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Increased extracellular matrix proteins thicken the glomerularbasement membrane, increasing the filtration barrier, thus reducingthe permeability to plasma.A. Aldosterone is produced in the zona glomerulosa of the adrenalcortex.B. Filtration fraction is defined as the glomerular filtration rate(GFR) divided by the renal plasma flow (RPF). The renal plasmaflow equals [RBF × (1-hematocrit)], or 600 mL/min. Because Cin isequated with the GFR, the filtration fraction is 125 mL/min ÷600 mL/min, or ∼20%.A.
Glucose is 100% reabsorbed in the proximal convoluted tubule,via Na+-glucose cotransporters.D. Plasma antidiuretic hormone has no direct effect on renal potassium handling, while the other conditions cause either secretion(high dietary or plasma potassium and aldosterone) or enhancedreabsorption (low dietary or plasma potassium and acidosis).D. Loop diuretics target the Na+-K+-2Cl− cotransporters on thethick ascending limb of Henle. When the transporters are blocked,the solutes are carried distally (where there is little sodium reabsorption in the absence of aldosterone) and most of the fluid is excretedas urine. Because this transporter is a key factor in the countercurrent multiplier system, an additional result of the use of loop diuretics is the washing out of the medullary interstitial concentrationgradient, which contributes to the sustained diuresis.E.
In the collecting duct cells, binding of ADH to V2 receptorsstimulates the insertion of aquaporins into the apical membranes.This causes sodium-free water reabsorption.A. Distal sodium reabsorption has no effect on the medullary concentration gradient, whereas the other factors all play significantroles in creating and maintaining the gradient.D. Free water clearance (CH2O) = V − [(Uosm/Posm) × V], or +1.The positive value implies that water was cleared in excess of theamount required for iso-osmotic excretion of solutes present inthe urine.C.
Angiotensin II has two direct actions on the kidneys, to increaseproximal tubular sodium reabsorption and to constrict renal afferent and efferent arterioles. These actions increase sodium and waterreabsorption. Renin is secreted from the juxtaglomerular cells inresponse to low sodium concentration and low tubular fluid flowrate in the distal tubule.
Aldosterone stimulates sodium reabsorption in the late distal tubules and collecting ducts.A. The reduction in vascular volume will stimulate sympatheticvasoconstriction and elevate sodium and fluid-retaining systems.Atrial natriuretic peptide is released from cardiac myocytes inresponse to increased atrial stretch during volume expansion. Thus,during dehydration, circulating ANP will be low.E.
Diabetes insipidus (DI) is usually of central origin (nephrogenicDI is rare), following trauma, disease, or surgery affecting the pituitary gland. Central DI involves the loss of ADH, so water channelsare not present in the apical membranes of the collecting ducts, andurine cannot be concentrated. This leads to massive excretion (3 to18 L/day) of hypotonic urine.C. Plasma bicarbonate is low in metabolic acidosis, and the α-intercalated cells of the collecting ducts will increase H+ secretion.C.
NAE is determined by the sum of urinary ammonium and titratable acids, minus any excreted bicarbonate. It does not depend onsodium excretion.B. The pH is <7.4, and therefore the condition is acidosis. Becausethe low plasma bicarbonate (<24 mEq/L) also reflects acidosis, thedisorder is metabolic in origin.
The anion gap can be used to determine whether the acidosis is from acid loading or base loss; AG =Na+ − (Cl− + HCO3−), or 136 − 114 = 22. The usual anion gap is ∼8to 12, and an increase in the gap reflects the addition of acid. Baseloss (as found with diarrhea) would show no change in AG, becausethe loss of HCO3− in the stool would be matched by an increase inplasma Cl−.17. D. In respiratory alkalosis, the high pH will be matched by low PCO2(indicating a reduction in plasma “acid” as the primary disturbance).
In uncompensated respiratory alkalosis, the plasma HCO3will be normal.18. A. Phosphoric acid (H2PO4−) is the primary TA excreted in theurine. Phosphates make very good buffers in urine because their pKis near the urine pH and there is a large amount (∼25% of the filtered phosphate load) of dibasic phosphate (HPO42−) available forconversion to TA and excretion.Section 6: Gastrointestinal Physiology1.
D. The GI tract does not directly regulate systemic blood flow.2. E. Growth hormone has no apparent effect on motility or secretionin the GI tract.3. D. The GI tract has an intrinsic nervous system (enteric nerves),which responds to signals from lumenal receptors and hormones,as well as extrinsic nerves. When the extrinsic nerve input is severed,motility and secretion continues, but not as efficiently.4. B.
Vagal afferent nerves regulate all of the actions EXCEPT primaryesophageal peristalsis, which is mediated by the swallowing centerin the medulla. The vagus does participate in secondary esophagealperistalsis, in conjunction with local enteric nerves.5. B. Action (or spike) potentials in the GI tract are caused by theinflux of calcium into the smooth muscle when the slow waves aredepolarized above −40 mV. This mechanism is active throughoutthe tract, and depolarization can result from local stretch (mechanoreceptors acting on enteric nerves), extrinsic nerves, andpeptides.6. C.
The undulations are a result of small changes in membranepotential due to the activity of the basolateral Na+/K+ ATPase. Theslow waves occur at different rates from the mid-stomach throughthe colon.7. D. Peristalsis follows the law of the intestines, where muscle is contracted proximal to the bolus and relaxed distal to the bolus, producing aboral movement of the chyme.8. B. There is no evidence for involvement of the parasympatheticnervous system in the MMC.9. D. Gastric inhibitory peptide (GIP) is an endocrine hormonesecreted from cells in the duodenum and jejunum of the smallintestine.
The other substances are secreted directly into the gastriclumen.10. C. Vagal afferent nerves stimulate gastric acid secretion directly atthe parietal cell, and indirectly by increasing gastrin-releasingpeptide (GRP) (and thus, gastrin release) and inhibiting somatostatin release.11. E.
Proton pump activity drives acid secretion and is the target forregulatory hormones, peptides, and nerves.12. B. Sodium enters the lumenal membrane by all of the mechanismsexcept the Na+ pump (Na+/K+ ATPase), which is located on thebasolateral membrane, and maintains the low intracellular Na+ concentrations that drive the lumenal Na+ transport mechanisms.13. C. Secretin is a hormone released from S-cells in the duodenumand early jejunum in response to acidic chyme.
The secretin bindsto receptors on the pancreatic acinar cells and stimulates release ofelectrolyte buffers into the pancreatic duct.14. B. The liver performs all of the functions noted, except vitaminproduction. It does store a variety of important vitamins and minerals, including vitamin B12 and iron.15. D. Obstruction of blood flow through the liver increases the pressure in the portal vein, which brings blood from the intestines toAnswers16.17.18.19.20.21.22.the liver.
This eventually results in portal hypertension. The obstruction also reduces bile secretion.A. The hepatocytes are the primary site of formation of lipoproteins, which carry fats, protein, and cholesterol to different tissuesfor processing and storage.B. Bile is amphipathic and essential for carrying the hydrophobiclipids through the unstirred water layer adjacent to the enterocytes.The ability to form micelles, with hydrophilic ends of bile orientedoutward, and lipophilic ends oriented inward and associated withthe lipids, allows the efficient transport of lipids to the intestinalcells.D.
Efficient digestion of proteins in the small intestine depends onthe pancreatic proteases, which have optimal catalytic activity nearphysiologic pH (between pH 7 to 8). Low lumenal pH will significantly decrease catalytic activity and reduce the amount of proteinsdigested to constituents that can be absorbed.D. Starch is the primary dietary carbohydrate, and digestion beginsin the mouth with salivary α-amylase. Only around 25% of digestion occurs preduodenally: pancreatic α-amylase digests the remaining starch to malto-oligosaccharides, and then intestinal brushborder saccharidases (maltase, isomaltase, sucrose, and lactase)digest oligosaccharides and disaccharides (sucrose and lactose) tomonosaccharides (glucose, galactose, and fructose).A.
The presence of chyme in the antrum of the stomach and theduodenum stimulates release of gastrin, which is a potent stimulatorof gastric acid secretion. It does not promote buffer secretion.C. Celiac sprue is an autoimmune reaction initiated by gluten proteins in wheat, resulting in the flattening of the villus lining andexpansion of the crypts. The reduction in the upper villi severelydecreases the brush border enzyme digestion of carbohydrates andproteins and reduces the surface area for absorption.B.
B12 is an essential nutrient and has multiple mechanisms protecting it from protease activity and facilitating entry into the intestinal cells. There is no limitation to absorption posed by intracellularB12 concentrations.Section 7: Endocrine Physiology1.
E. Thyroid hormone, vitamin D, and steroid hormones are lipophilic and readily diffuse into target cells, where they are bound bynuclear receptors, initiating gene transcription. Peptide hormonesand catecholamines are bound by membrane receptors, initiatingan intracellular signaling cascade that ultimately leads to regulationof cellular function.2. E.
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