Angiotensinogen is an $\alpha$-globulin protein produced by liver. Renin serves as an enzyme in the conversion of the plasma protein angiotensinogen into angiotensin. This protein stimulates the adrenal cortex to produce aldosterone which acts on the cells of the ascending limb of the loop of Henle and increases the rate of reabsorption of Na⁺ . Reabsorption of Na⁺ brings about the uptake of an osmotically equivalent amount of water. Absorption of sodium and water increases blood volume and pressure.