Nephrology
Renin–Angiotensin–Aldosterone System
The hormone cascade that defends blood pressure
The renin–angiotensin–aldosterone system (RAAS) is the body's master hormone cascade for defending blood pressure and blood volume. When the kidney senses that perfusion has fallen, its juxtaglomerular cells release renin into the blood. Renin cleaves liver-derived angiotensinogen into angiotensin I, which the angiotensin-converting enzyme (ACE) — concentrated in the lungs — converts into angiotensin II. Angiotensin II is a potent vasoconstrictor that immediately raises pressure and, in parallel, drives the adrenal gland to secrete aldosterone, which makes the kidney retain sodium and water. The result is restored volume and pressure within minutes to hours — and the same pathway, when chronically over-activated, becomes the central driver of hypertension and heart failure.
- Renin triggerRenal perfusion below ~90 mmHg
- ACE location~99% in pulmonary capillary endothelium
- Angiotensin II half-life~1-2 minutes
- Aldosterone onsetHours (genomic action)
- Primary aldosteronism5-10% of all hypertension
- Drug targetsACE-i, ARB, DRI, MRA
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The cascade, step by step
The RAAS is best understood as a relay: each molecule activates the next, and the signal is amplified at every handoff. It begins in the kidney, runs through the liver and the lungs, ends at the adrenal gland and the distal nephron, and closes the loop by changing the very blood pressure that started it.
1. Renin. The trigger molecule is renin, an enzyme — not a hormone in the classical sense — stored in the granular juxtaglomerular (JG) cells that wrap the afferent arteriole as it enters the glomerulus. Three signals open the renin tap: a drop in afferent arteriolar pressure (the cells act as intrarenal baroreceptors and fire when perfusion falls below roughly 90 mmHg); a fall in sodium chloride delivery sensed by the macula densa of the distal tubule; and sympathetic stimulation through beta-1 adrenergic receptors. Renin is the rate-limiting step of the entire cascade, which is why its plasma activity is the quantity clinicians measure to gauge how "switched on" the system is.
2. Angiotensinogen → angiotensin I. Renin's only meaningful substrate is angiotensinogen, a large glycoprotein the liver continuously secretes into the blood. Renin cleaves it to release the decapeptide angiotensin I, which is biologically nearly inert — a pro-hormone waiting to be activated.
3. ACE → angiotensin II. As blood passes through the pulmonary circulation, angiotensin-converting enzyme — bound to the luminal surface of capillary endothelium, with roughly 99% of the body's ACE activity in the lungs — snips two more amino acids off angiotensin I to make the octapeptide angiotensin II. This is the active effector. ACE simultaneously degrades bradykinin, a vasodilator; this dual role is the key to understanding the side-effect profile of ACE inhibitors.
4. Angiotensin II's actions. Acting through the AT1 receptor, angiotensin II works on multiple fronts within minutes. It constricts systemic arterioles to raise blood pressure. It selectively constricts the glomerular efferent arteriole to preserve filtration pressure when renal perfusion is low. It acts on the proximal tubule to reabsorb sodium and water directly. It stimulates thirst and the release of antidiuretic hormone (ADH/vasopressin) from the posterior pituitary. And it stimulates the adrenal cortex to make aldosterone.
5. Aldosterone. Produced in the zona glomerulosa of the adrenal cortex, aldosterone is a steroid that acts slowly — over hours — because it works by gene transcription. In the principal cells of the cortical collecting duct it increases epithelial sodium channels (ENaC) on the luminal side and Na⁺/K⁺-ATPase pumps on the basolateral side. The net effect is reabsorption of sodium (and, osmotically, water) in exchange for the secretion of potassium and hydrogen ions. This is the volume-expanding arm of the system.
The two effectors divide the labor neatly: angiotensin II is the fast, broad pressure-and-tone arm; aldosterone is the slow, focused volume-and-electrolyte arm. Together they raise blood pressure two ways at once — by squeezing the vessels and by filling the tank.
The numbers that matter
The RAAS operates across a wide dynamic range. Plasma renin activity in a normal, sodium-replete adult sits around 0.5–2.0 ng/mL/hr and can rise more than tenfold with volume depletion or upright posture. Angiotensin II is present at picomolar concentrations and has a plasma half-life of only one to two minutes, which is why its effects are tightly coupled to ongoing production rather than to a circulating reservoir. Aldosterone normally circulates at 5–20 ng/dL in an upright, sodium-replete person, and the body of a healthy adult secretes on the order of 100–150 micrograms per day.
The single most useful derived number in clinical practice is the aldosterone-to-renin ratio (ARR), the screening test for primary aldosteronism. When aldosterone is high but renin is suppressed — an ARR above roughly 20–30 depending on the assay — the adrenal gland is making aldosterone autonomously rather than in response to the cascade, and the clinician looks for a Conn adenoma or bilateral adrenal hyperplasia.
Where RAAS goes wrong — and how we block it
Almost everything that makes the RAAS clinically important comes from the same fact: a system built to rescue blood pressure in a hemorrhage is the wrong system to have chronically switched on in a person with a failing heart or stiff arteries. The angiotensin II that saves a trauma patient also drives the vascular and cardiac remodeling, fibrosis, and sodium retention that worsen chronic disease.
Hypertension. RAAS over-activity contributes to a large share of high blood pressure, and the cascade offers four distinct drug targets, each interrupting the relay at a different point.
- Direct renin inhibitors (aliskiren) block the first step.
- ACE inhibitors (lisinopril, ramipril, enalapril) block the conversion of angiotensin I to angiotensin II — and, because ACE also degrades bradykinin, they raise bradykinin and cause a dry cough in 10–20% of patients, with rare angioedema.
- Angiotensin receptor blockers (ARBs) (losartan, valsartan, candesartan) block the AT1 receptor so angiotensin II cannot act; they spare bradykinin and so rarely cause cough.
- Mineralocorticoid receptor antagonists (MRAs) (spironolactone, eplerenone) block aldosterone's receptor in the collecting duct, the cornerstone treatment for both resistant hypertension and primary aldosteronism.
Heart failure. In a failing heart, cardiac output falls, the kidney misreads this as low volume, and the RAAS switches on hard — raising afterload (angiotensin II vasoconstriction) and preload (aldosterone-driven volume retention), both of which further stress the heart. Breaking this vicious cycle with ACE inhibitors or ARBs, MRAs, and now the angiotensin-receptor-neprilysin inhibitor (ARNI, sacubitril/valsartan) is one of the best-proven mortality benefits in all of cardiology.
Diabetic and chronic kidney disease. Angiotensin II constricts the efferent arteriole, raising the pressure inside the glomerulus. Over years this intraglomerular hypertension damages the filter and drives proteinuria. ACE inhibitors and ARBs lower that pressure and are renoprotective — they slow the progression of diabetic nephropathy independent of their effect on systemic blood pressure.
Renal artery stenosis. When a kidney is starved of blood by a narrowed artery, it sustains its filtration entirely through angiotensin II–mediated efferent constriction. Give such a patient an ACE inhibitor or ARB and filtration can collapse — a sharp rise in creatinine after starting these drugs is a classic clue to bilateral renal artery stenosis.
The electrolyte signature. Because aldosterone trades sodium for potassium and hydrogen, the RAAS leaves an electrolyte fingerprint. Excess aldosterone produces hypokalemia and metabolic alkalosis; blocking the RAAS does the opposite, and hyperkalemia is the most important metabolic hazard of ACE inhibitors, ARBs, and MRAs — particularly in chronic kidney disease or when these agents are combined.
Primary vs secondary hyperaldosteronism
The clearest illustration of how the RAAS behaves as a loop is the comparison between the two ways the body can end up with too much aldosterone. The difference comes down to one question: is renin high or low?
| Feature | Primary hyperaldosteronism (Conn) | Secondary hyperaldosteronism |
|---|---|---|
| Driving lesion | Autonomous adrenal adenoma or bilateral hyperplasia | Low effective circulating volume activating the RAAS |
| Renin level | Suppressed (low) | Elevated (high) |
| Aldosterone level | High | High |
| Aldosterone-to-renin ratio | High (key diagnostic clue) | Normal or low |
| Blood pressure | Hypertension (often resistant) | Variable — high in renal artery stenosis, low/normal in heart failure or cirrhosis |
| Potassium | Often low (hypokalemia) | Often low; modified by underlying disease |
| Typical causes | Conn adenoma, idiopathic bilateral hyperplasia | Heart failure, cirrhosis, nephrotic syndrome, renal artery stenosis |
| First-line treatment | Adrenalectomy (adenoma) or MRA (hyperplasia) | Treat the underlying cause; MRA as adjunct |
Why the system is built this way
Evolution tuned the RAAS for a world of dehydration, hemorrhage, and salt scarcity, where the ability to clamp down vessels and hoard sodium meant survival. Every feature reflects that priority: redundant triggers (pressure, salt delivery, sympathetic tone) so the system rarely fails to activate; amplification at each enzymatic step so a small renin signal produces a large pressor response; and a built-in negative feedback loop, since the rise in blood pressure and volume that the system produces is exactly the signal that shuts renin off. The modern problem is mismatch — a salt-rich, sedentary environment keeps a survival system chronically engaged, and the pharmacology of cardiovascular medicine is largely the art of turning it back down.
This article is educational and is not medical advice. Diagnosis and treatment of hypertension, heart failure, and electrolyte disorders should be guided by a qualified clinician and individual testing.
Frequently asked questions
What triggers renin release?
Three independent signals converge on the juxtaglomerular cells of the afferent arteriole. First, a fall in renal perfusion pressure stretches these cells less, and they respond by secreting renin — a baroreceptor-like mechanism sensitive to pressures below roughly 90 mmHg. Second, the macula densa in the distal tubule senses low sodium chloride delivery (a proxy for low filtration) and signals renin release via prostaglandins and adenosine. Third, sympathetic nerves acting on beta-1 adrenergic receptors stimulate renin directly — which is why beta-blockers lower renin. Any state of low blood volume, low blood pressure, or sympathetic activation turns the system on.
What does angiotensin II actually do?
Angiotensin II is the active effector of the RAAS and acts within seconds to minutes through AT1 receptors. It is a powerful arteriolar vasoconstrictor, raising systemic vascular resistance and blood pressure. It stimulates the adrenal zona glomerulosa to release aldosterone, promotes thirst and ADH release through the hypothalamus, and constricts the efferent arteriole of the glomerulus to preserve filtration pressure when perfusion is low. It also drives proximal tubule sodium reabsorption directly and, over the long term, promotes cardiac and vascular remodeling and fibrosis — the maladaptive effects that drugs blocking the system are designed to prevent.
How is aldosterone different from angiotensin II?
Angiotensin II is a peptide that acts fast and broadly: vasoconstriction, thirst, and triggering aldosterone. Aldosterone is a steroid hormone that acts slowly — over hours — because it works by entering cells, binding the mineralocorticoid receptor, and changing gene transcription. Its main job is in the principal cells of the cortical collecting duct, where it increases the number of epithelial sodium channels (ENaC) and Na/K-ATPase pumps, reabsorbing sodium (and therefore water) while secreting potassium and hydrogen ions. So angiotensin II is the rapid pressure-and-tone arm; aldosterone is the slow volume-and-electrolyte arm of the same cascade.
How do ACE inhibitors and ARBs differ?
ACE inhibitors (lisinopril, enalapril, ramipril) block angiotensin-converting enzyme, so less angiotensin II is made. Because ACE also degrades bradykinin, ACE inhibitors raise bradykinin levels, producing the characteristic dry cough in about 10 to 20 percent of patients and, rarely, angioedema. ARBs (losartan, valsartan, candesartan) instead block the AT1 receptor directly, so angiotensin II can still form but cannot act; they do not raise bradykinin and therefore rarely cause cough. Both lower blood pressure, reduce proteinuria, and protect the kidney and heart, but they should not be combined, as dual blockade increases hyperkalemia and acute kidney injury without added benefit.
Why can RAAS blockers raise potassium and creatinine?
Aldosterone normally drives potassium excretion in the collecting duct, so blocking the RAAS reduces aldosterone and causes potassium to rise — hyperkalemia is the most important metabolic side effect, especially in chronic kidney disease or when combined with potassium-sparing diuretics. Creatinine can rise too because angiotensin II maintains glomerular filtration pressure by constricting the efferent arteriole; removing that constriction lowers the filtration fraction. A creatinine rise up to about 30 percent is expected and acceptable, but a larger jump can signal bilateral renal artery stenosis, in which the kidneys depend entirely on angiotensin II to maintain filtration.
What is the difference between primary and secondary hyperaldosteronism?
In primary hyperaldosteronism (Conn syndrome), an adrenal adenoma or bilateral hyperplasia autonomously overproduces aldosterone. Because volume is expanded and pressure is high, renin is suppressed — so the diagnostic clue is a high aldosterone-to-renin ratio with low renin. It is now thought to cause 5 to 10 percent of all hypertension and classically presents with resistant hypertension, sometimes with low potassium. In secondary hyperaldosteronism, the whole RAAS is activated by low effective circulating volume — as in heart failure, cirrhosis, or renal artery stenosis — so both renin and aldosterone are high. Distinguishing them rests on measuring renin alongside aldosterone.