Question

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

At which part urine is less osmotic?

• A. A
• B. B
• C. C
• D. D

Answer 1

The correct option is B B

A - renal artery; B - proximal tubule; C - loop of Henle; D - collecting tubule. The ability to excrete urine that is more concentrated than the extracellular fluid (ECF) depends on the creation of a hyperosmotic environment in the medulla. An increase in osmolarity reflects a decrease in water concentration. In humans, the osmotic gradient in the medulla allows the urine produced to be roughly 5 times as concentrated as the ECF. The first three steps in urine formation, filtration, reabsorption, and the establishment of an osmotic gradient, result in a fluid that is slightly hypotonic to blood. Electrolytes and water are absorbed by the distal tubule. The epithelial cells of the proximal tubule reabsorb ions, nutrients, and water and transport them to the blood vessels. Water flows from the epithelial cells into the blood via osmosis. Because osmosis occurs, the osmolarity of the filtrate remains isotonic. Hence the urine is less osmotic in the proximal tubule.

So, the correct answer is option B.