What is the role of ADH in urine concentration?

Antidiuretic hormone (ADH), also known as vasopressin, plays a crucial role in regulating the body's fluid balance and urine concentration. When ADH is released, it prompts the kidneys to reabsorb water from the filtrate back into the bloodstream. This action occurs primarily in the collecting ducts of the nephron, where ADH increases the permeability of the membranes, allowing water to be reabsorbed more efficiently. As a result, less water is lost in the urine, leading to the production of more concentrated urine.

When ADH is present, it effectively facilitates water absorption, promoting conservation of water during periods of dehydration or when the body needs to maintain blood pressure and volume. Thus, the correct answer highlights ADH's primary function in enhancing the kidney's ability to retain water, which is essential for maintaining homeostasis within the body.