The Renin-Angiotensin-Aldosterone System

Why the RAAS matters

• It is the body's primary long-term blood-pressure controller. While the baroreceptor reflex handles second-to-second corrections, the RAAS sets the sustained level of arterial pressure by controlling how much sodium and water the kidney keeps. Guyton's classic pressure-natriuresis model places renal sodium handling — and therefore the RAAS — at the center of chronic blood-pressure regulation.
• ACE inhibitors and ARBs are among the most-prescribed drugs in the world. Lisinopril alone is dispensed on the order of 80 to 90 million times a year in the United States. The RAAS is arguably the single most successfully drugged pathway in cardiovascular medicine, with four distinct blocking points in routine clinical use.
• It protects the kidney — and can wreck it. Angiotensin II preferentially constricts the efferent (downstream) glomerular arteriole to preserve filtration pressure when perfusion falls. That same mechanism is why ACE inhibitors reduce intraglomerular pressure and slow diabetic and hypertensive kidney disease, and also why they can precipitate acute kidney injury in bilateral renal artery stenosis.
• It drives the vicious cycle of heart failure. A failing heart lowers forward output; the kidney reads that as hypovolemia and cranks up the RAAS; the resulting salt retention and vasoconstriction increase congestion and afterload, worsening the failure. Interrupting this loop with ACE inhibitors, ARBs, aldosterone antagonists, and sacubitril-valsartan is a cornerstone of survival-improving therapy.
• Aldosterone links pressure to potassium and acid-base balance. By driving the epithelial sodium channel (ENaC) in the principal cells of the collecting duct, aldosterone couples sodium reabsorption to potassium and hydrogen ion secretion — which is why RAAS blockade tends to raise serum potassium, and why primary aldosteronism (Conn's syndrome) causes hypertension with hypokalemia.
• It explains a huge fraction of secondary hypertension. Renovascular hypertension (renal artery stenosis), primary aldosteronism, and renin-secreting tumors all act through this axis. Plasma renin activity and the aldosterone-to-renin ratio are frontline diagnostic tests for curable causes of high blood pressure.
• It intersected with COVID-19 biology. ACE2 — a homolog of ACE that converts angiotensin II into the vasodilatory angiotensin-(1-7) — is the cellular entry receptor for SARS-CoV-2, which put an obscure RAAS branch onto front pages and prompted intense study of whether ACE inhibitors and ARBs affected infection risk (large studies found no reason to stop them).

Common misconceptions

• "Renin is a hormone that raises blood pressure." Renin is an enzyme (an aspartyl protease), not a pressor hormone. It has essentially one physiological substrate — angiotensinogen — and does nothing to vessels directly. Its role is to be the rate-limiting, kidney-gated switch for the whole cascade; the actual pressor is angiotensin II.
• "ACE makes angiotensin from scratch." ACE performs one specific step: it removes the C-terminal dipeptide (His-Leu) from the decapeptide angiotensin I to yield the octapeptide angiotensin II. Renin has already done the committed, specific cleavage; ACE is a comparatively promiscuous dipeptidyl carboxypeptidase that also degrades bradykinin.
• "Aldosterone and ADH are the same thing." They are different hormones with different jobs. Aldosterone is a steroid from the adrenal cortex that drives sodium reabsorption (and potassium/H⁺ secretion) via ENaC and the Na⁺/K⁺-ATPase. ADH (vasopressin) is a peptide from the posterior pituitary that inserts aquaporin-2 channels to reabsorb pure water. Angiotensin II stimulates both, which is why they are often confused.
• "The lung just passes blood through." The pulmonary capillary endothelium is where the bulk of angiotensin I to II conversion happens, because ACE is densely expressed on that enormous vascular surface. The lungs are a major endocrine organ for the RAAS, not a passive conduit.
• "ACE inhibitors and ARBs do exactly the same thing." They converge on lower angiotensin II signaling but differ mechanistically. ACE inhibitors reduce angiotensin II production and raise bradykinin (hence cough); ARBs block the AT1 receptor and leave bradykinin alone (hence far less cough). ARBs also permit unopposed AT2-receptor signaling by circulating angiotensin II, a pharmacologically distinct situation.
• "The RAAS is one linear pathway." Modern physiology recognizes a counter-regulatory arm: ACE2 converts angiotensin II to angiotensin-(1-7), which signals through the Mas receptor to produce vasodilation and anti-fibrotic effects, opposing the classic ACE/AngII/AT1 axis. The system is a balance between a pressor arm and a protective arm.

How the RAAS works, step by step

The cascade is a relay of proteolytic clips, each performed by a different enzyme in a different tissue. Step 1 — the trigger. A fall in effective circulating volume is detected three ways at the kidney: reduced stretch of the afferent arteriole wall (the juxtaglomerular cells act as intrarenal baroreceptors), reduced NaCl delivery sensed by the macula densa of the distal tubule, and increased sympathetic tone acting on beta-1 adrenergic receptors. Any of these causes the granular juxtaglomerular (JG) cells to secrete renin. This is the rate-limiting step, so the entire system is controlled here.

Step 2 — angiotensinogen to angiotensin I. Renin, an aspartyl protease, cleaves circulating angiotensinogen — a 452-residue alpha-2-globulin continuously produced by the liver — between residues 10 and 11, releasing the inactive decapeptide angiotensin I (Asp-Arg-Val-Tyr-Ile-His-Pro-Phe-His-Leu). Angiotensinogen is normally present in large excess, so renin availability, not substrate, governs the rate.

Step 3 — angiotensin I to angiotensin II. As blood flows through the lungs (and across other endothelial beds), angiotensin-converting enzyme (ACE), a membrane-bound dipeptidyl carboxypeptidase, removes the C-terminal His-Leu dipeptide to produce the octapeptide angiotensin II. ACE is a zinc metalloprotease and is identical to kininase II, the enzyme that degrades the vasodilator bradykinin — the dual role that makes ACE inhibitors both effective and cough-prone.

Step 4 — angiotensin II acts on AT1 receptors. Angiotensin II binds the AT1 receptor, a Gq-coupled GPCR, on multiple targets. On vascular smooth muscle it triggers IP₃/DAG signaling, calcium influx, and powerful vasoconstriction, raising total peripheral resistance within seconds. On the proximal tubule it directly upregulates the Na⁺/H⁺ exchanger to reabsorb sodium. In the brain it stimulates thirst and vasopressin release, and it amplifies sympathetic outflow.

Step 5 — aldosterone and volume. Angiotensin II stimulates the zona glomerulosa of the adrenal cortex to synthesize and secrete aldosterone. Aldosterone, a mineralocorticoid steroid, enters principal cells of the distal tubule and collecting duct, binds the mineralocorticoid receptor, and upregulates the epithelial sodium channel (ENaC) and the basolateral Na⁺/K⁺-ATPase. The result is sodium and water retention (water follows sodium osmotically) and potassium/H⁺ secretion — expanding blood volume over hours. The combined effect of immediate vasoconstriction plus slower volume expansion restores blood pressure, and rising pressure and volume feed back to shut off renin, closing the loop.

The three enzymatic steps compared

Step	Renin	ACE	Aldosterone action
Molecule type	Aspartyl protease (enzyme)	Zn metalloprotease (kininase II)	Steroid hormone (mineralocorticoid)
Made / stored in	Juxtaglomerular cells (kidney)	Endothelium, esp. lung capillaries	Adrenal zona glomerulosa
Substrate / target	Angiotensinogen (liver globulin)	Angiotensin I → II; also bradykinin	Mineralocorticoid receptor / ENaC
Product / effect	Angiotensin I (decapeptide)	Angiotensin II (octapeptide)	Na⁺/water retention, K⁺/H⁺ loss
Rate-limiting?	Yes — gates whole system	No — usually in excess	Slow (gene-transcription) arm
Drug that blocks it	Aliskiren (direct renin inhibitor)	-pril ACE inhibitors	Spironolactone / eplerenone

ACE inhibitors vs ARBs vs aldosterone antagonists

Property	ACE inhibitors (-pril)	ARBs (-sartan)	Aldosterone antagonists
Molecular target	Angiotensin-converting enzyme	AT1 receptor	Mineralocorticoid receptor
Effect on angiotensin II	Less produced	Same level, but blocked at AT1	Unaffected (acts downstream)
Effect on bradykinin	Raised (kininase II blocked)	Unchanged	Unchanged
Dry cough	Common (~5–20%)	Rare	No
Angioedema risk	Small but real	Much lower	No
Signature side effect	Cough, hyperkalemia, teratogenic	Hyperkalemia, teratogenic	Hyperkalemia; gynecomastia (spironolactone)
Landmark example	Captopril (1st, 1981); lisinopril	Losartan (1st, 1995); valsartan	Spironolactone (RALES, 1999); eplerenone
Prime uses	HTN, HF, diabetic nephropathy	HTN, HF, ACEi-intolerant patients	HF, resistant HTN, Conn's syndrome

Famous experiments and history

• Tigerstedt and Bergman discover renin (1898). At the Karolinska Institute, Robert Tigerstedt and his student Per Bergman injected saline extracts of rabbit kidney into other rabbits and produced a durable rise in blood pressure. They named the responsible substance "renin" — the first demonstration that the kidney secretes a pressor factor, and the founding experiment of the entire field.
• The Goldblatt kidney (1934). Harry Goldblatt clamped the renal artery of a dog to reduce renal blood flow and produced sustained, reproducible hypertension — the "Goldblatt kidney" model. This proved that renal ischemia drives high blood pressure and set off the race to identify the humoral mediator, giving experimental teeth to Tigerstedt's renin.
• Two teams identify angiotensin (1939–1940). Working independently, Irvine Page and O.M. Helmer in Cleveland (calling it "angiotonin") and Eduardo Braun-Menéndez's group in Buenos Aires (calling it "hypertensin") showed that renin acted on a plasma substrate to generate a pressor peptide. The two names were later merged into the compromise term "angiotensin."
• Skeggs and the two forms of angiotensin (1950s). Leonard Skeggs and colleagues separated angiotensin I and angiotensin II, showed that a converting enzyme trimmed the first into the second, and established that angiotensin II is the active pressor — mapping the cascade that textbooks still teach.
• Snake venom to captopril (1965–1981). Sergio Ferreira showed that peptides in the venom of the Brazilian pit viper Bothrops jararaca potentiated bradykinin by inhibiting a converting enzyme. Using the venom peptide teprotide as a lead, David Cushman and Miguel Ondetti at Squibb rationally designed captopril in 1975 (FDA-approved 1981) — a landmark of structure-based drug design that made oral RAAS blockade possible and reshaped cardiovascular medicine.
• RALES proves aldosterone still matters (1999). The Randomized Aldactone Evaluation Study showed that adding low-dose spironolactone to standard heart-failure therapy cut mortality by about 30 percent, proving that residual aldosterone signaling ("aldosterone escape") is clinically harmful even in patients already on ACE inhibitors — and cementing aldosterone blockade as a fourth RAAS drug target.