Which hormone primarily regulates water reabsorption in the kidneys?

The primary hormone that regulates water reabsorption in the kidneys is antidiuretic hormone (ADH), also known as vasopressin. ADH is produced in the hypothalamus and released from the posterior pituitary gland in response to high osmolarity of the blood or low blood volume. When ADH is released into the bloodstream, it acts primarily on the kidneys by binding to specific receptors in the collecting ducts.

This binding promotes the insertion of aquaporin-2 channels into the membranes of kidney cells, which increases the permeability of the collecting ducts to water. Consequently, more water is reabsorbed back into the bloodstream, decreasing urine output and helping to concentrate the urine. This process is crucial for maintaining the body's fluid balance and regulating blood pressure.

In contrast, hormones like aldosterone primarily regulate sodium reabsorption, insulin is involved in glucose metabolism, and epinephrine influences the fight-or-flight response but does not have a direct role in water reabsorption in the kidneys.