USA Biology Olympiad (USABO) Practice Exam

What effect does antidiuretic hormone have in the kidneys?

1. Increases water absorption in the proximal tubule

2. Reduces free-water transfer to capillary blood

3. Increases free-water transfer from the distal convoluted tubule

4. Stimulates urine production

The correct answer is: Increases free-water transfer from the distal convoluted tubule

Antidiuretic hormone (ADH), also known as vasopressin, primarily functions to increase the permeability of the renal collecting ducts. This hormone promotes water reabsorption back into the bloodstream, specifically in the distal convoluted tubule and collecting ducts of the nephron. When ADH binds to its receptors in these areas, it triggers the insertion of aquaporin channels into the cell membranes, facilitating the reabsorption of free water into the bloodstream. This mechanism leads to the retention of water, resulting in more concentrated urine and minimizing urine output. In contrast to other choices, which either misrepresent the action of ADH or suggest effects not associated with this hormone, understanding the role of ADH highlights its crucial function in regulating the body's water balance and osmolarity.