Digestive System Chapter 25
Describe neural and hormonal control of stomach or gastric mixing, propulsion and emptying, and include description of the three phases of gastric motility: cephalic, gastric and intestinal.
. Gastric motility turns ingested food into chyme. (pp. 960-961) 1. Upon swallowing, the medulla relays signals to the stomach via the vagus nerves. 2. The stomach reacts with a receptive-relaxation response, in which the stomach briefly resists stretching but then relaxes to accept food. 3. The stomach soon shows a rhythm of peristaltic contractions about every 20 seconds that aregoverned by pacemaker cells in the muscularis externa. 4. After 30 minutes, these contractions become quite strong. 5. The antrum holds about 30 mL of chyme. The muscularis is thick in this area and acts as a strong antral pump that breaks up semidigested food into smaller particles. 6. A leading wave of antral contraction proceeds to the pyloric valve and closes it tightly. 7. A trailing wave then comes along, churning and breaking up the chyme. 8. The chyme jets backward through the trailing constriction into the gastric body, where it awaits thenext wave of contraction to drive it down again. 9. The repetitive downward propulsion and reverse jetting of chyme break the food into smaller andsmaller particles. 10. Only about 3 mL of chyme is squirted into the duodenum at a time. Receiving only small amounts ofchyme at a time allows the duodenum to neutralize the stomach acid and digest nutrients little by little. 11. A typical meal is emptied from the stomach in about 4 hours. The cephalic phase is the stage in which the stomach responds to the mere sight, smell, taste, orthought of food. The gastric phase is a period in which swallowed food and semidigested protein activate gastric activity. The intestinal phase is a stage in which the duodenum responds to arriving chyme and moderatesgastric activity through hormones and nervous reflexes.
Describe the process of vomiting. Explain the various stimuli that can trigger vomiting. Describe the protective effects of vomiting. Describe how vomiting alters extracellular pH balance.
. It is commonly induced by overstretching of the stomach or duodenum; chemical irritants such as alcohol and bacterial toxins; visceral trauma; intense pain; or psychological and sensory stimuli. Vomiting is usually preceded by nausea and retching. a. In retching, thoracic expansion and abdominal contraction create a pressure difference thatdilates the esophagus. i. The lower esophageal sphincter relaxes while the stomach and duodenum contract spasmodically. ii. Chyme enters the esophagus but then drops back into the stomach as the muscles relax; it does not get past the upper esophageal sphincter. iii. Retching is often accompanied by tachycardia, profuse salivation, and sweating. Vomiting occurs when abdominal contraction and rising thoracic pressure force the upperesophageal sphincter open, the esophagus and body of the stomach relax, and chyme is driven out of the stomach and mouth by strong abdominal contraction along with reverse peristalsis. c. Projectile vomiting is sudden vomiting with no prior nausea or retching. i. It may be caused by neurological lesions but is also common in infants afterfeeding. 4. Chronic vomiting can cause dangerous fluid, electrolyte, and acid-base imbalances. a. In cases of frequent vomiting, as in the eating disorder bulimia, tooth enamel becomesseverely eroded by the hydrochloric acid in the chyme. b. Aspiration of this acid is very destructive to the respiratory tract, and many have died fromaspiration of vomit when unconscious or semiconscious. c. Surgical anesthesia must be preceded by fasting until the stomach and small intestineare empty.
Describe the general histology of the alimentary canal wall.
. Most of the digestive tract follows a basic structural plan, with a wall composed of the followingtissue layers, in order from inner to outer surface: (Fig. 25.2) a. Mucosa, with epithelium, lamina propria, and muscularis mucosae. b. Submucosa. c. Muscularis externa, with inner circular layer and outer longitudinal layer. d. Serosa, with areolar tissue and mesothelium.
Define zymogen. Describe neural and hormonal control of the secretion of the pepsinogen (a zymogen) from chief cells and its subsequent conversion to pepsin (active form of the enzyme) in the presence of HCl and autocatalytic conversion by pepsin.
. Pepsin is one of the digestive enzymes secreted as an inactive protein called a zymogen, which is then converted to active form by removal of certain amino acids. a. In the stomach, chief cells secrete pepsinogen and hydrochloric acid removes some amino acids to convert it to pepsin. b. Because pepsin digests protein and pepsinogen is a protein, pepsin has an autocatalyticeffect—as some pepsin is formed, it converts pepsinogen to pepsin. (Fig. 25.15) c. The ultimate function of pepsin is to digest dietary proteins to shorter chains that are then completely digested in the small intestine.
Describe how water-soluble nutrients and drugs are absorbed into vascular capillaries and fat-soluble nutrients and drugs are absorbed into the lymphatic capillaries within the villi of the small intestine.
. The core of a villus is filled with areolar tissue of the lamina propria and contains anarteriole, blood capillaries, a venule, and a lymphatic capillary called a lacteal. v. Blood capillaries absorb most nutrients, but the lacteal absorbs most lipids, giving its contents a milky appearance. vi. The core of the villus also contains a few smooth muscle cells that contract periodically to mix chyme and to move lymph down the lacteal.
Stomach: Describe the main functions.
. The stomach is a muscular sac in the upper left abdominal cavity that receives food, stores it, breaks it up, and begins digestion to produce chyme. (p. 956)
Describe the gross and microscopic anatomy of stomach. List the cells of
. The stomach is divided into four regions. a. The cardiac region (cardia) is a small area within about 3 cm of the cardiac orifice. b. The fundic region (fundus) is the dome-shaped portion superior to the esophageal attachment. c. The body (corpus) makes up the greatest part of the stomach distal to the cardiac orifice. d. The pyloric region is a slightly narrower pouch at the inferior end that is subdivided into theantrum and the pyloric canal. i. The pyloric canal terminates at the pylorus, a narrow passage into the duodenum. ii. The pylorus is surrounded by a thick ring of smooth muscle, the pyloric(gastroduodenal) sphincter. 2. Between the esophagus and duodenum, the stomach has two margins called the greater and lessercurvatures. In the stomach, chief cells secrete pepsinogen and hydrochloric acid removes some amino acids to convert it to pepsin.
Describe digestion and transepithelial absorption of carbohydrates, proteins, and fats. Include identification of the specific monomers into which each macromolecule is digested for ultimate absorption.
Amino acid absorption is similar to that of monsaccharides. a. Several sodium-dependent amino acid cotransporters carry different classes of aminoacids across the membrane. b. Dipeptides and tripeptides may also be absorbed, but are hydrolyzed within theepithelial cells.
Describe the composition of bile, and discuss bile secretion, storage and digestive functions. Describe the composition, structure and function of micelles. Explain how the hydrophobic and hydrophilic portions of lecithin (a phospholipid) and bile salts (a.k.a. bile acids) are oriented so as to form micelles.
Bile is a green fluid containing minerals, cholesterol, neutral fats, phospholipids, bile pigments, and bile acids. a. The principal pigment is bilirubin, derived from the decomposition of hemoglobin. i. Bacteria of the large intestine metabolize bilirubin to urobilinogen, which givesfeces a brown color. ii. In the absence of bile secretion, the feces are grayish white and marked with streaks of undigested fat (acholic feces). b. Bile acids (bile salts) are steroids synthesized from cholesterol that, together with lecithin, aid in fat digestion and absorption. c. All other components of the bile are wastes destined for excretion; if these wastes becomeexcessively concentrated, they may form gallstones. 4. Bile gets into the gallbladder by first filling the bile duct, then overflowing into thegallbladder. a. Between meals, the gallbladder absorbs water and electrolytes from the bile and concentrates it by a factor of 5 to 20 times. b. The liver secretes about 500 to 1,000 mL of bile per day. 5. About 80% of bile acids are reabsorbed in the ileum, the last portion of the small intestine, andreturned to the liver, where hepatocytes absorb and resecrete them. The route of secretion, reabsorption, and resecretion is called the enterohepatic circulation. a. This process reuses the bile acids two or more times during the digestion of a meal. b. The 20% that is not reabsorbed is excreted in feces. c. This is the body's only way of eliminating excess cholesterol. d. New bile acids aresynthesized from cholesterol to replace the quantity lost in feces. The absorption of fatty acids, monoglycerides, and other lipids depends on minute droplets in the bilecalled micelles. a. Micelles consist of 20 to 40 bile acid molecules aggregated with hydrophilic side groupsfacing outward and hydrophobic steroid rings facing inward. b. Bile phospholipids and cholesterol diffuse into the center of the micelle to form its core. c. The micelles pass down the bile duct into the duodenum, where they absorb fat-solublevitamins, more cholesterol, and the products of fat digestion. d. Their hydrophilic surfaces allow them to remain suspended in water. e. They transport lipids to the surfaces of the intestinal absorptive cells, where the lipids leavethe micelles and diffuse into the cells. f. The micelles are then reused. g. Without micelles, the small intestine absorbs only about 40% to 50% of dietary fat and almost no cholesterol.
Describe the composition and functions of pancreatic secretions, including neural (parasympathetic, acetylcholine) and hormonal control (secretin and cholecystokinin) of release of pancreatic juices into the duodenum.
D. Secretion is regulated by responses to three types of stimuli that cause the release of pancreatic juiceand bile. (p. 970) 1. Acethylcholine (ACh) coming from the vagus and enteric nerves stimulates pancreatic acini to secrete enzymes even before food has been swallowed. a. The enzymes remain stored in the pancreatic acini and ducts, however, for release whenchyme enters the duodenum. 2. Cholecystokinin (CCK) is secreted by the mucosa of the duodenum and proximal jejunum in responseto fats in the small intestine. a. It stimulates the pancreatic acini to secrete enzymes. b. It has a strong stimulatory effect on the gallbladder, from which it gets its name. It induces contractions of the gallbladder and relaxation of the hepatopancreatic sphincter. 3. Secretin is produced in the small intestine in response to the acidity of chyme from the stomach. a. It stimulates ducts of the liver and pancreas to secrete sodium bicarbonate to buffer the HCl and protect the intestinal mucosa from stomach acid. b. This action also raises the intestinal pH to the optimum level for pancreatic and intestinaldigestive enzymes.
Describe the composition of gastric juice
Gastric juice has a high concentration of hydrochloric acid and a pH as low as 0.8. a. Parietal cells contain carbonic anhydrase (CAH), which catalyzes the first step in the following reaction (Fig. 25.14): CAH i. CO2 + H2O H2CO3 HCO3- + H+ b. The H+ is pumped into the lumen of a gastric gland by an active- transport protein called H+-K+ ATPase, an antiport. c. HCl secretion does not affect the pH within the parietal cell. d. Bicarbonate ions (HCO3-) are exchanged for chloride ions (Cl-) from the blood plasma, thesame chloride shift that occurs in the renal tubules and red blood cells, and the Cl- is pumped into the lumen to join the H+. e. Blood leaving the stomach has a higher pH when digestion is occurring than when thestomach is empty (the alkaline tide). f. Stomach acid has several functions. i. It activates the enzymes pepsin and lingual lipase. ii. It breaks up connective tissues and plant cell walls. iii. It converts ingested ferricions (Fe3+) to ferrous ions (Fe2+), which can be absorbed and used. iv. It contributes to nonspecific disease resistance by destroying ingested pathogens.
Small intestine: Describe the gross and microscopic anatomy.
In terms of gross anatomy, the small intestine is a coiled mass filling most of the abdominal cavity inferior to the stomach and liver; it is divided into three regions: duodenum, jejunum, and ileum. In terms of microscopic anatomy, the tissue layers of the small intestine are reminiscent of those in theesophagus and stomach with modifications for digestion and absorption. (pp. 971-972) 1. The lumen is lined with simple columnar epithelium. 2. The muscularis externa is notable for a thick inner circular layer and a thinner outer longitudinal layer. 3. Both the jejunum and ileum are intraperitoneal and thus covered with a serosa that is continuous with the folded mesentery that suspends the small intestine from the posterior abdominal wall. 4. A large internal surface area is produced by the small intestine's relatively great length and by threekinds of internal folds or projections: the circular folds, villi, and microvilli. 5. On the floor of the small intestine, between the bases of the villi, are numerous pores that open into tubular glands called intestinal crypts (crypts of Lieberkühn). 6. The duodenum has prominent duodenal (Brunner) glands in the submucosa that secretebicarbonate-rich mucus. 7. Throughout the small intestine, the lamina propria and submucosa have a large population oflymphocytes that intercept pathogens.
Review the hepatic portal circulation and its role in distribution and storage of nutrients post-absorption Download
In terms of innervation and circulation, the stomach receives parasympathetic nerve fibers from the vagus nervesand sympathetic fibers from the celiac ganglia. It is supplied with blood from the celiac trunk, and all blood drained from the stomach and intestines enters the hepatic portal circulation before returning to the heart. (p. 956)
Describe the process by which B-12 is absorbed in the ileum. Explain the roles of parietal cells, intrinsic factor, and receptor-mediated endocytosis in absorption of B-12 absorption.
Intrinsic factor, a glycoprotein secreted by parietal cells, is essential to the absorption of vitamin B12 in the small intestine. a. Without vitamin B12, hemoglobin cannot be synthesized and pernicious anemia develops. b. The secretion of intrinsic factor is the only indispensable function of the stomach. c. As people age, the gastric mucosa atrophies, and the risk of pernicious anemia rises. b. The B12-IF complex then binds to receptors on cells of the distal ileum, where it is taken up byreceptor-mediated endocytosis.
Describe the composition of saliva and explain the function of its major components: mucous salivary amylase lingual lipase lysozyme immunoglobulin-A pH
It is a hypotonic solution of 97.0% to 99.5% water, a pH of 6.8 to 7.0, and the following solutes: a. Mucus, which binds and lubricates the food and aids in swallowing. b. Electrolytes, including sodium, potassium, chloride, phosphate, and bicarbonate salts. c. Lysozyme, an enzyme that kills bacteria. d. Immunoglobulin A (IgA), an antibacterial antibody. e. Salivary amylase, an enzyme that begins starch digestion. f. Lingual lipase, an enzymethat begins fat digestion.
Describe the roles of the gastrocolic reflex and the duodenocolic reflex in regulating motility of the large intestine.
Mass movements are often triggered by the gastrocolic and duodenocolic reflexes, in which filling of the stomach and duodenum stimulates motility of the colon.
Explain the specific purpose of mechanical digestion and chemical digestion.
Mechanical digestion is the physical breakdown of food into smaller particles. i. It is achieved by the action of the teeth and the churning contractions of the stomachand small intestine. ii. It exposes more food surface to the action of digestive enzymes. b. Chemical digestion is a series of hydrolysis reactions that break macromolecules into monomers (residues). i. These monomers are monosaccharides, amino acids, monoglycerides and fattyacids, and nucleic acids. ii. Chemical digestion is carried out by digestive enzymes of the salivary glands, stomach, pancreas, and small intestine.
Name the nutritive macromolecules (minerals, vitamins, cholesterol, water) that are not digested by the human GI tract and are absorbed as is, and explain the biological importance that these molecules are absorbed in an undigested state. *Note: Dietary fiber, e.g., non-starch polysaccharides and other plant components, such as cellulose, inulin, lignins, chitins, pectins, and beta-glucans are not digested by human enzymes.
Minerals (electrolytes) are absorbed along the entire length of the small intestine. (p. 978) 1. Sodium ions are cotransported with sugars and amino acids. 2. Chloride ions are actively transported in the distal ileum by a pump that exchanges them forbicarbonate ions, the reverse of the exchange that takes place in the stomach. 3. Potassium ions are absorbed by simple diffusion. Vitamins are absorbed unchanged. (p. 978) 1. The fat-soluble vitamins A, D, E, and K are absorbed with other lipids; if they are ingested without fat-containing food, they are not absorbed at all. 2. Water-soluble vitamins (B complex and vitamin C) are absorbed by simple diffusion, with theexception of B12. a. This large molecule can only be absorbed if it binds to intrinsic factor from the stomach. b. The B12-IF complex then binds to receptors on cells of the distal ileum, where it is taken up byreceptor-mediated endocytosis.
Name the class of monomers into which each category of macronutrients (carbohydrates, lipids, proteins and nucleic acids) must be broken down for subsequent absorption.
Most of the digestible carbohydrate is starch; cellulose is indigestible. (pp. 974-975) 1. Starch is digested first to oligosaccharides up to eight glucose units long, then into the disaccharidemaltose, and finally to glucose, which is absorbed by the small intestine. 1. Fats are digested by enzymes called lipases. 2. Lingual lipase, secreted by the intrinsic salivary glands of the tongue, digests a small amount of fat while food is still in the mouth, but becomes more active in the acidic pH of the stomach. 3. Gastric lipase makes a much larger contribution to preduodenal fat digestion. 4. Antral pumping breaks up the fat into small droplets dispersed through the watery chyme, that is, it emulsifies the fat. Amino acids come from three protein sources: dietary proteins, digestive enzymes digested by each other, and sloughed epithelial cells. (pp. 975-976) Nucleic acids are present in only small quantities compared to other dietary macromolecules. (p. 978) 1. The nucleases (ribonuclease and deoxyribonuclease) of pancreatic juice hydrolyze these to nucleotides. 2. Nucleosidases and phosphatases of the brush border then decompose the nucleotides into phosphateions, ribose or deoxyribose, and nitrogenous bases. 3. These are transported by membrane carriers and enter the capillary blood.
Discuss its main functions.
Motility of the small intestine serves three functions: (1) to mix chyme with bile and with intestinal and pancreatic secretions; (2) to churn chyme and bring it into contact with the mucosa; and (3) to move residuetoward the large intestine. (pp. 972-974)
Discuss the digestion and absorption of nucleic acids.
Nucleic acids are present in only small quantities compared to other dietary macromolecules. (p. 978) 1. The nucleases (ribonuclease and deoxyribonuclease) of pancreatic juice hydrolyze these to nucleotides. 2. Nucleosidases and phosphatases of the brush border then decompose the nucleotides into phosphateions, ribose or deoxyribose, and nitrogenous bases. 3. These are transported by membrane carriers and enter the capillary blood.
Describe the mechanisms by which the following regulate digestive processes: pacemaker cells
Pacemaker cells of the muscularis externa set the rhythm of segmentation at about 12 times perminute in the duodenum and 8 to 9 times per minute in the ileum.
List and describe the function of the various components of pancreatic juice: sodium bicarbonate, zymogens - trypsinogen, chymotrypsinogen, and procarboxypeptidase, pancreatic lipase, deoxyribonuclease, and ribonuclease.
Pancreatic juice is an alkaline mixture of water, enzymes, zyomogens, sodium bicarbonate, and otherelectrolytes. a. The acini secrete the enzymes and zymogens, while the ducts secrete the sodium bicarbonate. b. The bicarbonate serves to buffer the hydrochloric acid arriving from the stomach. 5. The pancreatic zymogens are trypsinogen, chymotrypsinogen, and procarboxypeptidase. a. When secreted into the intestinal lumen, trypsinogen is converted to trypsin byenterokinase, which is secreted by the intestinal mucosa. (Fig. 25.23) b. Trypsin is autocatalytic and converts trypsinogen into more trypsin. c. Trypsin also converts the two other zymogens into chymotrypsin and carboxypeptidase. d. Trypsin's primary role is the digestion of dietary protein. 6. Other pancreatic enzymes include pancreatic amylase, which digests starch; pancreatic lipase,which digests fat; and ribonuclease and dexoyribonuclease, which digest RNA and DNA. 7. Theexocrinesecretions ofthepancreasaresummarized in Table25.2.
Describe the mechanisms by which the following regulate digestive processes: Paracrines, i.e., histamine
Paracrine secretions, such as histamine and prostaglandins, diffuse through tissue fluids to stimulatenearby target cells. Histamine is a paracrine secretion from enteroendocrine cells in gastric glands.
Describe the mechanisms by which the following regulate digestive processes: extrinsic neural control by the sympathetic and parasympathetic divisions of the autonomic nervous system (ANS)
Parasympathetic fibers stimulate the glands to produce abundant, thin saliva rich inenzymes. ii. Sympathetic stimulation briefly enhances salivation, but its primary effect is to produce thicker saliva with more mucus. Under conditions of stress the mouth may feel sticky or dry. In terms of innervation and circulation, the stomach receives parasympathetic nerve fibers from the vagus nervesand sympathetic fibers from the celiac ganglia. It is supplied with blood from the celiac trunk, and all blood drained from the stomach and intestines enters the hepatic portal circulation before returning to the heart. (p. 956)
Name the cells that secrete gastrin. Describe the hormonal (gastrin) regulation of gastric secretion of HCl and the mechanisms that mediate secretion of H+ and Cl- by the parietal cell, including hydration of carbon dioxide, carbonic anhydrase, the chloride shift, the alkaline tide, and the proton pump (H+, K+-ATPase).
Parietal cells contain carbonic anhydrase (CAH), which catalyzes the first step in the following reaction (Fig. 25.14): CAH i. CO2 + H2O H2CO3 HCO3- + H+ b. The H+ is pumped into the lumen of a gastric gland by an active- transport protein called H+-K+ ATPase, an antiport. c. HCl secretion does not affect the pH within the parietal cell. d. Bicarbonate ions (HCO3-) are exchanged for chloride ions (Cl-) from the blood plasma, thesame chloride shift that occurs in the renal tubules and red blood cells, and the Cl- is pumped into the lumen to join the H+. e. Blood leaving the stomach has a higher pH when digestion is occurring than when thestomach is empty (the alkaline tide). f. Stomach acid has several functions. i. It activates the enzymes pepsin and lingual lipase. ii. It breaks up connective tissues and plant cell walls. iii. It converts ingested ferricions (Fe3+) to ferrous ions (Fe2+), which can be absorbed and used. iv. It contributes to nonspecific disease resistance by destroying ingested pathogens.
Explain the essentiality of gastric secretion of HCl to digestion and intestinal absorption of specific nutrients, activation of enzymes, and non-specific protection against pathogens. Describe the three mechanisms that protect the stomach from potential detrimental effects of HCl.
Protection of the stomach from self-digestion is accomplished by three means. (p. 962) 1. Mucous coat. The thick, highly alkaline mucus that coats the stomach resists the action of acid and enzymes. 2. Tight junctions. The epithelial cells are joined by tight junctions that prevent gastric juice from seeping between them and digesting the underlying tissues. 3. Epithelial cell replacement. The stomach's epithelial cells live only 3 to 6 days and are then sloughed off. They are rapidly replaced by cell division in the gastric pits.
Describe the processes and functional importance of segmentation and peristalsis to digestion and absorption and propulsion.
Segmentation is a movement in which stationary, ringlike constrictions appear at several places along with the intestineand then relax as new constrictions form elsewhere Peristalsis begins as segmentation declines, after most nutrients have been absorbed. Most food passes by gravity when we are standing or sitting upright, but peristalsis ensures that food can be swallowed regardless of body position.
Describe the oral phase, pharyngeal phase, and esophageal stage of swallowing; include distinction between voluntary versus involuntary phases, and discuss the importance of cranial nerves (facial, glossopharyngeal, hypoglossal, and vagus nerves).
Swallowing, or deglutition, involves over 22 muscles in the mouth, pharynx, and esophagus, and iscoordinated by the swallowing center in the medulla oblongata. (p. 955) 1. The swallowing center, a pair of nuclei, communicates with muscles of the pharynx and esophagus through the trigeminal, facial, glossopharyngeal, and hypoglossal nerves (CN V, VII, IX, and XII). 2. Swallowing occurs in three phases. (Fig. 25.11) a. The oral phase is under voluntary control. i. The tongue collects food, presses it against the palate to form a bolus, and pushes it posteriorly. ii. The food accumulates in the oropharynx in front of the blade of the epiglottis. iii. When the bolus reaches a critical size, the epiglottis tips posteriorly and the food bolus slides around it through a space on each side, into the laryngopharynx. b. The pharyngeal phase is involuntary. i. The soft palate and root of the tongue prevent food and drink from reentering themouth or entering the nasal cavity. ii. To prevent choking, breathing is automatically suspended, the infrahyoid muscles pull the larynx up to meet the epiglottis and cover the laryngeal opening, and the vocal cordsadduct to close the airway. iii. The food bolus is driven downward by constriction of the upper, then middle, andfinally inferior pharyngeal constrictors. c. The esophageal phase is a wave of involuntary contractions called peristalsis, controlled by the swallowing centers and the myenteric plexus in the esophageal wall. i. The bolus stimulates a stretch receptor, which sends signals to the muscularisexterna above and below the bolus. ii. The circular muscle above the bolus constricts and pushes it downward. iii. Below the bolus, the circular muscle relaxes while the longitudinal muscle contracts, pulling the wall of the esophagus upward and making it wider. d. Most food passes by gravity when we are standing or sitting upright, but peristalsisensures that food can be swallowed regardless of body position.
Describe how the circular folds, villi and microvilli markedly increase surface area and the efficiency of digestion and absorption.
The circular folds are the largest folds and are transverse to spiral ridges up to 1 cm high.(Fig. 25.21) i. They occur from the duodenum to the middle of the ileum, altering the path ofchyme into a spiral and promoting mixing and absorption. ii. They are not found in the distal half of the ileum, but most absorption is complete bythe time chyme reaches that point. Each enterocyte of a villus has plasma membrane extensions, about 1 μm high, that form afuzzy brush border. i. The brush border increases the surface area and contains brush border enzymes in theplasma membrane. ii. These enzymes are not released into the lumen, but digest the chyme contents as itcontacts the brush border, a process called contact digestion.
List the fundamental functions of microbes in the colon.
The colon is that part of the large intestine between the ileocecal junction and rectum; it is divided into three regions. a. The ascending colon begins at the ileocecal valve and passes up the right side of theabdominal cavity, making a 90° turn at the right colic (hepatic) flexure near the right lobe of theliver. b. This is now the transverse colon; it passes horizontally across the upper abdominal cavityand turns 90° downward at the left colic (splenic) flexure near the spleen. c. Here it becomes the descending colon, which passes down the left side of the abdominalcavity.
Describe the location, gross and histological structures, and general functions of the organs of the digestive tract (a.k.a. alimentary canal or gastrointestinal tract).
The digestive tract (alimentary canal) is a muscular tube extending from mouth to anus. a. It is about 5 m (16 ft) long in a living person; it is about 9 m (30 ft) long in a cadaver. b. It includes the mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, and anus (know order) c. Part of this, the stomach and intestines, constitute the gastrointestinal (GI) tract.
Describe the mechanisms by which the following regulate digestive processes: enteric nervous system
The esophagus, stomach, and intestines have a nervous network called the enteric nervous system. a. This system regulates digestive tract motility, secretion, and blood flow. b. It is thought to have over 100 million neurons, more than the spinal cord. c. It can function independently of the CNS, although the CNS has a significant influence over it. d. The enteric nervous system is usually regarded as part of the autonomic nervous system, but opinions vary. e. It is composed of two networks of neurons: the submucosal (Meissner) plexus, and themyenteric (Auerbach) plexus of parasympathetic ganglia and nerve fibers between the layers ofthe muscularis externa. i. Parasympathetic preganglionic fibers terminate in the ganglia of the myentericplexus. ii. Postganglionic fibers arising in the plexus not only innervate the muscularis externa, but also pass through its inner layer to contribute to the submucosal plexus. iii. The myenteric plexus controls peristalsis and other contractions of the muscularis externa. iv. The submucosal plexus controls movements of the muscularis mucosae and glandularsecretion of the mucosa. f. The enteric nervous system also includes sensory neurons. D. The peritoneum is a serous membrane that lines the wall of the abdominal cavity and gives rise to connective tissue sheets called mesenteries. (pp. 947-948)
Describe the mechanisms by which the following regulate digestive processes: GI hormones, i.e., gastrin, secretin, cholecystokinin, GIP, somatostatin (more are explained with the organ)
The gastric phase is a period in which swallowed food and semidigested protein activate gastric activity.. Gastrin is a hormone produced by enteroendocrine G cells in the pyloric glands.As dietary protein is digested, it breaks down into smaller peptides and amino acids, which directly stimulate G cells to secrete more gastrin—a positive feedback loop. (Fig. 25.18)Amino acids in the chyme stimulate G cells of the duodenum to secrete more gastrin, which further stimulates the stomach. D. Secretion is regulated by responses to three types of stimuli that cause the release of pancreatic juiceand bile. (p. 970) 1. Acethylcholine (ACh) coming from the vagus and enteric nerves stimulates pancreatic acini to secrete enzymes even before food has been swallowed. a. The enzymes remain stored in the pancreatic acini and ducts, however, for release whenchyme enters the duodenum. 2. Cholecystokinin (CCK) is secreted by the mucosa of the duodenum and proximal jejunum in responseto fats in the small intestine. a. It stimulates the pancreatic acini to secrete enzymes. b. It has a strong stimulatory effect on the gallbladder, from which it gets its name. It induces contractions of the gallbladder and relaxation of the hepatopancreatic sphincter. 3. Secretin is produced in the small intestine in response to the acidity of chyme from the stomach. a. It stimulates ducts of the liver and pancreas to secrete sodium bicarbonate to buffer the HCl and protect the intestinal mucosa from stomach acid. b. This action also raises the intestinal pH to the optimum level for pancreatic and intestinaldigestive enzymes. Chyme also stimulates duodenal enteroendocrine cells to release secretin and cholecystokinin (CCK), which stimulate thepancreas and gallbladder, but also suppress gastric secretion and motility. The enteroendocrine cells also secrete glucose-dependent insulinotropic peptide (GIP),originally called gastric-inhibitory peptide. i. This peptide now appears to be more concerned with stimulating insulin secretion.
Describe the role of segmentation and peristalsis in mixing of food with digestive juices, optimization of digestion and absorption, and propulsion of food and residue through and out of the digestive system.
The most common type of colonic motility is segmentation called haustral contractions, which occur about every 30 minutes. a. Distension of a haustrum with feces stimulates it to contract, which churns and mixes theresidue, promotes water and salt absorption, and passes the residue distally. The esophageal phase is a wave of involuntary contractions called peristalsis, controlled bythe swallowing centers and the myenteric plexus in the esophageal wall. i. The bolus stimulates a stretch receptor, which sends signals to the muscularisexterna above and below the bolus. ii. The circular muscle above the bolus constricts and pushes it downward. iii. Below the bolus, the circular muscle relaxes while the longitudinal muscle contracts, pulling the wall of the esophagus upward and making it wider. d. Most food passes by gravity when we are standing or sitting upright, but peristalsisensures that food can be swallowed regardless of body position.
Mouth, pharynx and esophagus describe the main functions of each.
The mouth is also known as the oral, or buccal, cavity. (pp. 949-952) 1. Its functions include ingestion, taste and sensory responses to food, chewing, chemical digestion, swallowing, speech, and respiration. 2. The mouth is enclosed by the cheeks, lips, palate, and tongue. (Fig. 25.4) 3. The cheeks and lips retain food and push it between the teeth for chewing. 4. The tongue, although muscular, is a remarkably agile and sensitive organ.5. The palate separates the oral cavity from the nasal cavity, making it possible to breathe whilechewing food. 6. The teeth are collectively called the dentition; they serve to masticate the food, breaking it into smaller pieces. The oral phase is under voluntary control. i. The tongue collects food, presses it against the palate to form a bolus, and pushes it posteriorly. ii. The food accumulates in the oropharynx in front of the blade of the epiglottis. iii. When the bolus reaches a critical size, the epiglottis tips posteriorly and the food bolus slides around it through a space on each side, into the laryngopharynx. b. The pharyngeal phase is involuntary. i. The soft palate and root of the tongue prevent food and drink from reentering themouth or entering the nasal cavity. ii. To prevent choking, breathing is automatically suspended, the infrahyoid muscles pull the larynx up to meet the epiglottis and cover the laryngeal opening, and the vocal cordsadduct to close the airway. iii. The food bolus is driven downward by constriction of the upper, then middle, andfinally inferior pharyngeal constrictors. c. The esophageal phase is a wave of involuntary contractions called peristalsis, controlled by the swallowing centers and the myenteric plexus in the esophageal wall. i. The bolus stimulates a stretch receptor, which sends signals to the muscularisexterna above and below the bolus. ii. The circular muscle above the bolus constricts and pushes it downward. iii. Below the bolus, the circular muscle relaxes while the longitudinal muscle contracts, pulling the wall of the esophagus upward and making it wider. d. Most food passes by gravity when we are standing or sitting upright, but peristalsisensures that food can be swallowed regardless of body position.
Describe the location, gross and histological structures, and general functions of accessory digestive organs: teeth, tongue, salivary glands, liver, gall bladder, pancreas, appendix.
The mouth is enclosed by the cheeks, lips, palate, and tongue. (Fig. 25.4) a. Its anterior opening is the oral fissure and its posterior opening is the fauces. b. It is lined with stratified squamous epithelium that is keratinized in areas subject to abrasion, such as gums and hard palate, and is nonkeratinized in areas such as the floor of the mouth, thesoft palate, and the inside of the cheeks and lips. The tongue, although muscular, is a remarkably agile and sensitive organ. a. Its surface is covered with nonkeratinized stratified squamous epithelium and has bumps and projects called lingual papillae, where the taste buds reside. (Fig. 25.5) Tongue b. The anterior two-thirds of the tongue, called the body, occupies the oral cavity, while theposterior one-third, the root, occupies the oropharynx. c. The boundary between body and root is marked by a V-shaped row of vallate papillae, and behind these, a groove called the terminal sulcus. d. The body is attached to the floor of the mouth by a median fold called the lingual frenulum. The teeth are collectively called the dentition; they serve to masticate the food, breaking it into smaller pieces.. Regions of the tooth are defined by their relationship to the gingiva. i. The crown is the portion above the gum. ii. The root is the portion below the gum, embedded in alveolar bone. iii. The neck is the point where the crown, root, and gum meet. iv. The space between the tooth and gum is the gingival sulcus, and hygiene of this sulcus is important to dental health. Saliva moistens the mouth, digests some starch and fat, cleanses the teeth, inhibits bacterial growth, dissolves molecules for sensing by taste receptors, and moistens and binds food particles. (pp. 953-954) 1. It is a hypotonic solution of 97.0% to 99.5% water, a pH of 6.8 to 7.0, and the following solutes: a. Mucus, which binds and lubricates the food and aids in swallowing. b. Electrolytes, including sodium, potassium, chloride, phosphate, and bicarbonate salts. c. Lysozyme, an enzyme that kills bacteria. d. Immunoglobulin A (IgA), an antibacterial antibody. e. Salivary amylase, an enzyme that begins starch digestion. f. Lingual lipase, an enzymethat begins fat digestion. The liver is a reddish brown gland located inferior to the diaphragm, filling most of the right hypochondriacand epigastric regions. (pp. 965-966) (Fig. 25.19) 1. It is the body's largest gland, weighing about 1.4 kg (3 lb). 2. It has many functions, of which only one, the secretion of bile, contributes to digestion. 3. In terms of gross anatomy, the liver has four lobes: the right, left, quadrate, and caudate lobes. a. Anteriorly, the right and left lobes are visible and are separated from each other by thefalciform ligament. b. The round ligament (ligamentum teres), also visible anteriorly, is a remnant of the umbilical vein. c. From the inferior view, the quadrate lobe is seen next to the gallbladder, and the tail-likecaudate lobe is posterior to that. i. An irregular opening between these lobes, the porta hepatis, is a point of entry for thehepatic portal vein and proper hepatic artery. ii. It is also the point of exit for the bile passages, all of which travel in the lesseromentum. The gallbladder is a pear-shaped sac on the underside of the liver that stores and concentrates bile. The pancreas is a spongy retroperitoneal gland posterior to the greater curvature of the stomach, about 12 to 15 cm long and 2.5 cm thick. (pp. 968-969) (Fig. 25.21) 1. It has a globose head encircled by the duodenum, a midportion called the body, and a blunt, taperedtail on the left. 2. It is both an endocrine and exocrine gland; the endocrine part involves the pancreatic islets, which secrete insulin and glucagon. Attached to the lower end of the cecum is the appendix, a blind tube 2 to 7 cm long; it is a significant source of immunecells.
Also, include the location where digestion of each macromolecule category begins, where it may pause, and where it resume
The opening into the stomach is called the cardiac orifice (because of its proximity to the heart). a. Food pauses briefly at this point because of the lower esophageal sphincter (LES). b. The LES prevents stomach contents from regurgitating into the esophagus. c. "Heartburn" has nothing to do with the heart, but is the burning sensation caused by acidreflux.
Describe the process of defecation and include explanations of the roles of the myoenteric defecation reflex and the parasympathetic mediate defecation reflex. Explain the differences in the muscle composition and control of the internal and external anal sphincters.
The parasympathetic defecation reflex is a spinal reflex. (Fig. 25.33) a. Stretch signals are transmitted to the spinal cord. b. Motor signals return by way of the pelvic nerves to intensify peristalsis in the descending and sigmoid colon and rectum and to relax the internal anal sphincter. 3. These reflexes are involuntary and are the sole means of controlling defecation in infants andpeople with transecting spinal cord injuries. 4. The external anal sphincter, like the external urethral sphincter, is under voluntary control; defecation normally occurs only when the external anal sphincter and puborectalis muscle are voluntarily relaxed.
Describe parasympathetic and sympathetic regulation of salivation.
The salivatory nuclei send signals to the glands by way of autonomic fibers in the facialand glossopharyngeal nerves. i. Parasympathetic fibers stimulate the glands to produce abundant, thin saliva rich inenzymes. ii. Sympathetic stimulation briefly enhances salivation, but its primary effect is to produce thicker saliva with more mucus. Under conditions of stress the mouth may feel sticky or dry.
Briefly describe which nutrients and drugs are absorbed by the stomach.
The stomach does not absorb significant nutrients but does absorb aspirin and some lipid-solubledrugs.
List the major functions of the digestive system.
The study of digestive tract and its disorders = gastroenterology Its function is accomplished in five stages. a. Ingestion is the selective intake of food. b. Digestion is the mechanical and chemical breakdown of food into usable form. c. Absorption is the uptake of nutrient molecules into the epithelial cells of the digestive tract and then into the blood or lymph. d. Compaction is the absorption of water and consolidation of indigestible residue into feces. e. Defecation is the elimination of feces. 2. The digestive phase has two facets, mechanical and chemical. a. Mechanical digestion is the physical breakdown of food into smaller particles. i. It is achieved by the action of the teeth and the churning contractions of the stomachand small intestine. ii. It exposes more food surface to the action of digestive enzymes. b. Chemical digestion is a series of hydrolysis reactions that break macromolecules into monomers (residues). i. These monomers are monosaccharides, amino acids, monoglycerides and fattyacids, and nucleic acids. ii. Chemical digestion is carried out by digestive enzymes of the salivary glands, stomach, pancreas, and small intestine. 3. Some nutrients are already present in usable form without digestion: vitamins, free amino acids, minerals, cholesterol, and water.
Explain why the body cannot digest minerals, vitamins, cholesterol, single amino acids, cholesterol, and water, and describe the transport mechanisms for sodium, calcium, iron, water-soluble vitamins, fat-soluble vitamins, and vitamin B-12.
Unlike other food components, such as fats, proteins or carbohydrates — which your body breaks down and absorbs — fiber isn't digested by your body. Instead, it passes relatively intact through your stomach, small intestine and colon and out of your body.
Explain the process by which trypsinogen is converted to trypsin by enteropeptidase (a.k.a. enterokinase) and subsequently converts additional trypsinogen to trypsin, begins to digests proteins and converts chymotrypsinogen and procarboxypeptidase to their active forms of protein enzymes.
When secreted into the intestinal lumen, trypsinogen is converted to trypsin by enterokinase, which is secreted by the intestinal mucosa. (Fig. 25.23) b. Trypsin is autocatalytic and converts trypsinogen into more trypsin. c. Trypsin also converts the two other zymogens into chymotrypsin and carboxypeptidase. d. Trypsin's primary role is the digestion of dietary protein.
Discuss the pathology of pernicious anemia observed due to lack of intrinsic factor. Also discussed in Ch 12.
Without vitamin B12, hemoglobin cannot be synthesized and pernicious anemia develops.
Describe the digestive processes of ingestion, digestion, absorption, compaction, and defecation and the fundamental significance of each process to the efficient transfer of nutrients from food into the body with minimal loss of water from the body.
a. Ingestion is the selective intake of food. b. Digestion is the mechanical and chemical breakdown of food into usable form. c. Absorption is the uptake of nutrient molecules into the epithelial cells of the digestive tract and then into the blood or lymph. d. Compaction is the absorption of water and consolidation of indigestible residue into feces. e. Defecation is the elimination of feces.
Large intestine: Describe its main functions.
reabsorption of water and mineral ions such as sodium and chlorideformation and temporary storage of faecesmaintaining a resident population of over 500 species of bacteria (Singular: bacterium) Single-celled microorganisms that have no nucleus. bacterial fermentation of indigestible materials.
Describe how the anatomical organization of vascular and lymphatic capillaries in the villus along with the increased surface area enhances the efficiency of digestion of macromolecules and subsequent absorption of monomers (nutrients) via capillaries and lymphatic due to close proximity to the epithelial tissue.
v. The core of a villus is filled with areolar tissue of the lamina propria and contains anarteriole, blood capillaries, a venule, and a lymphatic capillary called a lacteal. v. Blood capillaries absorb most nutrients, but the lacteal absorbs most lipids, giving its contents a milky appearance. vi. The core of the villus also contains a few smooth muscle cells that contract periodically to mix chyme and to move lymph down the lacteal.