The Renin-Angiotensin system (RAS), also Renin-Angiotensin-Aldosterone system, is one of the mechanisms used by the body to regulate blood pressure. It comes into play when the blood pressure is too low, in disease it can happen when the blood pressure is not low which can lead to hypertension.
Low blood pressure is detected by the juxtaglomerular apparatus which secretes renin. This enzyme hydrolyses angiotensinogen in the blood stream to angiotensin I. Angiotensin I is further processed; angiotensin-converting-enzyme (ACE), released from the lungs, cleaves further amino acids from the protein resulting in angiotensin II. This protein help the body to raise the blood pressure by many mechanisms including vasoconstriction, ADH secretion and aldosterone secretion.