Gastrointestinal Physiology Flashcards
Which of the following substances is released from neurons in the GI tract and produces smooth muscle relaxation?
(a) Secretin
(b) Gastrin
(c) Cholecystokinin (CCK)
(d) Vasoactive intestinal peptide (VIP)
(e) Gastric inhibitory peptide (GIP)
The answer is D [II C 1].
Vasoactive intestinal peptide (VIP) is a gastrointestinal (GI) neurocrine that causes relaxation of GI smooth muscle. For example, VIP mediates the relaxation response of the lower esophageal sphincter when a bolus of food approaches it, allowing passage of the bolus into the stomach.
Which of the following is the site of secretion of intrinsic factor?
(a) Gastric antrum
(b) Gastric fundus
(c) Duodenum
(d) Ileum
(e) Colon
The answer is B [IV B 1; Table 6.3; Figure 6.7].
Intrinsic factor is secreted by the parietal cells of the gastric fundus (as is HCl). It is absorbed, with vitamin B12, in the ileum.
Vibrio cholerae causes diarrhea because it
(a) increases HCO3 − secretory channels in intestinal epithelial cells
(b) increases Cl− secretory channels in crypt cells
(c) prevents the absorption of glucose and causes water to be retained in the intestinal lumen isosmotically
(d) inhibits cyclic adenosine monophosphate (cAMP) production in intestinal epithelial cells
(e) inhibits inositol 1,4,5-triphosphate (IP3) production in intestinal epithelial cells
The answer is B [V D 4 c].
Cholera toxin activates adenylate cyclase and increases cyclic adenosine monophosphate (cAMP) in the intestinal crypt cells. In the crypt cells, cAMP activates the Cl−-secretory channels and produces a primary secretion of Cl− with Na+ and H2O following.
Cholecystokinin (CCK) has some gastrin-like properties because both CCK and gastrin
(a) are released from G cells in the stomach
(b) are released from I cells in the duodenum
(c) are members of the secretin-homologous family
(d) have five identical C-terminal amino acids
(e) have 90% homology of their amino acids
The answer is D [II A 2].
The two hormones have five identical amino acids at the C terminus. Biologic activity of cholecystokinin (CCK) is associated with the seven C-terminal amino acids, and biologic activity of gastrin is associated with the four C-terminal amino acids. Because this CCK heptapeptide contains the five common amino acids, it is logical that CCK should have some gastrin-like properties. G cells secrete gastrin. I cells secrete CCK. The secretin family includes glucagon.
Which of the following is transported in intestinal epithelial cells by a Na+-dependent cotransport process?
(a) Fatty acids
(b) Triglycerides
(c) Fructose
(d) Alanine
(e) Oligopeptides
The answer is D [V A-C; Table 6.4].
Fructose is the only monosaccharide that is not absorbed by Na+-dependent cotransport; it is transported by facilitated diffusion. Amino acids are absorbed by Na+-dependent cotransport, but oligopeptides (larger peptide units) are not. Triglycerides are not absorbed without further digestion. The products of lipid digestion, such as fatty acids, are absorbed by simple diffusion.
A 49-year-old male patient with severe Crohn disease has been unresponsive to drug therapy and undergoes ileal resection. After the surgery, he will have steatorrhea because
(a) the liver bile acid pool increases
(b) chylomicrons do not form in the intestinal lumen
(c) micelles do not form in the intestinal lumen
(d) dietary triglycerides cannot be digested
(e) the pancreas does not secrete lipase
Theanswer is C [IV D 4].
Ileal resection removes the portion of the small intestine that normally transports bile acids from the lumen of the gut and recirculates them to the liver. Because this process maintains the bile acid pool, new synthesis of bile acids is needed only to replace those bile acids that are lost in the feces. With ileal resection, most of the bile acids secreted are excreted in the feces, and the liver pool is significantly diminished. Bile acids are needed for micelle formation in the intestinal lumen to solubilize the products of lipid digestion so that they can be absorbed. Chylomicrons are formed within the intestinal epithelial cells and are transported to lymph vessels.
Cholecystokinin (CCK) inhibits
(a) gastric emptying
(b) pancreatic HCO3 − secretion
(c) pancreatic enzyme secretion
(d) contraction of the gallbladder
(e) relaxation of the sphincter of Oddi
The answer is A [II A 2 a; Table 6.1].
Cholecystokinin (CCK) inhibits gastric emptying and therefore helps to slow the delivery of food from the stomach to the intestine during periods of high digestive activity. CCK stimulates both functions of the exocrine pancreas— HCO3 − secretion and digestive enzyme secretion. It also stimulates the delivery of bile from the gallbladder to the small intestinal lumen by causing contraction of the gallbladder while relaxing the sphincter of Oddi.
Which of the following abolishes “receptive relaxation” of the stomach?
(a) Parasympathetic stimulation
(b) Sympathetic stimulation
(c) Vagotomy
(d) Administration of gastrin
(e) Administration of vasoactive intestinal peptide (VIP)
(f) Administration of cholecystokinin (CCK)
The answer is C [III C 1].
“Receptive relaxation” of the orad region of the stomach is initiated when food enters the stomach from the esophagus. This parasympathetic (vagovagal) reflex is abolished by vagotomy.
Secretion of which of the following substances is inhibited by low pH?
(a) Secretin
(b) Gastrin
(c) Cholecystokinin (CCK)
(d) Vasoactive intestinal peptide (VIP)
(e) Gastric inhibitory peptide (GIP)
The answer is B [II A 1; Table 6.1].
Gastrin’s principal physiologic action is to increase H+ secretion. H+ secretion decreases the pH of the stomach contents. The decreased pH, in turn, inhibits further secretion of gastrin—a classic example of negative feedback.
Which of the following is the site of secretion of gastrin?
(a) Gastric antrum
(b) Gastric fundus
(c) Duodenum
(d) Ileum
(e) Colon
The answer is A [II A 1 b; Table 6.3; Figure 6.7].
Gastrin is secreted by the G cells of the gastric antrum. HCl and intrinsic factor are secreted by the fundus.
Micelle formation is necessary for the intestinal absorption of
(a) glycerol
(b) galactose
(c) leucine
(d) bile acids
(e) vitamin B12
(f) vitamin D
The answer is F [V E 1; Table 6.4].
Micelles provide a mechanism for solubilizing fat-soluble nutrients in the aqueous solution of the intestinal lumen until the nutrients can be brought into contact with and absorbed by the intestinal epithelial cells. Because vitamin D is fat soluble, it is absorbed in the same way as other dietary lipids. Glycerol is one product of lipid digestion that is water soluble and is not included in micelles. Galactose and leucine are absorbed by Na+-dependent cotransport. Although bile acids are a key ingredient of micelles, they are absorbed by a specific Na+-dependent cotransporter in the ileum. Vitamin B12 is water soluble; thus, its absorption does not require micelles.
Which of the following changes occurs during defecation?
(a) Internal anal sphincter is relaxed
(b) External anal sphincter is contracted
(c) Rectal smooth muscle is relaxed
(d) Intra-abdominal pressure is lower than when at rest
(e) Segmentation contractions predominate
The answer is A [III E 3].
Both the internal and external anal sphincters must be relaxed to allow feces to be expelled from the body. Rectal smooth muscle contracts and intra-abdominal pressure is elevated by expiring against a closed glottis (Valsalva maneuver). Segmentation contractions are prominent in the small intestine during digestion and absorption.
Which of the following is characteristic of saliva?
(a) Hypotonicity relative to plasma
(b) A lower HCO3 − concentration than plasma
(c) The presence of proteases
(d) Secretion rate that is increased by vagotomy
(e) Modification by the salivary ductal cells involves reabsorption of K+ and HCO3 −
The answer is A [IV A 2 a; Table 6.2].
Saliva is characterized by hypotonicity and a high HCO3 − concentration (relative to plasma) and by the presence of α-amylase and lingual lipase (not proteases). The high HCO3 − concentration is achieved by secretion of HCO3 − into saliva by the ductal cells (not reabsorption of HCO3 −). Because control of saliva production is parasympathetic, it is abolished by vagotomy.
Which of the following substances is secreted in response to an oral glucose load?
(a) Secretin
(b) Gastrin
(c) Cholecystokinin (CCK)
(d) Vasoactive intestinal peptide (VIP)
(e) Glucose-dependent insulinotropic peptide (GIP)
The answer is E [II A 4; Table 6.4].
Glucose-dependent insulinotropic peptide (GIP) is the only gastrointestinal (GI) hormone that is released in response to all three categories of nutrients—fat, protein, and carbohydrate. Oral glucose releases GIP , which, in turn, causes the release of insulin from the endocrine pancreas. This action of GIP explains why oral glucose is more effective than intravenous glucose in releasing insulin.
Which of the following is true about the secretion from the exocrine pancreas?
(a) It has a higher Cl− concentration than does plasma
(b) It is stimulated by the presence of HCO3 − in the duodenum
(c) Pancreatic HCO3 − secretion is increased by gastrin
(d) Pancreatic enzyme secretion is increased by cholecystokinin (CCK)
(e) It is hypotonic
The answer is D [II A 2, 3; Table 6.2].
The major anion in pancreatic secretions is HCO3 − (which is found in higher concentration than in plasma), and the Cl− concentration is lower than in plasma. Pancreatic secretion is stimulated by the presence of fatty acids in the duodenum. Secretin (not gastrin) stimulates pancreatic HCO3 − secretion, and cholecystokinin (CCK) stimulates pancreatic enzyme secretion. Pancreatic secretions are always isotonic, regardless of flow rate.
