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Renal and Fluid-Electrolyte Physiology

Counter-current Mechanism Practice Questions

20 free Counter-current Mechanism practice questions for the Physiology. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.

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Question 1 of 20 Medium

Which of the following best describes the primary function of the counter-current multiplier system in the kidney?

  1. A To maximize the rate of glomerular filtration
  2. B To transport glucose against its concentration gradient
  3. C To secrete hydrogen ions into the distal tubule
  4. D To create a hyperosmotic medullary interstitium

Correct answer: To create a hyperosmotic medullary interstitium

The counter-current multiplier uses the energy-dependent transport of solutes in the thick ascending limb to build a high osmotic pressure in the renal medulla. This gradient is essential for the subsequent reabsorption of water from the collecting ducts.

Question 2 of 20 Medium

Which segment of the nephron is responsible for the 'single effect' that initiates the counter-current multiplication process?

  1. A Thick ascending limb of the loop of Henle
  2. B Descending limb of the loop of Henle
  3. C Proximal convoluted tubule of the nephron
  4. D Cortical collecting duct of the nephron

Correct answer: Thick ascending limb of the loop of Henle

The 'single effect' is the active transport of NaCl out of the thick ascending limb into the interstitium, coupled with its impermeability to water. This creates a 200 mOsm/L gradient between the tubule and the interstitium at any horizontal level.

Question 3 of 20 Medium

The vasa recta act as 'counter-current exchangers.' What is the physiological significance of this specific arrangement?

  1. A They actively pump urea directly into the inner medulla
  2. B They provide the metabolic energy required for sodium reabsorption
  3. C They prevent the washout of the medullary osmotic gradient
  4. D They increase the hydrostatic pressure inside the glomerulus

Correct answer: They prevent the washout of the medullary osmotic gradient

Because the vasa recta are highly permeable and have a U-shaped loop, they gain solutes and lose water as they descend, then reverse the process as they ascend. This passive exchange ensures that nutrients are delivered to the medulla without destroying the high osmolarity gradient.

Question 4 of 20 Medium

In the counter-current multiplier system, the descending limb of the loop of Henle is characterized by which of the following?

  1. A High permeability to water and low permeability to solutes
  2. B Active transport of sodium and chloride ions out of the lumen
  3. C Presence of the NKCC2 symporter in its membrane
  4. D Impermeability to water and high permeability to urea molecules

Correct answer: High permeability to water and low permeability to solutes

The descending limb allows water to move out of the tubule into the hypertonic interstitium via osmosis. As a result, the tubular fluid becomes increasingly concentrated as it reaches the bend of the loop.

Question 5 of 20 Medium

Which substance contributes approximately 40-50% of the total osmolarity of the inner medullary interstitium during periods of maximum antidiuresis?

  1. A Sodium chloride salt
  2. B Potassium ions, a minor contributor
  3. C Urea, a nitrogen waste product
  4. D Creatinine, a filtered waste product

Correct answer: Urea, a nitrogen waste product

Urea is recycled from the inner medullary collecting ducts into the interstitium, especially when ADH levels are high. This urea recycling significantly boosts the medullary osmotic gradient, facilitating further water reabsorption.

Question 6 of 20 Medium

What is the effect of Antidiuretic Hormone (ADH) on the counter-current mechanism in the inner medulla?

  1. A It inhibits the NKCC2 transporter in the thick ascending limb
  2. B It decreases the permeability of the vasa recta walls to solutes
  3. C It prevents water from leaving the thin descending limb
  4. D It increases urea recycling by activating UT-A1 transporters

Correct answer: It increases urea recycling by activating UT-A1 transporters

ADH increases the permeability of the medullary collecting ducts to urea. This allows urea to flow into the interstitium, where it adds to the osmotic gradient used to concentrate urine.

Question 7 of 20 Medium

Which of the following would lead to a 'washout' of the medullary osmotic gradient and a subsequent decrease in urine concentrating ability?

  1. A Increased blood flow through the vasa recta
  2. B Decreased dietary protein intake over time
  3. C Activation of the renin-angiotensin-aldosterone system
  4. D Inhibition of the renal myogenic reflex response

Correct answer: Increased blood flow through the vasa recta

If blood flow through the vasa recta increases too much (e.g., during hypertension), solutes are carried away from the medulla faster than they can be replaced. This weakens the osmotic gradient, making it harder for the kidney to produce concentrated urine.

Question 8 of 20 Medium

The fluid entering the distal convoluted tubule is typically ________ relative to plasma due to the action of the counter-current multiplier.

  1. A Isosmotic
  2. B Hyperosmotic
  3. C Equiosmotic
  4. D Hyposmotic

Correct answer: Hyposmotic

The thick ascending limb removes solutes without water, a process called 'diluting.' By the time the tubular fluid reaches the start of the distal tubule, its osmolarity has dropped to about 100 mOsm/L, which is significantly lower than plasma.

Question 9 of 20 Medium

Which transporter is the primary molecular engine of the counter-current multiplier in the thick ascending limb?

  1. A Na+/K+ ATPase pump on the basolateral membrane
  2. B SGLT2 glucose carrier
  3. C Aquaporin-2 water channel in the collecting duct
  4. D Na+/K+/2Cl- cotransporter (NKCC2)

Correct answer: Na+/K+/2Cl- cotransporter (NKCC2)

NKCC2 moves one sodium, one potassium, and two chloride ions into the cells of the thick ascending limb. This active movement of salt is what 'multiplies' the concentration gradient in the medulla.

Question 10 of 20 Medium

What happens to the osmolarity of the blood as it flows down the descending limb of the vasa recta?

  1. A It stays the same at 300 mOsm/L
  2. B It decreases as it picks up excess water from the loop of Henle
  3. C It increases as it picks up solutes from the interstitium
  4. D It decreases as it loses salt to the medulla

Correct answer: It increases as it picks up solutes from the interstitium

As the vasa recta descend into the salty medulla, they equilibrate with the environment by losing water and gaining solutes. By the time the blood reaches the tip of the vasa recta, its osmolarity matches the local interstitial concentration.

Question 11 of 20 Medium

A person on a very low-protein diet may have difficulty concentrating their urine. This is primarily because of a reduction in:

  1. A Sodium filtration at the glomerulus
  2. B Urea available for the medullary osmotic gradient
  3. C ADH secretion from the posterior pituitary gland
  4. D Blood flow through the renal peritubular capillaries

Correct answer: Urea available for the medullary osmotic gradient

Urea is a metabolic byproduct of protein breakdown. Low protein intake leads to lower urea production, which reduces the osmotic strength of the inner medulla and impairs the ability to reabsorb water.

Question 12 of 20 Medium

In the context of the counter-current system, the term 'multiplication' refers to which phenomenon?

  1. A The division of nephrons into cortical and juxtamedullary types
  2. B The increase in GFR due to efferent arteriolar constriction
  3. C Turning a small horizontal effect into a large vertical gradient
  4. D The rapid rise in circulating antidiuretic hormone levels during dehydration

Correct answer: Turning a small horizontal effect into a large vertical gradient

The 'single effect' only creates a 200 mOsm/L difference horizontally. However, because of the counter-current flow of fluid, this effect is multiplied vertically, resulting in an interstitium that is 1200 mOsm/L at the papilla.

Question 13 of 20 Medium

Which nephron type is most essential for the counter-current mechanism due to its long loops of Henle extending deep into the medulla?

  1. A Cortical nephrons
  2. B Mid-cortical zone nephrons
  3. C Juxtamedullary nephrons
  4. D Superficial cortical nephrons

Correct answer: Juxtamedullary nephrons

Juxtamedullary nephrons have loops that reach all the way to the renal papilla. These long loops are necessary to create the high osmotic gradients required for concentrated urine production.

Question 14 of 20 Medium

Which of the following describes the permeability of the 'Thin Ascending Limb' of the loop of Henle?

  1. A Permeable to water, impermeable to NaCl
  2. B Impermeable to water, permeable to NaCl
  3. C Permeable to both water and NaCl
  4. D Impermeable to both water and NaCl

Correct answer: Impermeable to water, permeable to NaCl

The thin ascending limb is impermeable to water but allows NaCl to diffuse out passively into the interstitium. This step is a passive part of the counter-current system in the inner medulla.

Question 15 of 20 Medium

During maximal diuresis (low ADH levels), what happens to the medullary osmotic gradient?

  1. A It increases to 1400 mOsm/L
  2. B It remains unchanged
  3. C It decreases because urea recycling is reduced
  4. D It disappears entirely as the kidney shuts down

Correct answer: It decreases because urea recycling is reduced

When ADH is low, the collecting ducts are impermeable to urea and water. The lack of urea recycling into the interstitium causes the medullary osmotic gradient to dissipate or weaken significantly.

Question 16 of 20 Medium

The active transport of ions in the thick ascending limb is inhibited by which class of drugs, effectively disrupting the counter-current multiplier?

  1. A Thiazide diuretics
  2. B Loop diuretics
  3. C Potassium-sparing diuretics
  4. D ACE inhibitors

Correct answer: Loop diuretics

Loop diuretics (like furosemide) block the NKCC2 transporter. This stops the active transport of salt into the medulla, collapsing the osmotic gradient and preventing water reabsorption in the collecting duct.

Question 17 of 20 Medium

As blood flows back UP the ascending limb of the vasa recta toward the cortex, it:

  1. A Gains water and loses solutes
  2. B Gains solutes and loses water
  3. C Gains both water and solutes
  4. D Loses both water and solutes

Correct answer: Gains water and loses solutes

The ascending vasa recta move from a hyperosmotic environment to a less concentrated one. They reabsorb water that left the descending limb and the collecting duct, while solutes diffuse back out into the interstitium.

Question 18 of 20 Medium

The 'U-turn' at the bottom of the loop of Henle allows for counter-current flow. What is the osmolarity of the tubular fluid at this specific point?

  1. A 100 mOsm/L
  2. B 300 mOsm/L
  3. C 1500 mOsm/L
  4. D 1200 mOsm/L

Correct answer: 1200 mOsm/L

At the very tip of the long loops of juxtamedullary nephrons, the tubular fluid has equilibrated with the surrounding inner medullary interstitium, reaching its highest concentration of approximately 1200 mOsm/L.

Question 19 of 20 Medium

Which of the following serves as the 'sink' for water reabsorbed from the collecting ducts under the influence of ADH?

  1. A Loop of Henle
  2. B The vasa recta
  3. C The renal pelvis
  4. D The macula densa

Correct answer: The vasa recta

Water reabsorbed from the collecting ducts enters the medullary interstitium and is then carried away by the vasa recta. This prevents the reabsorbed water from diluting the interstitial osmotic gradient.

Question 20 of 20 Medium

The counter-current mechanism allows the human kidney to produce urine with a maximum concentration of approximately:

  1. A 300 mOsm/L
  2. B 600 mOsm/L
  3. C 1200 mOsm/L
  4. D 2500 mOsm/L

Correct answer: 1200 mOsm/L

Due to the limits of the counter-current multiplier and the concentration of urea, the maximum concentration of human urine is about 4 to 5 times the osmolarity of plasma.

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