at any one time is very small =*Ascites*: buildup of fluid creates a characteristic abdominal swelling--caused by liver disease, kidney disease, and heart failure; the fluid can distort internal organs and cause symptoms like heartburn, indigestion, and lower back pain

Pharynx

-Throat -Anatomical space that serves as a common passageway for solid food, liquids, and air -Food normally passes through the oropharynx and laryngopharynx on its way to the esophagus -These regions have stratified squamous epithelium similar to that of oral cavity -Lamina propria contains scattered mucous glands and lymphatic tissue of pharyngeal, palatal, and lingual tonsils -Deep to lamina propria lies dense layer of elastic fibers, bound to underlying skeletal muscles -*Pharyngeal constrictor muscles*: push bolus toward and into esophagus -*Palatopharyngeus and stylopharyngeus muscles* elevate the larynx -*Palatal muscles* elevate the soft palate and adjacent portions of pharyngeal wall -These muscles work with muscles of oral cavity and esophagus to start swallowing, which pushes bolus along the esophagus and into stomach

Teeth

-Tongue movements pass food across opposing surfaces (occlusal surfaces) of the teeth--these surfaces carry out chewing or *mastication* of food -Mastication breaks down tough CT in meat and plant fibers in vegetables and also helps saturate the food with salivary secretions and enzymes -*Dentin*: mineralized matrix in the bulk of each tooth that is similar to bone--but doesn't contain cells -*Pulp cavity*: inferior chamber, receives blood vessels and nerves through *root canal* (narrow tunnel located at the *root* or base of the tooth) -Blood vessels and nerves enter the root canal through an opening called the *apical foramen* to supply the pulp cavity -Root of tooth sits in a bony cavity or socket called tooth socket or alveolus -Collagen fibers of *periodontal ligament* extend from dentin of root to alveolar bone, creating strong articulation known as gomphosis -Layer of *cementum* covers dentin of the root -Cementum protects and firmly anchors to the periodontal ligament--less resistant to erosion than dentin -*Neck* of tooth marks boundary between root and *crown* (exposed portion of tooth that projects beyond the soft tissue of gingiva) -*Gingival sulcus* (shallow groove) surrounds neck of each tooth -Mucosa of gingival sulcus is very thin and not tightly bound to periosteum -Epithelium is bound to the tooth at the base of sulcus--this epithelial attachment prevents bacterial access to lamina propria of gingiva and relatively soft cementum of root -*Gingivitis*: inflammation of the gingivae caused by bacterial infection, can occur if attachment breaks down -Layer of *enamel* covers dentin of crown--contains calcium phosphate in a crystalline form =It's the hardest biologically manufactured substance

Gastroileal Reflex

-Triggers the opening of the ileocecal valve, allowing materials to pass from small intestine into large intestine -*Ileocecal valve*: controls passage of materials into large intestine

Swallowing Process

1. *Buccal phase*: begins with compression of the bolus against the hard palate -Retraction of the tongue then forces the bolus into the oropharynx and assists in the elevation of the soft palate, thereby sealing off the nasopharynx -Once bolus enters the oropharynx, reflex responses begin and bolus is moved toward stomach 2. *Pharyngeal phase*: begins as bolus comes into contact with the palatoglossal and palatopharyngeal arches and the posterior pharyngeal wall -Elevation of larynx and folding of epiglottis direct bolus past the closed glottis--at same time, uvula and soft palate block passage back to nasopharynx 3. *Esophageal phase*: begins as contraction of pharyngeal muscles forces the bolus through the entrance to the esophagus -Once in the esophagus, the bolus is pushed toward the stomach by peristalsis 4. Bolus enters stomach: approach of bolus triggers opening of lower esophageal sphincter -Bolus then continues into stomach

Colon is subdivided into 4 regions

1. *Ascending Colon*: begins at the superior border of the cecum and ascends along the right lateral and posterior wall of the peritoneal cavity to the inferior surface of the liver--there the colon bends sharply to the left at the *right colic flexure*, this bend marks the end 2. *Transverse Colon*: curves anteriorly from the right colic flexure and crosses the abdomen from right to left -It is supported by the transverse mesocolon -It is separated from anterior abdominal wall by the layers of greater omentum -Near the spleen, the colon makes a 90 degree turn at the *left colic flexure* and becomes the descending colon 3. *Descending Colon*: proceeds inferiorly along the left side until reaching iliac fossa formed by inner surface of left ilium -Descending colon is retroperitoneal and firmly attached to the abdominal wall -Curves at the *sigmoid flexure* and becomes the sigmoid colon 4. *Sigmoid Colon*: is an S-shaped segment, lies posterior to the urinary bladder, suspended from the sigmoid mesocolon, empties into the *rectum*

4 Regions of Stomach

1. *Cardia*: smallest part of stomach -Contains abundant mucous glands -Their secretions coat the connection with the esophagus and help protect that tube from the acid and enzymes of the stomach 2. *Fundus*: portion of the stomach that is superior to the junction between the stomach and esophagus -Contacts the inferior, posterior surface of the diaphragm 3. *Body*: area of the stomach between the fundus and curve of the J -Largest region of the stomach -Acts as a mixing tank for ingested food and secretions produced in the stomach -Gastric glands in the fundus and body secrete most of the acid and enzymes involved in gastric digestion 4. *Pylorus*: forms the sharp curve of the J -Divided into a *pyloric antrum* (connected to the body) and a *pyloric canal* (which empties into the duodenum) -As mixing movements take place during digestion, pylorus frequently changes shape -Muscular *pyloric sphincter* regulates release of chyme into duodenum -Glands in pylorus secrete mucus and important digestive hormones, including *gastrin* (hormone that stimulates gastric glands)

Types of Teeth

1. *Incisors*: blade-shaped teeth located at front of mouth -Useful for clipping or cutting -Have a single root 2. *Cuspids*: or canines, are conical, with a sharp ridgeline and a pointed tip -Useful for tearing or slashing -Have a single root 3. *Bicuspids or premolars*: have flattened crowns with prominent ridges -They crush, mash, and grind -Have one or two roots 4. *Molars*: have very large, flattened crows with prominent ridges adapted for crushing and grinding -Molars in upper jaw have 3 roots -Molars in lower jaw have 2 roots

Functions of the Digestive System

1. *Ingestion*: takes place when materials enter the oral cavity of digestive tract -Ingestion is an active process involving conscious choice and decision making 2. *Mechanical Processing*: is crushing and shearing that makes materials easier to propel along the digestive tract -It increases surface area, making them more susceptible to attack by enzymes -It may or may not be required before ingestion--swallow liquids immediately, but must chew solids first -Preliminary mechanical processing: tearing and mashing with the teeth, followed by squashing and compacting by the tongue -Mechanical processing after ingestion: swirling, mixing, and churning motions of the stomach and intestines 3. *Digestion*: the chemical breakdown of food into small organic fragments suitable for absorption by the digestive epithelium -Simple molecules in food, glucose, can be absorbed intact -Epithelial cells have no way to absorb molecules with the size and complexity of proteins, polysaccharides, or triglycerides -Digestive enzymes must first disassemble these molecules 4. *Secretion*: the release of water, acids, enzymes, buffers, and salts by the epithelium of the digestive tract and by glandular organs 5. *Absorption*: the movement of organic molecules, electrolytes (inorganic ions), vitamins, and water across the digestive epithelium and into the interstitial fluid of the digestive tract 6. *Excretion*: the removal of wastes from the body -Digestive tract and glandular organs discharge wastes in secretions that enter the lumen of the tract -Most of these wastes mix with indigestible residue of the digestive process and then leave the body -*Defecation*: ejects materials from the digestive tract, eliminating them as matter called *feces*

Secretion of Hydrochloric Acid (HCl)

1. Hydrogen ions are generated inside a parietal cell as the enzyme carbonic anhydrase converts CO2 and H2O to carbonic acid (H2CO3), which then dissociates 2. Countertransport mechanism ejects the bicarbonate ions into the interstitial fluid and imports chloride ions into the cell 3. Chloride ions then diffuse across the cell and exit through open chloride channels into the lumen of gastric gland 4. Hydrogen ions are actively transported into lumen of gastric gland -Summary: initial step is formation of carbonic acid within parietal cells; dissociation of carbonic acid releases bicarbonate and hydrogen ions; H+ are actively transported into lumen of gastric gland; bicarbonate ions are exchanged for chloride ions from interstitial fluid; when gastric glands are actively secreting, enough bicarbonate ions diffuse into bloodstream from interstitial fluid to increase pH of blood; this sudden influx of bicarbonate ions has been called *alkaline tide*

Histology of the Digestive Tract

4 major layers of the digestive tract are: 1) Mucosa 2) Submucosa 3) Muscularis Externa 4) Serosa

Lining of the digestive tract plays a protective role

It safeguards surrounding tissues against (1) the corrosive effects of digestive acids and enzymes; (2) mechanical stresses, such as abrasion; and (3) bacteria that either are swallowed with food or live in the digestive tract -Digestive epithelium and its secretions provide a nonspecific defense against these bacteria--when bacteria reach the underlying layer of areolar tissue, the lamina propria, macrophages and other cells of the immune system attack them

Retroperitoneal

Organs located behind the peritoneum of the abdominal cavity

Digestive system works with the cardiovascular and lymphatic systems in order to

Provide the needed organic molecules -Digestive system supplies the fuel that keeps body's cells running and building blocks needed for cell growth and repair

Gastroenteric Reflex

Stimulates motility and secretion along the entire small intestine

During embryonic development

The digestive tract and accessory organs are suspended within the peritoneal cavity by dorsal and ventral mesenteries -The ventral mesentery later disappears along most of the digestive tract -It persists in adults in only 2 places: on the ventral surface of the stomach, between the stomach and the liver (lesser omentum), aand between the liver and the anterior abdominal wall (falciform ligament)

Carbohydrate Digestion and Absorption

This is through 2 steps: carbohydrases produced by salivary glands and pancreas AND brush border enzymes

Short Reflex

Triggers series of peristaltic contractions in the rectum that move feces toward the anus