The Renin-Angiotensin-Aldosterone System (RAAS) is a hormone system that regulates blood pressure and fluid balance. It begins when the kidneys release renin in response to low blood pressure or low sodium levels. Renin converts angiotensinogen, a protein produced by the liver, into angiotensin I, which 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, and prompts the kidneys to retain sodium and water. Together, these actions help restore blood pressure and maintain fluid balance in the body.