The functions of the gastrointestinal tract are digestion and absorption of nutrients. To serve these functions, there are four major activities of the gastrointestinal tract. (1) Motility propels ingested food from the mouth toward the rectum and mixes and reduces the size of the food. The rate at which food is propelled through the gastrointestinal tract is regulated to optimize the time for digestion and absorption. (2) Secretions from the salivary glands, pancreas, and liver add fluid, electrolytes, enzymes, and mucus to the lumen of the gastrointestinal tract. These secretions further aid in digestion and absorption. (3) Ingested foods are digested into absorbable molecules. (4) Nutrients, electrolytes, and water are absorbed from the intestinal lumen into the bloodstream.
The gastrointestinal tract is innervated by both the parasympathetic and sympathetic nervous systems, which converge on the intrinsic nervous system in the myenteric and submucosal plexuses.
Gastrointestinal peptides are secreted by cells of the gastrointestinal tract and include the hormones gastrin, CCK, secretin, and GIP, which are released into the circulation; the paracrines somatostatin and histamine, which act locally; and neurocrines, which are released from nerves.
Slow waves in gastrointestinal smooth muscle cells are spontaneous depolarizations and repolarizations of the membrane potential. Action potentials are fired if the membrane potential reaches threshold as a result of a slow wave. Thus, the frequency of slow waves determines the frequency of action potentials and, consequently, the frequency of contractions.
Gastric motility includes mixing and grinding of ingested food. Small intestinal motility includes segmentation contractions, which mix chyme with digestive enzymes, and peristaltic contractions, which move the chyme in the caudad direction. In the large intestine, mass movements push the fecal material over long distances and eventually into the rectum, where it is stored until defecation occurs.
Salivary secretion is utilized for buffering and dilution of foods and for initial digestion of starch and lipids. Saliva is hypotonic and is produced by a two-step process involving formation of an initial saliva by acinar cells and its modification by ductal epithelial cells.
Pancreatic secretion contains HCO3− for the neutralization of H+ from the stomach and enzymes for digestion of carbohydrates, proteins, and lipids. Pancreatic juice is isotonic and is produced by a two-step process. The acinar cells secrete the enzymatic component, centroacinar and ductal epithelial cells secrete the aqueous HCO3−-containing component, and ductal cells modify the secretion.
Bile salts, the major constituents of bile, are used for emulsification and solubilization of lipids, aiding in their digestion and absorption. Bile is produced by hepatocytes, stored in the gallbladder, and secreted into the intestine when the gallbladder contracts. Bile salts solubilize and form micelles with the products of lipid digestion. Approximately 95% of bile acids are recirculated to the liver via the enterohepatic circulation.
Carbohydrates must be digested to monosaccharides for absorption. The digestive steps are carried out by salivary and pancreatic amylases and disaccharidases in the intestinal brush border. Glucose and galactose are absorbed by intestinal epithelial cells by Na+-dependent cotransporters, and fructose is absorbed by facilitated diffusion.
Proteins are digested to amino acids, dipeptides, and tripeptides for absorption. The digestive steps are carried out by pepsin, trypsin, and other pancreatic and brush-border proteases. Amino acids, dipeptides, and tripeptides are absorbed by intestinal epithelial cells by Na+- or H+-dependent cotransporters.
Lipids are digested to monoglycerides, fatty acids, cholesterol, and lysolecithin by pancreatic enzymes. The products of lipid digestion are solubilized in micelles with bile acids. At the apical membrane of intestinal epithelial cells, the lipids are released from the micelles and diffuse into the cells. Within the cells, they are packaged in chylomicrons and transferred into lymph vessels by exocytosis.
Approximately 9 L of fluid is absorbed daily by the gastrointestinal tract. The volume of fluid absorbed is approximately equal to the sum of the volume ingested and the volume secreted in salivary, gastric, pancreatic, and intestinal juices. Diarrhea results if absorption is decreased or if secretion is increased.
The liver conjugates bilirubin, a metabolite of hemoglobin, with glucuronic acid to form conjugated bilirubin, which is excreted into urine and bile. In the intestine, conjugated bilirubin is converted to urobilinogen, which recirculates to the liver, and to urobilin and stercobilin, which are excreted in the stool.
Answer each question with a word, phrase, sentence, or numerical solution. When a list of possible answers is supplied with the question, one, more than one, or none of the choices may be correct. Correct answers are provided at the end of the book.
1 Which of the following is/are (an) action(s) of cholecystokinin (CCK): contraction of the gallbladder, acceleration of gastric emptying, stimulation of HCO3− secretion, stimulation of pancreatic enzyme secretion?
2 A patient with a duodenal ulcer is treated with cimetidine, a drug that inhibits H+ secretion in parietal cells. Which of the following is the mechanism of cimetidine’s action: inhibition of H+-K+ATPase, inhibition of muscarinic receptors, stimulation of muscarinic receptors, decreased intracellular cyclic AMP (cAMP) levels, inhibition of somatostatin?
3 During the rising phase (upstroke) of a slow wave, which change in membrane potential is occurring: more negative, less negative, more positive, less positive?
4 Which phenomenon in salivary ducts explains why the final salivary secretion is hypotonic relative to the primary secretion of the acinar cells: secretion of water, absorption of water, absorption of more solute than water, secretion of more solute than water?
5 Cholera toxin has which of the following direct or indirect actions: opens Na+ channels, closes Cl− channels, increases cAMP levels, activates αs subunit of GTP-binding protein, increases GTPase activity?
6 Which of the following substances must be digested before being absorbed by the small intestine: Ca2+, alanine, fructose, sucrose, cholesterol?
7 What is the correct sequence of events in lipid absorption: formation of cholesterol ester, action of pancreatic lipase, emulsification of lipids in the intestinal lumen, micelles, chylomicrons?
8 As pancreatic flow rate increases, which of the following has/have increased concentration in pancreatic juice: Na+, K+, HCO3−, Cl−, osmolarity?
9 Which reaction(s) is/are catalyzed by trypsin: pepsinogen to pepsin, trypsinogen to trypsin, procarboxypeptidase to carboxypeptidase?
10 Where is the frequency of slow waves the highest: stomach, duodenum, ileum?
11 Where am I? For each item in the following list, give its correct location in the gastrointestinal system. The location may be anatomic, a graph or portion of a graph, or a concept.
Na+-bile salt cotransport
Intrinsic factor secretion
Secondary bile acids (or bile salts)
12 A patient with H. pylori infection develops a gastric ulcer and is treated with omeprazole. Which of the following is/are mechanism(s) of action of omeprazole? Inhibition of ACh action on parietal cells, stimulation of somatostatin action on parietal cells, inhibition of CCKB receptors, inhibition of H+-K+ ATPase, inhibition of Na+-K+ ATPase.
13 Which of the following suppresses appetite? Increased body fat, increased insulin levels, increased ghrelin levels.
14 In the peristaltic reflex, which of the following occurs orad to (behind) the food bolus? Release of 5-hydroxytryptamine (5-HT) from IPAN neurons, contraction of circular muscle, contraction of longitudinal muscle, action of acetylcholine on circular muscle, action of vasoactive intestinal peptide (VIP) on circular muscle.
Johnson LR: Physiology of the Gastrointestinal Tract, 2nd ed. New York, Raven Press, 1987.
Johnson LR: Gastrointestinal Physiology, 6th ed. St Louis, Mosby, 2001.
Schultz SG, Wood JD, Raunder BB: Handbook of Physiology: The Gastrointestinal System. Bethesda, Md, American Physiological Society, 1989.