MCAT Biology Q&A CH10

Sarin is a potent organophosphate that can be used in chemical warfare. As an inhibitor of acetylcholinesterase, sarin causes excessive buildup of acetylcholine in all synapses where it is the neurotransmitter. Which of the following symptoms would be most likely to be seen in an individual with sarin poisoning? A. Increased urination and increased sweating B. Increased urination and decreased sweating C. Decreased urination and increased sweating D. Decreased urination and decreased sweating

(A) An excess of acetylcholine will lead to activation of all parasympathetic neurons, preganglionic sympathetic neurons, and the postganglionic sympathetic neurons that innervate sweat glands. Because the parasympathetic nervous system causes contractions of the bladder, one would expect increased urination. The increased activation of sweat glands would lead to increased sweating as well.

Which of the following sequences correctly shows the passage of blood through the vessels of the kidney? A. Renal artery → afferent arterioles → glomerulus → efferent arterioles → vasa recta → renal vein B. Afferent arterioles → renal artery → glomerulus → vasa recta → renal vein → efferent arterioles C. Glomerulus → renal artery → afferent arterioles → efferent arterioles → renal vein → vasa recta D. Renal vein → efferent arterioles → glomerulus → afferent arterioles → vasa recta → renal artery

(A) Blood enters the kidney through the renal artery, which divides into many afferent arterioles that run through the medulla and into the cortex. Each afferent arteriole branches into a convoluted network of capillaries called a glomerulus. Rather than converging directly into a vein, the capillaries converge into an efferent arteriole, which divides into a fine capillary network known as the vasa recta. The vasa recta capillaries enmesh the nephron tubule, where they reabsorb various ions, and then converge into the renal vein. This arrangement of tandem capillary beds is known as a portal system.

Diabetic nephropathy is commonly detected by finding protein in the urine of a patient. In such a disease, where is the likely defect in the nephron? A. Glomerulus B. Proximal convoluted tubule C. Loop of Henle D. Collecting duct

(A) The glomerulus is the most likely location of pathology if large proteins are detected in the urine. This is because large proteins should not be able to pass through the filter of the glomerulus in the first place. Once large proteins are in the filtrate, no other nephron structure can reabsorb them. Thus, the only likely source of protein in the urine is glomerular pathology.

On a very cold day, a man waits for over an hour at the bus stop. Which of the following structures helps his body set and maintain a normal temperature? A. Hypothalamus B. Kidneys C. Posterior pituitary D. Brain stem

(A) The hypothalamus functions as a thermostat that regulates body temperature. When it's cold outside, nervous stimulation to the blood vessels in the skin is increased, causing the vessels to constrict. This constriction diminishes blood flow to the skin surface and prevents heat loss. Sweat glands are turned off to prevent heat loss through evaporation. Skeletal muscles are stimulated to shiver (rapidly contract), which increases the metabolic rate and produces heat. The hypothalamus is also involved in other processes, including the release of endocrine hormones, regulation of appetite, and circadian rhythms.

Under normal physiological circumstances, the primary function of the nephron is to create urine that is: A. hypertonic to the blood. B. hypotonic to the blood. C. isotonic to the filtrate. D. hypotonic to the vasa recta.

(A) The kidneys function to eliminate wastes such as urea, while reabsorbing various important substances such as glucose and amino acids for reuse by the body. Generation of a solute concentration gradient from the cortex to the medulla allows a considerable amount of water to be reabsorbed. Excretion of concentrated urine serves to limit water losses from the body and helps to preserve blood volume. Thus, the primary function of the nephron is to create urine that is hypertonic to the blood, making choice (A) the correct answer and eliminating choices (B) and (D). Water should be reabsorbed from the filtrate, so urine should be hypertonic to the filtrate, eliminating choice (C).

Which region of the kidney has the lowest solute concentration under normal physiological circumstances? A. Cortex B. Outer medulla C. Inner medulla D. Renal pelvis

(A) The region of the kidney that has the lowest solute concentration is the cortex, where the proximal convoluted tubule and a part of the distal convoluted tubule are found. The solute concentration increases as one descends into the medulla, and concentrated urine can be found in the renal pelvis.

A drug is used that prevents the conversion of angiotensin I to angiotensin II. What is a likely effect of this drug? A. Increased sodium reabsorption B. Increased potassium reabsorption C. Increased blood pressure D. Increased blood pH

(B) Normally, angiotensin II causes secretion of aldosterone from the adrenal cortex. Aldosterone serves to increase reabsorption of sodium, while promoting excretion of potassium and hydrogen ions. Thus, blocking the release of aldosterone should result in decreased reabsorption of sodium, while decreasing excretion of potassium and hydrogen ions. This eliminates choices (A) and (D) and makes choice (B) the correct answer. In the absence of aldosterone, less water reabsorption will occur, eliminating choice (C).

In which of the following segments of the nephron is sodium NOT actively transported out of the nephron? A. Proximal convoluted tubule B. Thin portion of the ascending limb of the loop of Henle C. Distal convoluted tubule D. Thick portion of the ascending limb of the loop of Henle

(B) Sodium is actively transported out of the nephron in the proximal and distal convoluted tubules, where the concentration of sodium outside of the nephron is higher than inside; thus, energy is required to transport the sodium molecules against their concentration gradient, eliminating choices (A) and (C). In the inner medulla, however, sodium and other ions (such as chloride) diffuse passively down their concentration gradients from the thin ascending limb of the loop of Henle, making choice (B) the correct answer. The thick ascending limb of the loop of Henle is thick because its cells contain many mitochondria—which produce the ATP needed for active transport of sodium and chloride out of the filtrate, eliminating choice (D).

Which of the following would most likely be filtered through the glomerulus into Bowman's space? A. Erythrocytes B. Monosaccharides C. Platelets D. Proteins

(B) The glomerulus functions like a sieve; small molecules dissolved in the fluid will pass through the glomerulus, including glucose, which is later reabsorbed. Large molecules, such as proteins, and cells, such as erythrocytes and platelets, will not be able to pass through the glomerular filter.

Which of the following statements is FALSE? A. ADH increases water reabsorption in the kidney. B. Aldosterone indirectly increases water reabsorption in the kidney. C. ADH acts directly on the proximal convoluted tubule. D. Aldosterone stimulates reabsorption of sodium from the collecting duct.

(C) All of the answer choices describe ADH or aldosterone. These two hormones ultimately act to increase water reabsorption in the kidney; their respective mechanisms of action, however, are different. ADH increases water reabsorption by increasing the permeability of the collecting duct to water, whereas aldosterone stimulates reabsorption of sodium from the distal convoluted tubule and collecting duct. Using this knowledge, we can now attack the answer choices. Choice (C) is the correct answer because ADH does not act on the proximal convoluted tubule, but rather the collecting duct.

Glucose reabsorption in the nephron occurs in the: A. loop of Henle. B. distal convoluted tubule. C. proximal convoluted tubule. D. collecting duct.

(C) The filtrate enters Bowman's capsule and then flows into the proximal convoluted tubule, where virtually all glucose, amino acids, and other important organic molecules are reabsorbed via active transport.

A laceration cuts down into a layer of loose connective tissue in the skin. Which layer of the skin is this? A. Stratum corneum B. Stratum lucidum C. Papillary layer D. Reticular layer

(C) The layer of the skin that is predominantly loose connective tissue is the papillary layer of the dermis. The stratum corneum and stratum lucidum, choices (A) and (B), contain dead keratinocytes, while the reticular layer, choice (D), consists of dense connective tissue.

In which layer of the skin can the stem cells of keratinocytes be found? A. Stratum lucidum B. Stratum granulosum C. Stratum basale D. Stratum corneum

(C) The stratum basale contains the stem cells that proliferate to form keratinocytes, which then ascend through the other layers of skin until they are shed from the stratum corneum.

In the nephron, amino acids enter the vasa recta via the process of: A. filtration. B. secretion. C. excretion. D. reabsorption

(D) Essential substances, such as glucose, salts, amino acids, and water, are reabsorbed from the filtrate and returned to the blood in the vasa recta. In general, reabsorption refers to the movement of solutes from the filtrate back into the blood.

When the pH of the blood is high, which substance is likely to be excreted in larger quantities in the urine? A. Urea B. Ammonia C. Hydrogen ions D. Bicarbonate ions

(D) When the pH of the blood is high, this indicates that the blood is alkalemic. In order to correct the pH of the blood, the kidney will increase the excretion of a base, namely bicarbonate. Excretion of urea would have little effect on the pH, eliminating choice (A). While ammonia is a base, it is quite toxic and is generally converted to urea before excretion, eliminating choice (B). Excretion of hydrogen ions would result in exacerbation of the alkalemia, eliminating choice (C).