The main difference between angiotensin I and II is the different structure and function. 1. Structure: Angiotensin I is a 10-peptide produced by angiotensinogen in the liver under the action of renin (a protein hydrolyzing enzyme); Angiotensin II is an 8-peptide produced by angiotensin I under the action of angiotensin-converting enzyme in the lungs and kidneys. 2. Function: Angiotensin I can stimulate the adrenal glands to secrete adrenaline, inhibit the increase of vascular endothelial permeability, etc., almost no direct vasoconstrictive effect. Angiotensin II stimulates aldosterone secretion, constricts small arteries throughout the body, and stimulates sympathetic nerves. Angiotensin I and II both belong to the angiotensin, both have synergistic effects, together maintain the body’s water-electrolyte balance, maintain the stability of the internal environment.