The Renin-Angiotensin System (RAS) is a hormone system that regulates blood pressure and fluid balance in the body. It begins when the kidneys release an enzyme called renin in response to low blood pressure or low sodium levels. Renin converts a protein called angiotensinogen, produced by the liver, into angiotensin I. Angiotensin I is then converted into angiotensin II by an enzyme in the lungs. Angiotensin II has several effects: it constricts blood vessels to increase blood pressure, stimulates the release of aldosterone from the adrenal glands to promote sodium retention, and triggers thirst to increase fluid intake.