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Hypertension – Study Notes

Definition

• Hypertension = high blood pressure

  • Systolic BP > 140 mmHg → systolic hypertension

  • Diastolic BP > 90 mmHg → diastolic hypertension

  • Normal: 120/80 mmHg

Sensors – Baroreceptors

• Location:

  • Carotid sinus (internal carotid bifurcation) → CN IX (glossopharyngeal)

  • Aortic sinus (aortic arch) → CN X (vagus)

• Mechanism:

  • Increased BP stretches vessel walls → activates stretch-sensitive Na⁺ channels → depolarization → action potentials → medulla (Nucleus Tractus Solitarius, NTS).

Brainstem Control Centers

1. Cardiac Accelerator Center (sympathetic)

   • Normally ↑ HR & contractility.

   • In hypertension → inhibited.

2. Cardiac Inhibitory Center (parasympathetic; vagus)

   • Acts on SA node (right vagus) & AV node (left vagus).

   • Releases ACh → binds M2 receptors → Gi pathway:

     • ↓ cAMP → ↓ Ca²⁺ entry.

     • Opens K⁺ channels → hyperpolarization.

   • Result: negative chronotropic effect (↓ HR).

   • ↓ HR → ↓ cardiac output (CO) → ↓ BP.

3. Vasomotor Center

   • C1 area = vasoconstriction.

   • A1 area = vasodilation.

   • In hypertension → C1 inhibited, A1 stimulated.

   • ↓ Sympathetic outflow → ↓ NE release → vasodilation (relaxation of tunica media).

   • ↑ Radius → ↓ Total Peripheral Resistance (TPR) → ↓ BP (mainly diastolic).

Sympathetic Suppression

• ↓ Sympathetic drive → ↓ NE/Epi release from adrenal medulla.

• Less α1 stimulation on vessels → vasodilation.

• Less β1 stimulation on heart → ↓ HR & ↓ contractility.

Hormonal Control

1. Renin-Angiotensin-Aldosterone System (RAAS) – in low BP

   • Renin → Ang I → Ang II → ↑ BP (via vasoconstriction, aldosterone, ADH, thirst).

2. In Hypertension: Atrial Natriuretic Peptide (ANP)

   • Released by atrial stretch (↑ left atrial pressure).

   • Blocks Ang II actions:

     • Inhibits vasoconstriction → vasodilation.

     • Inhibits aldosterone → ↓ Na⁺/water reabsorption in kidneys → natriuresis + diuresis.

     • Inhibits ADH → ↓ water reabsorption in collecting ducts.

     • Inhibits thirst → ↓ fluid intake.

   • Net effect: ↓ blood volume → ↓ EDV → ↓ stroke volume → ↓ CO → ↓ BP.

Key Equations

• BP = CO × TPR

• CO = HR × SV

• Poiseuille’s Law: Resistance ∝ 1 / r⁴

  • ↑ Radius → ↓ Resistance → ↓ BP

Summary of Compensation in High BP

1. Baroreceptors sense stretch → send signals to NTS.

2. Inhibition of sympathetic centers:

   • ↓ HR (cardiac accelerator off).

   • Vasodilation (vasomotor C1 off).

   • ↓ NE/Epi release.

3. Stimulation of parasympathetic center:

   • Vagus → ACh on SA/AV nodes → ↓ HR (negative chronotropy).

4. Hormonal adjustment:

   • ANP release → blocks Ang II, aldosterone, ADH, thirst → ↓ volume and resistance.

Net effect: Lower cardiac output + lower resistance → blood pressure normalizes.