Hormones that act on the kidneys
The brain and kidneys regulate the amount of water excreted by the body. When the blood volume is low, the concentration of solutes in the blood is high. When the hypothalamus senses this low blood volume and increased serum osmolality it synthesizes ADH, a small peptide molecule. The pituitary gland then releases ADH into the bloodstream and causes the kidneys to retain water by concentrating the urine and reducing urine volume. Water retention boosts blood volume and decreases serum osmolality.
Increased blood volume prevents the release of ADH. As a result, the kidneys retain less water, which dilutes the urine and increases urine output. As fluid leaves the body, blood volume decreases and serum osmolality increases. This stimulates the release of ADH and the cycle begins anew.
It is important for the body to keep sodium and water balances in check, and also to maintain healthy blood volume and blood pressure. The renin-angiotensin-aldosterone system assists with this in the following manner:
NOTE: If blood flow to the kidneys or the amount of sodium increases, less renin is produced in an attempt to normalize blood pressue.
When blood volume and blood pressure increase, the stretched atria release Atrial Natiuretic Peptide (ANP), a cardiac hormone. ANP promotes natriurisis by shutting down the renin-angiotensin-aldosterone system and causing vasodilation. As the blood vessels expand, urine excretion of sodium and water increases, stabilizing blood volume and blood pressure.