The renin-angiotensin-aldosterone system is a series of reactions designed to help regulate blood pressure. When blood pressure falls (for systolic, to 100 mm Hg or lower), the kidneys release the enzyme renin into the bloodstream. Renin splits angiotensinogen, a large protein that circulates in the bloodstream, into pieces. One piece is angiotensin I. Angiotensin I, which is relatively inactive, is split into pieces by angiotensin-converting enzyme (ACE). One piece is angiotensin II, a hormone, which is very active. Angiotensin II causes the muscular walls of small arteries (arterioles) to constrict, increasing blood pressure. Angiotensin II also triggers the release of the hormone aldosterone from the adrenal glands and vasopressin (antidiuretic hormone) from the pituitary gland. Aldosterone and vasopressin cause the kidneys to retain sodium (salt). Aldosterone also causes the kidneys to excrete potassium. The increased sodium causes water to be retained, thus increasing blood volume and blood pressure.