Which hormone primarily stimulates water reabsorption in the kidneys?

Vasopressin, also known as antidiuretic hormone (ADH), is the primary hormone responsible for stimulating water reabsorption in the kidneys. It acts on the collecting ducts of the nephrons, where it increases the permeability of the duct walls to water. This allows more water to be reabsorbed from the urine back into the bloodstream, effectively concentrating the urine and helping to maintain hydration and electrolyte balance in the body.

When the body needs to conserve water, such as during dehydration or in response to high plasma osmolality, vasopressin is released from the posterior pituitary gland into the bloodstream. It binds to specific receptors (V2 receptors) on the cells of the collecting ducts, triggering a cascade of events that leads to the insertion of water channels called aquaporins into the cell membranes, enhancing water reabsorption.

Other hormones, while important in renal function and fluid balance, do not primarily focus on water reabsorption. Insulin is mainly involved in glucose metabolism. Cortisol plays a role in the stress response and has various effects on metabolism and inflammation but is not directly involved in promoting water reabsorption. Aldosterone primarily regulates sodium reabsorption and potassium secretion in the kidneys, indirectly