What hormone is primarily responsible for promoting water reabsorption in the kidneys?

The hormone primarily responsible for promoting water reabsorption in the kidneys is ADH, or antidiuretic hormone. ADH is produced in the hypothalamus and stored in the posterior pituitary gland. When the body senses dehydration or an increase in blood osmolality, ADH is released into the bloodstream.

Once in the kidneys, ADH acts on the collecting ducts and increases their permeability to water. This allows more water to be reabsorbed back into the bloodstream, thus concentrating the urine and retaining water. The overall effect of ADH is to decrease urine output and help maintain fluid balance in the body, especially during times of dehydration.

The other hormones listed have different primary functions. Insulin primarily regulates glucose levels in the blood, cortisol is involved in stress response and metabolism, and aldosterone mainly regulates sodium reabsorption and potassium excretion in the kidneys, indirectly affecting water retention but not directly promoting water reabsorption like ADH does.