RAAS in blood pressure control

RAAS Triggers - The Pressure Alarm

Renin release from juxtaglomerular (JG) cells is stimulated by three main triggers:

• ↓ Renal Blood Flow: JG cells act as intra-renal baroreceptors, sensing low perfusion.
• ↓ NaCl to Distal Tubule: Macula densa cells sense decreased sodium chloride delivery.
• ↑ Sympathetic Tone: β1-adrenergic receptors on JG cells are activated by norepinephrine.

⭐ The macula densa senses low NaCl delivery to the distal tubule, signaling JG cells to release renin.

RAAS Cascade - A Chain Reaction

The liver produces angiotensinogen. Kidneys release renin, which converts it to angiotensin I. Angiotensin-Converting Enzyme (ACE) from the lungs then transforms it into the potent vasoconstrictor, angiotensin II, which acts on various tissues to raise blood pressure.

$Angiotensinogen \xrightarrow{Renin} Angiotensin\ I \xrightarrow{ACE} Angiotensin\ II$

⭐ ACE, found primarily in pulmonary and renal endothelium, also breaks down bradykinin, a vasodilator. This dual action further elevates blood pressure.

Angiotensin II Effects - The Master Squeezer

Acts via Gq-coupled AT1 receptors to orchestrate a systemic "squeeze" on blood vessels and fluid volume, raising blood pressure.

• Vascular System: Potent vasoconstriction of arterioles → ↑ Systemic Vascular Resistance (SVR).
• Adrenal Gland: Stimulates aldosterone release from the zona glomerulosa → ↑ Na⁺ and H₂O reabsorption.
• Brain:
  • ↑ ADH (vasopressin) secretion from the posterior pituitary.
  • Stimulates the hypothalamic thirst center.
• Kidneys:
  • Constricts the efferent arteriole.

⭐ Angiotensin II preferentially constricts the efferent arteriole, which helps maintain GFR during low renal blood flow states.

Aldosterone's Role - The Salt Saver

• Source: Released from the adrenal cortex (zona glomerulosa).
• Target: Acts on principal cells within the kidney's collecting ducts.
• Mechanism:
  • Upregulates epithelial Na+ channels (ENaC) on the apical side.
  • Boosts Na+/K+ ATPase pumps on the basolateral side.
• Net Effect:
  • Promotes robust Na+ (salt) and water retention.
  • Increases K+ excretion into the urine.
  • Ultimately raises blood volume and blood pressure.

⭐ Aldosterone works on the principal cells of the collecting duct to increase Na+ reabsorption and K+ secretion.

RAAS Pharmacology - Hacking the System

📌 'A-pril' fools the ACE; '-sartan' blocks the 'czar' Angiotensin II.

Targets key pathway steps to lower blood pressure. Major classes:

• ACE Inhibitors (-pril): Block conversion of Angiotensin I to II.
• ARBs (-sartan): Block Angiotensin II receptors directly.
• Aldosterone Antagonists (Spironolactone): Block aldosterone effects.
• Direct Renin Inhibitors (Aliskiren): Inhibit renin's activity.

⭐ ACE inhibitors' characteristic dry cough is due to the accumulation of bradykinin; ARBs do not have this side effect.

Key Risks: Hyperkalemia, hypotension, and Angioedema (especially with ACEi).

High‑Yield Points - ⚡ Biggest Takeaways

• Renin release from the kidney's juxtaglomerular apparatus is the rate-limiting step, triggered by ↓ renal perfusion.
• Angiotensin II is the primary effector, causing potent vasoconstriction (↑ SVR) and stimulating aldosterone release from the adrenal cortex.
• Aldosterone acts on the distal convoluted tubule & collecting duct to ↑ Na+ and H₂O reabsorption, expanding blood volume.
• ACE from the lungs converts Ang I to Ang II and also breaks down bradykinin (implicated in ACE inhibitor cough).
• The system defends against hypotension, making it a key target for antihypertensive drugs (e.g., ACE inhibitors, ARBs).