1. The renin – angiotensin – aldosterone system or RAAS system regulates the blood pressure and fluid balance in the body. It also regulates the blood volume . Renin is responsible for elevating the blood pressure when it falls abruptly . When the blood pressure falls , renin is released from the kidney into the bloodstream . Renin acts to cleave angiotensinogen into angiotensin 1 and angiotensin 1 is converted into angiotensin II . Angiotensin II directly acts on the blood vessels causing the constriction and thus raising the blood pressure . The presence of aldosterone and vasopressin causes the kidney to retain sodium and thus increased sodium increases the blood pressure and blood volume .