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PHYSIOLOGY RENAL QUESTIONS AND ANSWERS
- Secretion of K+ by the distal tubule will be decreased by (A) metabolic alkalosis (B) a high-K+ diet (C) hyperaldosteronism (D) spironolactone administration (E) thiazide diuretic administration ✔✔The answer is D Distal K+secretion is decreased by factors that decrease the driving force for passive diffusion of K+ across the luminal membrane. Because spironolactone is an aldosterone antagonist, it reduces K+ secretion. Alkalosis, a diet high in K+, and hyperaldosteronism all increase [K+] in the distal cells and thereby increase K+ secretion. Thiazide diuretics increase flow through the distal tubule and dilute the luminal [K+] so that the driving force for K+ secretion is increased.
- Jared and Adam both weigh 70 kg. Jared drinks 2 L of distilled water, and Adam drinks 2 L of isotonic NaCl. As a result of these ingestions, Adam will have a (A) greater change in intracellular fluid (ICF) volume (B) higher positive free-water clearance CH O2 ( ) (C) greater change in plasma osmolarity (D) higher urine osmolarity (E) higher urine flow rate ✔✔The answer is D. After drinking distilled water, Jared will have an increase in intracellular fluid (ICF) and extracellular fluid (ECF) volumes, a decrease in plasma osmolarity, a suppression of antidiuretic hormone (ADH) secretion, and a positive free-water clearance (CH O2 ), and will produce dilute urine with a high flow rate. Adam, after drinking the same volume of isotonic NaCl, will have an increase in ECF volume only and no change in plasma osmolarity. Because Adam's ADH will not be suppressed, he will have a higher urine osmolarity, a lower urine flow rate, and a lower CH O2 than Jared. Use the values below to answer the following question: Glomerular capillary hydrostatic pressure = 47 mm Hg
Bowman space hydrostatic pressure = 10 mm Hg Bowman space oncotic pressure = 0 mm Hg At what value of glomerular capillary oncotic pressure would glomerular filtration stop? (A) 57 mm Hg (B) 47 mm Hg (C) 37 mm Hg (D) 10 mm Hg (E) 0 mm Hg ✔✔The answer is C Glomerular filtration will stop when the net ultrafiltration pressure across the glomerular capillary is zero; that is, when the force that favors filtration (47 mm Hg) exactly equals the forces that oppose filtration (10 mm Hg + 37 mm Hg). The reabsorption of filtered HCO3 - (A) results in reabsorption of less than 50% of the filtered load when the plasma concentration of HCO3- is 24 mEq/L (B) acidifies tubular fluid to a pH of 4. (C) is directly linked to excretion of H+ as NH4+ (D) is inhibited by decreases in arterial Pco (E) can proceed normally in the presence of a renal carbonic anhydrase inhibitor ✔✔The answer is D Decreases in arterial Pco2 cause a decrease in the reabsorption of filtered HCO3- by diminishing the supply of H+ in the cell for secretion into the lumen. Reabsorption of filtered HCO3- is nearly 100% of the filtered load and requires carbonic anhydrase in the brush border to convert filtered HCO3- to CO2 to proceed normally. This process causes little acidification of the urine and is not linked to net excretion of H+ as titratable acid or NH4+ The following information was obtained in a 20-year-old college student who was participating in a research study in the Clinical Research Unit: Plasma Urine [Inulin] = 1 mg/mL [Inulin] = 150 mg/mL [X] = 2 mg/mL [X] = 100 mg/mL Urine flow rate = 1 mL/min Assuming that X is freely filtered, which of the following statements is most correct?
(Tm), the (A) clearance of glucose is zero (B) excretion rate of glucose equals the filtration rate of glucose (C) reabsorption rate of glucose equals the filtration rate of glucose (D) excretion rate of glucose increases with increasing plasma glucose concentrations (E) renal vein glucose concentration equals the renal artery glucose concentration ✔✔The answer is D At concentrations greater than at the transport maximum (Tm) for glucose, the carriers are saturated so that the reabsorption rate no longer matches the filtration rate. The difference is excreted in the urine. As the plasma glucose concentration increases, the excretion of glucose increases. When it is greater than the Tm, the renal vein glucose concentration will be less than the renal artery concentration because some glucose is being excreted in urine and therefore is not returned to the blood. The clearance of glucose is zero at concentrations lower than at Tm (or lower than threshold) when all of the filtered glucose is reabsorbed but is greater than zero at concentrations greater than Tm. Which of the following would produce an increase in the reabsorption of isosmotic fluid in the proximal tubule? (A) Increased filtration fraction (B) Extracellular fluid (ECF) volume expansion (C) Decreased peritubular capillary protein concentration (D) Increased peritubular capillary hydrostatic pressure (E) Oxygen deprivation ✔✔The answer is A Increasing filtration fraction means that a larger portion of the renal plasma flow (RPF) is filtered across the glomerular capillaries. This increased flow causes an increase in the protein concentration and oncotic pressure of the blood leaving the glomerular capillaries. This blood becomes the peritubular capillary blood supply. The increased oncotic pressure in the peritubular capillary blood is a driving force favoring reabsorption in the proximal tubule. Extracellular fluid (ECF) volume expansion, decreased peritubular capillary protein concentration, and increased peritubular capillary hydrostatic pressure all inhibit proximal reabsorption. Oxygen deprivation would also inhibit reabsorption by stopping the Na+ - K+ pump in the basolateral membranes. Compared with a person who ingests 2 L of distilled water, a person with water deprivation will have a (A) higher free-water clearance CH O2 ( ) (B) lower plasma osmolarity (C) lower circulating level of antidiuretic hormone (ADH) (D) higher tubular fluid/plasma (TF/P) osmolarity in the proximal tubule (E) higher rate of H2O reabsorption in the collecting ducts ✔✔The answer is E.
The person with water deprivation will have a higher plasma osmolarity and higher circulating levels of antidiuretic hormone (ADH). These effects will increase the rate of H2O reabsorption in the collecting ducts and create a negative free-water clearance (-CH2O). Tubular fluid/plasma (TF/P) osmolarity in the proximal tubule is not affected by ADH.
- Which of the following would cause an increase in both glomerular filtration rate (GFR) and renal plasma flow (RPF)? (A) Hyperproteinemia (B) A ureteral stone (C) Dilation of the afferent arteriole (D) Dilation of the efferent arteriole (E) Constriction of the efferent arteriole ✔✔The answer is C Dilation of the afferent arteriole will increase both renal plasma flow (RPF) (because renal vascular resistance is decreased) and glomerular filtration rate (GFR) (because glomerular capillary hydrostatic pressure is increased). Dilation of the efferent arteriole will increase RPF but decrease GFR. Constriction of the efferent arteriole will decrease RPF (due to increased renal vascular resistance) and increase GFR. Both hyperproteinemia (↑ π in the glomerular capillaries) and a ureteral stone (↑ hydrostatic pressure in Bowman space) will oppose filtration and decrease GFR. Which of the following would best distinguish an otherwise healthy person with severe water deprivation from a person with the syndrome of inappropriate antidiuretic hormone (SIADH)? (A) Free-water clearance CH O2 ( ) (B) Urine osmolarity (C) Plasma osmolarity (D) Circulating levels of antidiuretic hormone (ADH) (E) Corticopapillary osmotic gradient ✔✔The answer is C Both individuals will have hyperosmotic urine, a negative free-water clearance (−CH O2 ), a normal corticopapillary gradient, and high circulating levels of antidiuretic hormone (ADH). The person with water deprivation will have a high plasma osmolarity, and the person with syndrome of inappropriate antidiuretic hormone (SIADH) will have a low plasma osmolarity (because of dilution by the inappropriate water reabsorption). A woman has a plasma osmolarity of 300 mOsm/L and a urine osmolarity of 1200 mOsm/L. The correct diagnosis is (A) syndrome of inappropriate antidiuretic hormone (SIADH) (B) water deprivation
Which of the following causes hyperkalemia? (A) Exercise (B) Alkalosis (C) Insulin injection (D) Decreased serum osmolarity (E) Treatment with β-agonists ✔✔The answer is A Exercise causes a shift of K+ from cells into blood. The result is hyperkalemia. Hyposmolarity, insulin, β-agonists, and alkalosis cause a shift of K+ from blood into cells. The result is hypokalemia. Which of the following is a cause of metabolic alkalosis? (A) Diarrhea (B) Chronic renal failure (C) Ethylene glycol ingestion (D) Treatment with acetazolamide (E) Hyperaldosteronism (F) Salicylate poisoning ✔✔The answer is E A cause of metabolic alkalosis is hyperaldosteronism; increased aldosterone levels cause increased H+ secretion by the distal tubule and increased reabsorption of "new" HCO3-. Diarrhea causes loss of HCO3- from the gastrointestinal (GI) tract and acetazolamide causes loss of HCO3- in the urine, both resulting in hyperchloremic metabolic acidosis with normal anion gap. Ingestion of ethylene glycol and salicylate poisoning leads to metabolic acidosis with increased anion gap. Which of the following is an action of parathyroid hormone (PTH) on the renal tubule? (A) Stimulation of adenylate cyclase (B) Inhibition of distal tubule K+ secretion (C) Inhibition of distal tubule Ca2+ reabsorption (D) Stimulation of proximal tubule phosphate reabsorption (E) Inhibition of production of 1,25-dihydroxycholecalciferol ✔✔The answer is A Parathyroid hormone (PTH) acts on the renal tubule by stimulating adenyl cyclase and generating cyclic adenosine monophosphate (cAMP). The major actions of the hormone are inhibition of phosphate reabsorption in the proximal tubule, stimulation of Ca2+ reabsorption in the distal tubule, and stimulation of 1,25-dihydroxycholecalciferol production. PTH does not alter the renal handling of K+. At which nephron site is the tubular fluid/plasma (TF/P) osmolarity lowest in a person who has been deprived of water?
(A) Site A (B) Site B (C) Site C (D) Site D (E) Site E (see diagram) ✔✔The answer is D. A person who is deprived of water will have high circulating levels of antidiuretic hormone (ADH). The tubular fluid/plasma (TF/P) osmolarity is 1. throughout the proximal tubule, regardless of ADH status. In antidiuresis, TF/P osmolarity is greater than 1.0 at site C because of equilibration of the tubular fluid with the large corticopapillary osmotic gradient. At site E, TF/P osmolarity is greater than 1.0 because of water reabsorption out of the collecting ducts and equilibration with the corticopapillary gradient. At site D, the tubular fluid is diluted because NaCl is reabsorbed in the thick ascending limb without water, making TF/P osmolarity less than 1.0. At which nephron site is the tubular fluid inulin concentration highest during antidiuresis? (A) Site A (B) Site B (C) Site C (D) Site D (E) Site E (See diagram) ✔✔The answer is E Because inulin, once filtered, is neither reabsorbed nor secreted, its concentration in tubular fluid reflects the amount of water remaining in the tubule. In antidiuresis, water is reabsorbed throughout the nephron (except in the thick ascending limb and cortical diluting segment). Thus, inulin concentration in the tubular fluid progressively rises along the nephron as water is reabsorbed, and will be highest in the final urine. At which nephron site is the tubular fluid inulin concentration lowest? (A) Site A (B) Site B (C) Site C (D) Site D (E) Site E (see diagram) ✔✔The answer is A
c. Inability of the distal nephron to secrete hydrogen d. Inability of the distal nephron to secrete pot ✔✔The answer is b. The patient is treated with amiloride, a potassium sparing diuretic, which blocks sodium channels in the principal cells of the cortical collecting ducts thus limiting sodium reabsorption. Sodium reabsorption in the cortical collecting ducts is normally under the control of aldosterone. In patients with Liddle syndrome, the cortical collecting ducts reabsorb excess Na+ despite low levels of aldosterone and renin in the plasma, because of a mutation in the genes for the renal ENaCs, which increases ENaC activity and sodium retention. Metabolic alkalosis, hypokalemia, and hypertension are also present secondary to the increased sodium (and water) reabsorption. An inability of the distal nephron to secrete hydrogen (choice d) would cause RTA type I. An inability to concentrate urine (choice e) occurs when patients are treated with loop diuretics like furosemide, which prevents the kidney from developing medullary hypertonicity, thus limiting the reabsorption of water and the production of concentrated urine. The amount of sodium reabsorbed in the proximal tubules is relatively constant (choice a) at roughly 60% of the filtered amount, primarily as a result of Na+/H+ exchange. The inability of the distal nephron to secrete potassium ion (choice c) would result in hyperkalemia, not hypokalemia as described in the case presentation. A 69-year-old man presents with symptoms of thirst and dizziness, and physical evidence of orthostatic hypotension and tachycardia, decreased skin turgor, dry mucous membranes, reduced axillary sweating, and reduced jugular venous pressure. He was recently placed on an angiotensin-converting enzyme (ACE) inhibitor for his hypertension. Urinalysis reveals a reduction in the fractional excretion of sodium and the presence of acellular hyaline casts. The internist suspects acute renal failure of prerenal origin associated with increased renin secretion by the kidney. A stimulus for increasing renal renin secretion is an increase in which of the following? a. Angiotensin II b. Atrial natriuretic peptide (ANP) c. GFR d. Mean blood pressure e. Sympathetic nerve activity ✔✔The answer is e. (Barrett, pp 644, 705-706. Le, p 485. Widmaier, pp 497 - 499.) Renin secretion is stimulated by the sympathetic nerves innervating the juxtaglomerular apparatus. Sympathetic nerve activity increases when baroreceptors sense low blood pressure. Increasing mean blood pressure (choice d) decreases sympathetic activity, thereby decreasing renin secretion. Angiotensin II (choice a) decreases renin release through a negative feedback loop by binding to AT1 receptors on the juxtaglomerular cells to increase intracellular Ca2+ concentration, which inhibits renin secretion. ANP (choice b) also decreases renin release. Increases in GFR (choice c) sensed by the macula densa lead to the secretion of a mediator, perhaps adenosine or ATP, which contracts the afferent arteriole (tubuloglomerular feedback) and decreases renin release. Decreases in GFR lead to an increase in renin release.
A patient with uncontrolled hypertension is placed on a new diuretic targeted to act on the Na+ reabsorption site from the basolateral surface of the renal epithelial cells. Which of the following transport processes is the new drug affecting? a. Facilitated diffusion b. Na+/H+ exchange c. Na+-glucose cotransport d. Na+ - K+ pump e. Solvent drag ✔✔The answer is d. Na+ is pumped out of renal epithelial cells by the Na+ - K +pump located on the basolateral surface of the cells. The Na+/H+ exchanger and the Na+ - glucose cotrans-porter are located on the apical surface of the epithelial cells. Na+ is transported from the peritubular spaces to the capillaries by solvent drag. A 32-year-old man complaining of fatigue and muscle weakness is seen by his physician. Blood tests reveal a serum glucose level of 325 mg/dL and serum creatinine of 0.8 mg/dL. Results of a 24-hour urine analysis are as follows: Total volume = 5L Total glucose = 375 g Total creatinine = 2.4 g The patient's GFR is approximately which of the following? a. 75 mL/min b. 100 mL/min c. 125 mL/min d. 200 mL/min e. 275 mL/min ✔✔The answer is d. GFR is approximately equal to the clearance of creatinine. In this case, Creatine clearence= creatine excreted/plasma creatine concentration 2.4/.8 d= 300L/day After conversion= 200 mL/min ✔✔The answer is d. If a substance disappears from the circulation during its passage through the kidney, it usually indicates that it has been totally secreted into the nephron, in which case the clearance of the substance equals RPF. The clearance would not equal the GFR (choice c) because the normal filtration fraction is 20%, which would not totally clear the plasma concentration of substance. None of the substance is reabsorbed
pressure (choice b). The decreased extracellular osmolarity causes water to flow from the extracellular fluid compartment into the intracellular fluid compartment, increasing intracellular volume (choice a). Because more water is being reabsorbed, less is excreted and urine flow (choice e) is decreased. A 46-year-old man presents to his physician with a 12-week history of frontal headaches. CT of the brain shows a mass in the posterior pituitary, and the posterior pituitary "bright spot" is absent on MRI. The patient also complains of increased thirst and waking up frequently during the night. Which of the following best describes his urine? a. A higher-than-normal flow of hypertonic urine b. A higher-than-normal flow of hypotonic urine c. A lower-than-normal flow of hypertonic urine d. A lower-than-normal flow of hypotonic urine e. A normal flow of hypertonic urine ✔✔The answer is b.
- The presence of a mass in the posterior pituitary, coupled with the presentation of thirst and nocturia, suggests that the patient has a central diabetes insipidus with inadequate pituitary secretion of ADH. As a result of decreased ADH, the urine will have a low tonicity. A patient with diabetes insipidus often presents with polyuria, polydipsia, and dehydration. Due to the inability to reabsorb water, the patient may also have serum hyperosmolarity due to hypernatremia. On MRI, the absence of the normal bright spot in the region of the posterior pituitary further supports the diagnosis. A 52-year-old man presents to his internist for a 6-month checkup following diuretic therapy and recommended diet changes for his essential hypertension. His blood pressure is 145/95 mm Hg and serum aldosterone levels are increased. Aldosterone secretion is increased when there is a decrease in the plasma concentration of which of the following? a. Adrenocorticotropic hormone (ACTH) b. Angiotensin II c. Potassium d. Renin e. Sodium ✔✔The answer is e. A decrease in plasma sodium increases aldosterone secretion. Aldosterone secretion increases in response to an increase in all of the other answer choices. The effects of sodium on aldosterone secretion are mediated via the renin-angiotensin system. Hyponatremia, as may occur with a low-sodium diet, is associated with a decrease in extracellular volume, which increases renin secretion, probably due to a reflex increase in renal sympathetic nerve activity. Increased renin leads to increased production of angiotensin II, which binds to AT receptors in the zona glomerulosa, which act via a G protein to activate phospholipase C. The resultant
increase in protein kinase C fosters the conversion of cholesterol to pregnenolone and facilitates the action of aldosterone synthase, resulting in the conversion of deoxycorticosterone to aldosterone. Increased potassium concentration directly stimulates aldosterone secretion. Like angiotensin II, K+ stimulates the conversion of cholesterol to\ pregnenolone and the conversion of deoxycorticosterone to aldosterone by aldosterone synthase. Potassium exerts effect on aldosterone secretion by depolarizing the zona glomerulosa cells, which opens voltage-gated Ca 2+ channels, increasing intracellular Ca2+. ACTH stimulates aldosterone synthesis and secretion via increases in cAMP and protein kinase A. The stimulatory effect of ACTH on aldosterone secretion is usually transient, declining in 1 to 2 days, but persists in patients with glucocorticoid-remediable aldosteronism, an autosomal dominant disorder in which the 5′ regulatory region of the 11β-hydroxylase gene is fused to the coding region of aldosterone synthase gene, producing an ACTH-sensitive aldosterone synthase. A 92-year-old man presents with dehydration following 4 days of persistent diarrhea. Under this circumstance, hypotonic fluid would be expected in which of the following? a. Glomerular filtrate b. Proximal tubule c. Ascending limb of the loop of Henle d. Cortical collecting tubule e. Distal collecting duct ✔✔The answer is c. When a person is dehydrated, the decrease in extracellular fluid volume is sensed by stretch receptors in the low pressure receptors in the great veins, right and left atria, and pulmonary vessels, leading to an increase in vasopressin (ADH) secretion from the posterior pituitary. The ascending limb of the loop of Henle is not affected by ADH and remains impermeable to water; thus, as sodium and other electrolytes are reabsorbed from the ascending limb, its filtrate becomes hypotonic. The glomerular filtrate and proximal tubular fluid remain isotonic to plasma, which in the case of dehydration is higher than normal. In the presence of ADH, the cortical and medullary collecting tubules become permeable to water due to the insertion of aquaporin channels in the luminal membrane, and the filtrate within these portions of the nephron reaches osmotic equilibrium with the interstitial fluid surrounding them. A 63-year-old hospitalized woman becomes oliguric and confused. A blood sample is drawn to measure her glucose concentration, which is found to be 35 mg/dL. An IV access is obtained and an ampule of 50% dextrose is given followed by a continuous infusion of 10% dextrose. Most of the glucose that is filtered through the glomerulus undergoes reabsorption in which of the following areas of the nephron? a. Proximal tubule b. Descending limb of the loop of Henle c. Ascending limb of the loop of Henle d. Distal tubule e. Collecting duct ✔✔The answer is a.
- sparing diuretics such as amiloride act in this fashion. Aldosterone increases the intracellular potassium concentration by augmenting the activity of the Na-K pump and increasing the potassium permeability of the luminal membrane. Increasing dietary intake increases the plasma potassium concentration, which in turn stimulates aldosterone production. Increasing the rate of distal tubular flow increases the rate of K+ secretion. The high flow maintains a low tubular K+ concentration and therefore increases the electrochemical gradient for K+ secretion. Low-dose thiazide diuretics, such as hydrochlorothiazide, are often used as first- line antihypertensive agents, and are often combined with a potassium-sparing diuretic to prevent hypokalemia. Health-promoting lifestyle modifications are recommended for individuals with prehypertension and as an adjunct to therapy in hypertensive individuals. A 23-year-old woman presents with burning epigastric pain. A careful history reveals that the burning is exacerbated by fasting and improved with meals. The woman is prescribed the H2-receptor antagonist, cimeti-dine, for suspected peptic ulcer disease. Cimetidine may also have an adverse effect on proximal tubular function. Which of the following substances is more concentrated at the end of the proximal tubule than at the beginning of the proximal tubule? a. Bicarbonate b. Creatinine c. Glucose d. Phosphate e. Sodium ✔✔The answer is b. Because creatinine is not reabsorbed from the tubule, its concentration rises as water is reabsorbed. The H receptor antagonist, cimetidine, competes with creatinine for proximal tubule transport by the organic cation pathways. This may elevate serum creatinine levels, but this change does not reflect changes in GFR. Phosphate (choice d) is almost completely reabsorbed in the proximal tubule, so its concentration decreases along the length of the tubule. The concentrations of glucose (choice c) and bicarbonate (choice a) are also less at the end of the proximal tubule than at the beginning. Sodium is isosmotically reabsorbed from the proximal tubule; that is, when sodium is reabsorbed, water flows out of the proximal tubule to maintain a constant osmolarity; thus, the concentration of sodium (choice e) does not normally change as the filtrate flows through the proximal tubule. A 36-year-old African American man presents with low renin essential hypertension. Renin release from the juxtaglomerular apparatus is normally inhibited by which of the following? a. Aldosterone b. β-Adrenergic agonists c. Increased pressure within the afferent arterioles d. Prostaglandins e. Stimulation of the macula densa ✔✔The answer is c.
Juxtaglomerular cells are sensitive to changes in afferent arterial intraluminal pressure. Increased pressure within the afferent arteriole leads to a decrease in renin release, whereas decreased pressure tends to increase renin release. Angiotensin appears to inhibit renin release by initiating the flow of calcium into the juxtaglomerular cells. Renin release is increased in response to increased activity in the sympathetic neurons innervating the kidney. Prostaglandins, particularly PGI2 and PGE2, stimulate renin release. Stimulation of the macula densa leads to an increase in renin release, and although the mechanism is not fully understood, it appears that increased delivery of NaCl to the distal nephron is responsible for stimulating the macula densa. Aldosterone does not appear to have any direct effect on renin release. 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. 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 [IIA2].The two hormones have five identical amino acids at the C terminus. Biologic activity of cholecystokinin (CCK) is associated with the seven C-terminal
(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. 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 HCO − 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 HCO ✔✔the answer is a [IV A 2 a; Table 6.2]. Saliva is characterized by hypotonicity and a high HCO − concentration (relative to plasma) and by the presence of α-amylase and lingual 3 lipase (not proteases). The high HCO − concentration is achieved by secretion of HCO − 33 into saliva by the ductal cells (not reabsorption of HCO −). Because control of saliva 3 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 HCO − in the duodenum 3 (c) Pancreatic HCO − secretion is increased by gastrin 3 (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 HCO − 3 (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.
- Which of the following substances must be further digested before it can be absorbed by specific carriers in intestinal cells? (a) Fructose (B) Sucrose (c) Alanine (d) Dipeptides (e) Tripeptides ✔✔16. The answer is B [V A, B; Table 6-4]. Only monosaccharides can be absorbed by intestinal epithelial cells. Disaccharides, such as sucrose, must be digested to monosaccharides before they are absorbed. On the other hand, proteins are hydrolyzed to amino acids, dipeptides, or tripeptides, and all three forms are transported into intestinal cells for absorption.
- Slow waves in small intestinal smooth muscle cells are (A) action potentials (B) phasic contractions (C) tonic contractions (D) oscillating resting membrane potentials (E) oscillating release of cholecystokinin (CCK) ✔✔17. The answer is D [III A; Figure 6-3]. Slow waves are oscillating resting membrane potentials of the gastrointestinal (GI) smooth muscle. The slow waves bring the membrane potential toward or to threshold, but are not themselves action potentials. If the membrane potential is brought to threshold by a slow wave, then action potentials occur, followed by contraction.
