AAMC FL Practice Exam

What is the role of vasopressin (ADH) in the kidneys?

1. Stimulates the filtration of blood

2. Stimulates water reabsorption during low plasma osmolality

3. Stimulates water reabsorption during high plasma osmolality

4. Increases urine production

The correct answer is: Stimulates water reabsorption during high plasma osmolality

Vasopressin, also known as antidiuretic hormone (ADH), plays a crucial role in regulating the body’s water balance. In the kidneys, its primary function is to enhance water reabsorption in the collecting ducts when plasma osmolality is high, which typically indicates dehydration or increased solute concentration in the blood. When plasma osmolality rises, the osmoreceptors in the hypothalamus detect this increase and trigger the release of vasopressin from the posterior pituitary gland. Once released into the bloodstream, vasopressin travels to the kidneys and binds to specific receptors on the collecting duct cells. This action promotes the translocation of aquaporin-2 water channels to the cell membranes, increasing permeability to water. As a result, more water is reabsorbed back into the bloodstream, reducing urine output and concentrating the urine to help correct the high plasma osmolality. This mechanism is vital for maintaining homeostasis in the body, particularly during times of dehydration or high solute loads. In contrast, when plasma osmolality is low, vasopressin secretion decreases, leading to reduced water reabsorption, increased urine output, and further dilution of the body's plasma. Thus, the role