Which one of the following elements is involved in the control of water content of the blood?

Aldosterone is a key steroid hormone critical for maintaining salt and water balance. It regulates the concentration of minerals, like sodium and potassium, in the fluid outside your cells. When your sodium levels drop, or potassium levels rise, your body signals your adrenal glands to release aldosterone into your blood. Aldosterone signals your kidneys to reabsorb sodium into the blood, and acts on your sweat glands to reduce sodium loss through sweat.