Renin-angiotensin-aldosterone system
Low blood pressure in, sodium and water back.
Vasoconstriction, then sodium and water retained
This mnemonic, unpacked word by word →
A blood-pressure loop rather than a fuel pathway. The kidney senses low perfusion, and the answer comes back as a constricted vessel and a reabsorbed sodium ion. Renin release is the controlled step, so every drug that blocks this system works at or after it.
Renin comes from the juxtaglomerular cells of the afferent arteriole, released when renal perfusion pressure falls, when the macula densa senses less sodium chloride, or on sympathetic β₁ stimulation. Angiotensinogen itself is made constantly by the liver, so renin release is what sets the pace.
Angiotensin-converting enzyme sits on pulmonary capillary endothelium, so most of this happens in the lung. ACE also degrades bradykinin, which is why ACE inhibitors cause a dry cough and ARBs do not.
A stimulus rather than a conversion. Angiotensin II acts on the zona glomerulosa, which builds aldosterone from cholesterol like any steroid. Angiotensin II also constricts vessels directly, triggers ADH and thirst, and constricts the efferent arteriole to hold up the filtration rate.