The RASS system is the renin-angiotensin-aldosterone system, which is an important endocrine system for regulating blood pressure in the body. When the body bleeds heavily or blood pressure decreases, the renal blood flow decreases, stimulating the renal parietal cells to secrete more renin, which can convert angiotensinogen into angiotensin I, which has a weak blood pressure-raising effect. In the blood vessels, angiotensin I is converted into angiotensin II, which can strongly constrict small blood vessels and increase blood pressure while promoting secretion of aldosterone, which has the effect of sodium and potassium retention and increases blood volume. Angiotensin II can also constrict small veins and increase the return blood volume. Under the effect of angiotensin II, the return blood volume increases, blood pressure rises and the effective circulating blood volume is improved, which is the mechanism of action of the RASS system of the body, that is, the renin-angiotensin-aldosterone system.