Renin angiotensin aldosterone system is a major blood pressure regulating mechanism. Renin is released from the kidney that binds to angiotensinogen that is release by liver into blood stream, and coverts it into angiotensin I. Lungs produce ACE that binds to angiotensin I and coverts it to Angiotensin II. That is a potent vasoconstrictor and increase salt and water retention by increasing the level of aldosterone. ACE inhibitors bind to Angiotensin I and prevent the binding of ACE preventing its conversion to Angiotensin II.