ANGIOTENSIN II
As blood pressure decreases, the cells of the juxtaglomerular apparatus release the enzyme renin and activate the renin-angiotensin-aldosterone pathway. The juxtaglomerular apparatus is a small group of cells situated in the area where the renal tubule links up with the afferent and efferent arterioles. Renin converts angiotensinogen into angiotensin I. Angiotensin converting enzyme (ACE) then converts angiotensin I into angiotensin II, a peptide hormone that is the active form. Angiotensin II has the following effects:

• Increases the synthesis and release of aldosterone
• Raises blood pressure directly by constricting blood vessels
• Stimulates sodium reabsorption by the proximal convoluted tubules
• May stimulate the posterior pituitary to release ADH

These changes assist in restoring extracellular fluid volume and in stabilizing blood pressure.

ACE inhibitors are sometimes administered to block the production of angiotensin II in people with elevated blood pressure.