(a) Aldosterone (salt-retaining hormone) is the principal mineralocorticoid in humans. Its main function is to regulate the sodium content of the body. It is secreted when the sodium level is low.
(b) It acts on the kidneys to cause more sodium to be returned to the blood and more potassium to be excreted.
(c) As the sodium concentration in the blood increases, water follows it by osmosis, so the blood volume also increases. Thus, the effect of aldosterone is to increase both sodium and water in the blood.