The function of renin is degradation of angiotensinogen. When the blood pressure falls ,the hormone renin secreted by juxtaglomerular cells of kidney plays an important role in reabsorption of sodium. Renin on reaching the blood, changes angiotensinogen (inactive compound released from liver) of plasma into angiotensin II via angiotensin I. Angiotensin II being a powerful vasaconstrictor, increases the geomerular blood pressure and thereby GFR.