& x20;Which structure in the nephron functions as the counter-current multiplier to help concentrate urine? - INI-CET EXAM -2026

INI-CET EXAM -2026

Applied Physiology

Easy

Appeared in: INI-CET EXAM -2026

Explanation

The thick ascending loop of Henle is called the counter-current multiplier because it actively transports ions (Na+, K+, Cl-) out of the filtrate into the surrounding medullary interstitium.
This segment is uniquely impermeable to water, so as salts are pumped out, water is left behind in the tubule.
This separation of solute and water movement establishes a strong hyperosmotic gradient (high salt concentration) in the deep renal medulla.
This gradient is the driving force that allows the kidney to reabsorb water from the collecting ducts (under ADH influence) and produce concentrated urine.

Option B: The thin ascending loop of Henle is permeable to salts but impermeable to water. However, the movement of salt here is passive diffusion, not active transport. It contributes to the medullary gradient but does not actively 'multiply' it.
Option C: The collecting duct is where the final concentration of urine occurs. It uses the hyperosmotic gradient created by the loop of Henle to reabsorb water, but it does not create the gradient itself. Its permeability to water is regulated by Antidiuretic Hormone (ADH).
Option D: The vasa recta are the capillary networks that surround the loop of Henle. They function as counter-current exchangers, not multipliers. Their role is to preserve the medullary gradient by removing reabsorbed water and solutes, preventing the gradient from being 'washed out' by blood flow.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Counter-current multiplier mechanism in the nephron for urine concentration as background academic context rather than a clinical decision trigger.
Understanding the counter-current multiplier is crucial for comprehending how loop diuretics (e.g., furosemide) work. They inhibit the Na+-K+-2Cl- cotransporter in the thick ascending limb, disrupting the creation of the medullary gradient and leading to potent diuresis (increased urine output).
This physiological mechanism is fundamental to the body's ability to conserve water. In states of dehydration, the system works maximally to produce concentrated urine and prevent further water loss.
What if? - If a patient has central diabetes insipidus, they lack ADH. The counter-current multiplier (loop of Henle) would still create the medullary gradient, but the collecting duct would remain impermeable to water. The result would be the excretion of large volumes of dilute urine, as the gradient cannot be utilized for water reabsorption.

How to Approach the Question

First, identify the key physiological term in the question: 'counter-current multiplier'.
Recall that the kidney's counter-current system has two main components: the 'multiplier' (which actively creates the concentration gradient) and the 'exchanger' (which passively maintains it).
Analyze the function of each option in the context of this system. The term 'multiplier' implies an active, energy-dependent process that builds up the concentration.
Evaluate the options: The thick ascending loop of Henle uses active transport to pump out salts while being impermeable to water. This perfectly fits the definition of the multiplier.
Contrast this with the vasa recta (exchanger), the collecting duct (uses the gradient), and the thin ascending limb (passive role) to confirm the correct choice.

Concept Tested & Keywords

Concept Tested: Counter-current multiplier mechanism in the nephron for urine concentration.
Stem keywords: nephron, counter-current multiplier, concentrate urine
Lead-in keywords: Which structure
Negative lead-in flag: false

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E6 Physiology Guyton 4SAE Part 2B pp. 42-44, 41-43, 43-45