What hormone is responsible for increasing blood volume by stimulating water reabsorption in the kidneys?

Vasopressin, also known as antidiuretic hormone (ADH), plays a crucial role in regulating blood volume and osmolarity. It is produced in the hypothalamus and released by the posterior pituitary gland. Its primary function is to increase water reabsorption in the kidneys, specifically in the collecting ducts. When vasopressin is released into the bloodstream, it binds to receptors on kidney cells, leading to the insertion of aquaporins (water channels) in the cell membranes. This process allows more water to be reabsorbed from the urine back into the bloodstream, effectively increasing blood volume and helping to concentrate urine.

This mechanism is vital for maintaining fluid balance in the body, especially in situations where fluid loss may lead to dehydration or low blood pressure. By enhancing water retention, vasopressin ensures that the body can maintain appropriate blood pressure and prevent excessive loss of fluids. In contrast, other hormones mentioned do not primarily regulate water reabsorption in this way. For instance, adrenaline is more associated with the fight-or-flight response, insulin regulates glucose levels in the blood, and aldosterone primarily focuses on sodium reabsorption and potassium excretion rather than directly influencing water reabsorption by itself.