What hormone is primarily responsible for increasing blood volume and sodium reabsorption when blood volume is low?

The hormone primarily responsible for increasing blood volume and sodium reabsorption when blood volume is low is aldosterone. Aldosterone is a steroid hormone produced by the adrenal cortex, and its primary role is to regulate sodium and potassium levels in the body, which directly affects blood volume and blood pressure.

When blood volume decreases, the kidneys contribute to the secretion of renin, which ultimately leads to the production of angiotensin II. Angiotensin II then stimulates the adrenal glands to release aldosterone. Once aldosterone is in circulation, it acts on the renal tubules in the kidneys, promoting the reabsorption of sodium from the urine back into the bloodstream. Since water follows sodium osmotically, this reabsorption of sodium leads to an increase in blood volume, helping to restore blood pressure.

Antidiuretic hormone (ADH), also known as vasopressin, does play a role in regulating water balance and can impact blood volume, but its primary mechanism is through the reabsorption of water in the kidneys rather than directly influencing sodium levels, as aldosterone does. Cortisol is a glucocorticoid hormone that has various functions, including metabolism regulation and immune response; however, it does not primarily serve to increase sodium