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Difficulty: HardExcretion and Excretory Organs

In a physiological experiment investigating mammalian kidney function, fluid samples were collected from various sections of the nephron. Analysis revealed that active reabsorption of sodium and chloride ions occurs without the accompanying movement of water, rendering the tubular fluid progressively hypotonic to the surrounding interstitium. Which segment of the nephron is responsible for this selective ion transport?

  1. Ascending limb of the loop of HenleAnswer
  2. B
    Descending limb of the loop of Henle
  3. C
    Proximal convoluted tubule
  4. D
    Bowman's capsule

Answer

Ascending limb of the loop of Henle
The ascending limb of the loop of Henle actively pumps electrolytes (Na+Na^+ and Cl−Cl^-) out of the tubule into the medullary tissue while being completely impermeable to water. Because solute is removed while water remains trapped inside the lumen, the filtrate becomes hypotonic by the time it reaches the distal convoluted tubule.

Step-by-Step Solution

1
Analyze the functional permeability characteristics of nephron segments.
The ascending limb of the loop of Henle actively transports Na+Na^+ and Cl−Cl^- out of the tubular lumen while remaining strictly impermeable to water.
Active transport of electrolytes without water flow reduces the solute concentration of the remaining tubular fluid.
2
Determine the impact on filtrate osmolarity.
As salts exit and water is retained, the filtrate becomes progressively hypotonic relative to the renal medulla and blood plasma.
This differential permeability is central to maintaining the medullary osmotic gradient without diluting interstitial concentration.

Key Concept

Differential segment permeability and electrolyte transport in the loop of Henle
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Topics

Excretion and Excretory Organs

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