Antidiuretic hormone (ADH) primarily acts on which area of the nephron?

Antidiuretic hormone (ADH), also known as vasopressin, plays a crucial role in the regulation of water balance in the body by primarily acting on the collecting duct of the nephron. When ADH is released from the posterior pituitary gland in response to high plasma osmolality or low blood volume, it promotes the insertion of aquaporin-2 water channels into the cell membranes of the collecting duct's principal cells.

This insertion of aquaporins allows water to be reabsorbed back into the bloodstream, leading to concentrated urine and a decrease in urine volume. The action of ADH significantly affects the body’s ability to maintain osmotic balance and prevent dehydration. By increasing the permeability of the collecting duct to water, ADH effectively enhances water reabsorption, demonstrating its critical function in the renal system.

The roles of other segments of the nephron are different. For instance, the proximal convoluted tubule is primarily responsible for the reabsorption of sodium, chloride, and water, while the loops of Henle are involved in the concentration of urine and creating a gradient in the renal medulla. Thus, the specific action of ADH on the collecting duct makes it the correct answer for this question.