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 the precursor protein angiotensinogen in the blood stream to angiotensin I. Angiotensin I is further processed by angiotensin-converting-enzyme (ACE) which is released from the lungs, and cleaves further amino acids from the protein resulting in angiotensin II. This protein helps the body to raise the blood pressure by many mechanisms including vasoconstriction, ADH secretion and aldosterone secretion. Aldosterone acts on the collecting duct and the distal tubule of the kidneys and increases the activity of ENaC channels. This will then allow for an increase in sodium uptake which will pass into the blood, with water following via osmosis, helping to raise blood pressure.