Question

This question is based on the diagram of a mammalian nephron given below.

In which of the following part urine is becoming hypertonic, as it goes down?

• A. A
• B. B
• C. C
• D. D

Answer 1

The correct option is C C

A - renal artery; B - proximal tubule; C - loop of Henle; D - collecting tubule. The ability to concentrate urine depends on upon the generation of a hyperosmolar environment in the medulla. The loops of Henle are set up so as to concentrate osmolarity in the deepest part of the medulla. This occurs because the ascending and descending limbs have different permeabilities to salt and water. The loop of Henle has three distinct regions, the thin-walled descending limb, the thin-walled lower portion of the ascending limb and the thick-walled upper portion of the ascending limb. The descending limb of the loop is highly permeable to water but almost completely impermeable to solutes. As the filtrate travels down the descending loop, water will flow from the loop into the surrounding medium via osmosis. When the filtrate reaches the hairpin turn, it is isotonic with the surrounding medium (about 1200 most/L). The lower portion of the ascending branch of the loop of Henle is highly permeable to Na+ and Cl-, moderately permeable to urea, and almost completely impermeable to water. As it travels up into the less concentrated regions of the medulla, Na+, and Cl- will passively diffuse across the membrane. As, the filtrate continues up the thick portion of the loop of Henle, Na+, and Cl- are actively pumped out of the filtrate into the surrounding medium. This requires energy but helps to maintain the osmotic concentration gradient in the medulla.

So, the correct answer is option C.