MCAT Biology : Excretory Regulation Mechanisms

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #1 : Excretory Regulation Mechanisms

Aldosterone is a key hormone used by the kidneys during urine formation.

What is the function of aldosterone in the kidneys?

Possible Answers:

Aldosterone increases the reabsorption of sodium from nephron filtrate

Aldosterone decreases the reabsorption of sodium from nephron filtrate

Aldosterone helps to decrease blood pressure

Aldosterone increases the amount of urine production

Correct answer:

Aldosterone increases the reabsorption of sodium from nephron filtrate

Explanation:

Aldosterone increases the reabsorption of sodium from nephron filtrate.

The reabsorption of sodium leads to reabsorption of water, which makes the urine more concentrated. Increasing aldosterone production would lead to increased blood pressure, since more water is retained in the blood stream. 

Example Question #1 : Excretory Regulation Mechanisms

What is the role of antidiuretic hormone (ADH)?

Possible Answers:

ADH decreases the reabsorption of water from the loop of Henle

ADH decreases the reabsorption of water from the collecting duct

ADH increases the reabsorption of water from the collecting duct

ADH increases the reabsorption of water from the loop of Henle

Correct answer:

ADH increases the reabsorption of water from the collecting duct

Explanation:

Antidiuretic hormone, also known as vasopressin, increases the reabsorption of water from the collecting duct. It increases the permeability of the collecting duct, which allows water to be reabsorped and makes the urine more concentrated.

You can remember what antidiuretic hormone does by remembering that diuetics increase urine production; therefore an ANTIdiuretic will decrease urine production.

Example Question #2 : Excretory Regulation Mechanisms

Which of the following would most likely NOT happen in the excretory system if a person has not drunk water for an extended period of time?

Possible Answers:

Increase in antidiuretic hormone secretion

Decrease in sodium

Increase in aldosterone secretion

Increase in blood volume

Decrease in urine volume

Correct answer:

Increase in blood volume

Explanation:

The signal to increase water reabsorption in the nephrons comes from antidiuretic hormones and aldosterone. The urine volume also decrease in an attempt to retain the fluids already present in the body. Since the body is trying to conserve the fluids it has and there are no incoming fluids, the blood volume should not increase.

Example Question #3 : Excretory Regulation Mechanisms

The juxtaglomerular cells of the nephron regulate __________.

Possible Answers:

blood osmolarity

blood pH

urine acidity

blood pressure

Correct answer:

blood osmolarity

Explanation:

Juxtaglomerular cells respond to low levels of sodium and secrete renin in response, which results in the release of aldosterone from the adrenal cortex. Aldosterone, as a result, will increase sodium reabsorption from the collecting duct. This will in turn increase blood pressure, however, the direct role of the cells themselves is to regulate blood osmolarity.

Example Question #4 : Excretory Regulation Mechanisms

The effect of the hormone vasopressin (ADH) on the kidney is best described by which of the following?

Possible Answers:

It increases water reabsorption in the afferent convoluted tubule

It increases the permeability of the collecting duct to water

It increases sodium reabsorption in the distal convoluted tubule

It decreases the glomerular filtration rate

Correct answer:

It increases the permeability of the collecting duct to water

Explanation:

Vasopressin acts on the collecting duct in order to increase its permeability to water. This results in more water being reabsorbed, and increases blood pressure.

Example Question #43 : Excretory System

Which of the following conditions would you not predict in a severely dehydrated patient?

Possible Answers:

The collecting ducts would be permeable to water

Aldosterone would be elevated

Antidiuretic hormone (ADH) would be elevated

Renin would be secreted at low levels

Correct answer:

Renin would be secreted at low levels

Explanation:

In a severely dehydrated patient, the kidneys will be acting to preserve water in the body. Renin is secreted by the kidneys, and is the starting enzyme for a cascade that stimulates the release of aldosterone. Aldosterone raises the blood pressure of the body by acting on the distal tubule, and antidiuretic hormone (ADH) is responsible for making the collecting ducts permeable to water, thus concentrating the urine. Because of this, we would expect that renin levels would be higher than normal in a dehydrated patient.

Example Question #44 : Excretory System

Which of the following is the function of aldosterone?

Possible Answers:

Water retention

Sodium and water retention

Sodium retention

Sodium and water excretion

Sodium excretion

Correct answer:

Sodium and water retention

Explanation:

Aldosterone is released from the adrenal cortex and acts on nephrons to increase water and sodium retention. Aldosterone directly affects the synthesis of sodium ion channels and sodium-potassium pump proteins in the nephron, actively leading to sodium retention and indirectly leading to water retention based on increased blood osmolarity. Antidiuretic hormone (ADH), on the other hand, works by just retaining water without directly affecting sodium retention.

Example Question #6 : Excretory Regulation Mechanisms

The interaction between blood pressure and kidney function in humans requires coordination by the renin-angiotensin-aldosterone system (RAAS). This system involves the dynamic interplay of the kidneys, lungs, and blood vessels to carefully regulate sodium and water balance.

A normal human kidney has cells adjacent to the glomerulus called juxtaglomerular cells. These cells sense sodium content in urine of the distal convoluted tubule, releasing renin in response to a low level. Renin is an enzyme that converts angiotensinogen to angiotensin I (AI). AI is converted to angiotensin II (AII) by angiotensin converting enzyme (ACE) in the lung.

AII stimulates aldosterone secretion in the zona glomerulosa of the adrenal gland. Aldosterone then acts to upregulate the sodium-potassium pump on the basolateral side of distal tubule epithelial cells to increase sodium reabsorption from the urine, as well as increasing potassium excretion.

A doctor is examining a patient in a dialysis center. She notices that the patient's blood pressure is high. A common treatment of high blood pressure is a class of drugs called ACE inhibitors. After administering an ACE inhibitor, which of the following is likely to be true?

Possible Answers:

Aldosterone levels will remain high

Sodium reabsorption in the distal tubule will increase

Renin levels will immediately decrease

Angiotensin I levels will immediately decrease

Renin levels will remain elevated

Correct answer:

Renin levels will remain elevated

Explanation:

The passage outlines the role of angiotensin converting enzyme (ACE) in the renin-angiotensin-aldosterone system (RAAS). Because ACE inhibitors act on the enzyme that converts angiotensin I to angiotensin II, we would expect renin and angiotensin I to remain high because they are present before the ACE step in the RAAS pathway. The hormones present after ACE action, including angiotensin II and aldosterone, however would decrease with ACE inactivation, as would sodium reabsorption. The result is generally lower blood pressure.

We can see that inhibiting the action of ACE would cause buildup of renin and angiotensin I, and a decrease in angiotensin II, which would result in a failure to retain water.

Example Question #5 : Excretory Regulation Mechanisms

What substance is produced by the juxtaglomerular apparatus in the kidney to help regulate blood volume and pressure?

Possible Answers:

Angiotensinogen

Angiotensin II

Renin

Aldosterone

Correct answer:

Renin

Explanation:

The juxtaglomerular apparatus in the distal tubule monitors the filtrate that passes by to sense increases or decreases in blood volume. Granular cells in the apparatus secrete the enzyme renin, which initiates a cascade that ultimately produces aldosterone. Remember that aldosterone acts on the distal tubule to stimulate sodium reabsorption and potassium secretion.

Secretion of renin allows the conversion of angiotensinogen to angiotensin I. Angiotensin-converting enzymes (ACE) change angiotensin I to angiotensin II in the lungs, which then stimulates the adrenal cortex to release aldosterone and increase blood volume.

Example Question #8 : Excretory Regulation Mechanisms

Carbonic anhydrase is a very important enzyme that is utilized by the body. The enzyme catalyzes the following reaction:

A class of drugs that inhibits this enzyme is carbonic anhydrase inhibitors (eg. acetazolamide, brinzolamide, dorzolamide). These drugs are commonly prescribed in patients with glaucoma, hypertension, heart failure, high altitude sickness and for the treatment of basic drugs overdose.   

In patients with hypertension, carbonic anhydrase inhibitors will prevent the reabsorption of sodium chloride  in the proximal tubule of the kidney. When sodium is reabsorbed back into the blood, the molecule creates an electrical force. This electrical force then pulls water along with it into the blood. As more water enters the blood, the blood volume increase. By preventing the reabsorption of sodium, water reabsorption is reduced and the blood pressure decreases. 

When mountain climbing, the atmospheric pressure is lowered as the altitude increases. As a result of less oxygen into the lungs, ventilation increases. From the equation above, hyperventilation will result in more  being expired. Based on Le Chatelier’s principle, the reaction will shift to the left. Since there is more bicarbonate than protons in the body, the blood will become more basic (respiratory alkalosis). To prevent such life threatening result, one would take a carbonic anhydrase inhibitor to prevent the reaction from shifting to the left.  

Carbonic anhydrase inhibitors are useful in patients with a drug overdose that is acidic. The lumen of the collecting tubule is nonpolar. Due to the lumen's characteristic, molecules that are also nonpolar and uncharged are able to cross the membrane and re-enter the circulatory system. Since carbonic anhydrase inhibitors alkalize the urine, acidic molecules stay in a charged state.

Which of the following side effects is/are expected in a drug that overstimulates carbonic anhydrase?

I. Increased in the blood's pH

II. Increased in the urine's pH

III. Increased bicarbonate level in the blood

Possible Answers:

I and III

I only

II only

I and II

I, II, and III

Correct answer:

I and III

Explanation:

As mentioned from the passage, carbonic anhydrase inhibitors will alkalize the urine and make the blood more acidic. Therefore, the opposite will occur if one were to overstimulate the enzyme.  

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