Notes · Physiology · 12 November 2026

Renin‑Angiotensin‑Aldosterone System (RAAS) regulation

Definition

• The Renin‑Angiotensin‑Aldosterone System (RAAS) is a hormone cascade that maintains circulatory volume, arterial pressure, and electrolyte balance.
• It is activated when renal perfusion pressure falls, when there is a decrease in sodium chloride delivery to the macula densa, or when sympathetic nervous activity rises.

Components of the System

Regulation – Stimuli for Renin Release

1. Baroreceptor (pressure) control

   • Decreased afferent arteriolar pressure → stretch‑sensitive JG cells sense low pressure → increased renin secretion.
   • Normal renal perfusion pressure ≈ 90‑100 mm Hg; a drop of ≥ 10 mm Hg markedly raises renin output.

2. Macula densa (tubular) control

   • The macula densa, part of the distal convoluted tubule, detects NaCl concentration in the tubular fluid.
   • Low NaCl (≤ 10 mEq/L) → reduced Na⁺/K⁺‑2Cl⁻ cotransporter activity → signals JG cells (via prostaglandin E₂) → renin release.
   • High NaCl (> 20 mEq/L) → adenosine release → afferent arteriole vasoconstriction and renin inhibition.

3 Sympathetic (neural) control

• Baroreceptor‑mediated sympathetic activation → β1‑adrenergic receptors on JG cells → cyclic AMP (cAMP) rise → renin granule exocytosis.
• Stress, exercise, or hemorrhage increase sympathetic tone and thus renin.

4. Hormonal and paracrine modulators
   • Prostaglandins (PGE₂, PGI₂): vasodilatory; stimulate renin.
   • Nitric oxide (NO): inhibits renin release.
   • Atrial natriuretic peptide (ANP): released from atrial myocytes when atrial stretch occurs; directly suppresses renin and opposes Ang II actions.
   • Angiotensin II (feedback inhibition): high circulating Ang II binds AT₁ receptors on JG cells → reduces renin secretion (negative feedback).

Step‑by‑Step Cascade

1. Trigger – ↓ renal perfusion pressure, ↓ NaCl at macula densa, or ↑ sympathetic activity.
2. Renin release – JG cells secrete renin (≈ 1 µg/min at baseline).
3. Angiotensinogen cleavage – Renin cleaves the single‑bond between leucine‑10 and valine‑11 of angiotensinogen, producing the decapeptide Ang I.
4. Conversion to Ang II – ACE, a zinc‑metallopeptidase, removes the C‑terminal dipeptide His‑Leu from Ang I, yielding the octapeptide Ang II.
5. Ang II actions –
   • Binds AT₁ receptors on vascular smooth muscle → vasoconstriction (↑ systemic vascular resistance).
   • Stimulates adrenal zona glomerulosa → aldosterone synthesis (↑ Na⁺ reabsorption, ↑ K⁺ excretion).
   • Acts on the hypothalamus → thirst and ADH release (↑ water reabsorption).
   • Enhances sympathetic outflow → further vasoconstriction and renin release (positive loop).
6. Aldosterone effects – Binds mineralocorticoid receptors in principal cells of the distal tubule and collecting duct → up‑regulates epithelial Na⁺ channel (ENaC) and Na⁺/K⁺‑ATPase → ↑ Na⁺ reabsorption, ↑ water retention, ↓ K⁺ excretion.
7. Feedback – Elevated arterial pressure and increased Ang II suppress further renin secretion (negative feedback).

Interaction with Other Systems

• Kidney‑Heart Axis – Ang II–mediated vasoconstriction raises afterload; chronic activation contributes to left‑ventricular hypertrophy.
• Renal Autoregulation – Myogenic response and tubuloglomerular feedback operate alongside RAAS; when RAAS is overactive, autoregulation may be overridden, leading to hyperfiltration.
• Endocrine Crosstalk – Cortisol can bind AT₁ receptors; 11β‑hydroxysteroid dehydrogenase type 2 (11β‑HSD2) in the distal nephron converts cortisol to cortisone, protecting mineralocorticoid receptors from cortisol excess.

Clinical Relevance (Why MBBS Students Must Know RAAS)

• Hypertension – Overactivity of Ang II (e.g., due to renal artery stenosis) is a common cause of secondary hypertension.
• Heart Failure – Compensatory RAAS activation initially maintains perfusion but later causes maladaptive remodeling and fluid overload.
• Nephrotic Syndrome & Chronic Kidney Disease – Sodium retention driven by aldosterone worsens edema; ACE inhibitors slow progression of renal disease.
• Pharmacology – ACE inhibitors, angiotensin‑II receptor blockers (ARBs), direct renin inhibitors, and aldosterone antagonists target specific steps of the cascade.

High‑Yield Exam Points

• Renin release is stimulated by ↓ perfusion pressure, ↓ NaCl at macula densa, and ↑ β1‑adrenergic activity.
• Angiotensin‑II acts on AT₁ receptors to cause vasoconstriction, aldosterone release, ADH secretion, and thirst.
• Negative feedback: High Ang II and high arterial pressure inhibit renin secretion.
• ACE is most abundant in pulmonary endothelium; this explains why ACE inhibitors cause cough (bradykinin accumulation).
• Aldosterone’s principal renal effect is to increase Na⁺ reabsorption via ENaC in the collecting duct; this also promotes K⁺ excretion (hyperkalaemia risk when blocked).
• ANP opposes RAAS by inhibiting renin and aldosterone; remember the “pressure‑natriuresis” concept.
• In renal artery stenosis, the affected kidney perceives low pressure → renin ↑ → systemic hypertension that is resistant to diuretics but responds to ACE inhibitors/ARBs.

Summary Flow Diagram (textual)

↓ Renal perfusion pressure / ↓ NaCl (macula densa) / ↑ Sympathetic → Renin → Angiotensinogen → Ang I → ACE → Ang II →

• Vasoconstriction (↑ SVR)
• Aldosterone (↑ Na⁺ reabsorption, ↓ K⁺)
• ADH (↑ water reabsorption)
• Thirst & SNS activation

↑ Arterial pressure & ↑ Ang II → Negative feedback on renin.

Practice questions

1. Which of the following is the most potent direct stimulus for renin release?

   • A) Increased atrial natriuretic peptide
   • B) Elevated plasma Angiotensin II
   • C) Decreased NaCl delivery to the macula densa
   • D) High circulating aldosterone

   Answer: C — low NaCl at the macula densa triggers prostaglandin‑mediated renin secretion.

2. ACE inhibitors commonly cause a dry cough because they:

   • A) Increase bradykinin degradation
   • B) Decrease bradykinin breakdown, leading to its accumulation
   • C) Directly stimulate cough receptors in the airway
   • D) Block β1‑adrenergic receptors

   Answer: B — ACE also degrades bradykinin; inhibition raises bradykinin levels, irritating the airway.

3. In a patient with renal artery stenosis, which laboratory finding is most characteristic?

   • A) Low plasma renin activity
   • B) High plasma renin activity
   • C) Low aldosterone with normal renin
   • D) Normal renin and aldosterone

   Answer: B — the ischemic kidney perceives low perfusion, markedly raising renin secretion.

More in Physiology

AI-assisted study material for exam revision. Always confirm with standard textbooks and current guidelines; not medical advice.