Which hormone is responsible for the reabsorption of water in the kidneys?

Vasopressin, also known as antidiuretic hormone (ADH), plays a crucial role in regulating the body’s water balance by promoting the reabsorption of water in the kidneys. It does this primarily by acting on the collecting ducts of the nephrons, where it increases the permeability of the tubule walls to water. When the body is dehydrated or when blood osmolality is high, vasopressin is released from the posterior pituitary gland, leading to more water being reabsorbed back into the bloodstream instead of being excreted in urine. This mechanism helps to concentrate urine and maintain overall fluid balance in the body.

The other hormones listed have different functions: Aldosterone primarily regulates sodium and potassium balance and helps control blood pressure but does not directly focus on water reabsorption. Cortisol is a glucocorticoid involved in metabolism and the stress response and does not directly contribute to water reabsorption. Insulin is primarily involved in glucose metabolism and the regulation of blood sugar levels and does not directly influence water reabsorption in the kidneys. Thus, vasopressin is uniquely responsible for promoting water reabsorption, making it the correct choice in this context.