ADH controls water permeability of:

Question

Accepted Answer

The correct option is 2: DCT (Distal Convoluted Tubule).

Antidiuretic hormone (ADH), also known as vasopressin, plays a crucial role in regulating water reabsorption in the kidneys, thereby maintaining the body's water balance and blood pressure. ADH primarily acts on the collecting ducts and, significantly, on the Distal Convoluted Tubule (DCT). Its main mechanism of action is to increase the water permeability of the cells in these segments by stimulating the insertion of aquaporin-2 water channels into the apical membrane. When ADH levels are high, more water is reabsorbed from the filtrate back into the bloodstream, leading to the production of concentrated urine. Conversely, when ADH levels are low, these tubules become less permeable to water, resulting in more dilute urine. This precise control over water permeability in the DCT and collecting ducts is vital for the body's ability to conserve water when dehydrated.

The incorrect option is 1: Bowman's capsule.
Bowman's capsule is the starting point of filtration in the nephron, where blood plasma is filtered to form a filtrate. Its primary function is filtration, not the reabsorption of water, and it is not directly affected by ADH in terms of water permeability.

The incorrect option is 3: PCT (Proximal Convoluted Tubule).
The Proximal Convoluted Tubule (PCT) is where the bulk of reabsorption of water, solutes, and nutrients occurs, but this reabsorption is largely unregulated by hormones. Water reabsorption in the PCT is obligatory and follows the reabsorption of solutes, rather than being under the direct control of ADH.

The incorrect option is 4: All of the above.
As explained, ADH primarily affects the DCT and collecting ducts. It does not significantly control water permeability in Bowman's capsule or the PCT. Therefore, stating "All of the above" would be incorrect because not all parts of the nephron listed are regulated by ADH for water permeability.