The hormone produced by the kidneys that helps regulate blood pressure is renin.
Renin is released mainly by specialized juxtaglomerular cells in the kidney when blood pressure falls, sodium levels decline, or the sympathetic nervous system signals that circulation needs support. It begins the renin–angiotensin–aldosterone system, usually abbreviated RAAS, one of the body’s major blood-pressure control mechanisms.
Renin acts on a liver-produced protein called angiotensinogen, converting it into angiotensin I. An enzyme called angiotensin-converting enzyme, or ACE, then forms angiotensin II, which narrows blood vessels and stimulates aldosterone release from the adrenal glands. Aldosterone causes the kidneys to retain sodium and water, increasing blood volume and helping raise blood pressure.
Renin is often described as a hormone, although it is technically an enzyme that triggers a hormonal cascade. Angiotensin and aldosterone are related but are not the kidney-produced starting signal: angiotensin II is formed through enzymatic steps, while aldosterone is made by the adrenal cortex. ADH is produced in the hypothalamus and released by the pituitary.