Renin-angiotensin-aldosterone system

Adrenal cortex (mnemonic GFR - ACD)

• zona glomerulosa (on outside): mineralocorticoids, mainly aldosterone
• zona fasciculata (middle): glucocorticoids, mainly cortisol
• zona reticularis (on inside): androgens, mainly dehydroepiandrosterone (DHEA)

Renin

• an enzyme that is released by the renal juxtaglomerular cells in response to reduced renal perfusion
• other factors that stimulate renin secretion include hyponatraemia, sympathetic nerve stimulation
• hydrolyses angiotensinogen to form angiotensin I

Angiotensin II

• angiotensin-converting enzyme (ACE) in the lungs converts angiotensin I → angiotensin II
• angiotensin II has a wide variety of actions:
• causes vasoconstriction of vascular smooth muscle leading to raised blood pressure and vasoconstriction of efferent arteriole of the glomerulus → increased filtration fraction (FF) to preserve GFR. Remember that FF = GFR / renal plasma flow
• stimulates thirst (via the hypothalamus)
• stimulates aldosterone and ADH release
• increases proximal tubule Na⁺/H⁺ activity

Aldosterone

• released by the zona glomerulosa in response to raised angiotensin II, potassium, and ACTH levels
• causes retention of Na⁺ in exchange for K⁺/H⁺ in distal tubule