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The kidneys sense low blood pressure and low blood volume and release an enzyme called Renin. Renin converts angiotensinogen (a plasma protein made by the liver) into angiotensin I.
Angiotensin Converting Enzyme (ACE) converts angiotensin I into angiotensin II.
Angiotensin II has three main effects:
1. constricts arterioles to raise blood pressure.
2. stimulates the adrenal cortex to release aldosterone. Aldosterone then causes the kidneys to reabsorb more salt. This causes water to be retained and increases blood volume and thus blood pressure.
3. stimulates the posterior pituitary gland to release antidiuretic hormone, also known as vasopressin. Vasopressin causes the kidneys to retain more water, which raises blood volume and therefore raises blood pressure.

Drug Targets to Lower Blood Pressure in RAAS:
1. ACE inhibitors block ACE action
2. ARA (angiotensin receptor antagonists) block Angiotensin II from binding its targets
3. Aldo Antagonists
4. Vaptans inhibit vasopressin
5. Renin inhibitors