Blood Pressure Compensation Mechanisms (Hypotension)

Definition

• Hypotension = systolic BP < 100 mmHg.

  • Severe hypotension = ~80 mmHg (e.g., hypovolemic shock).

• Formula:
  Blood Pressure (BP) = Cardiac Output (CO) × Total Peripheral Resistance (TPR)

  • CO = HR × Stroke Volume (SV)

Detection of Low BP

• Baroreceptors (stretch/pressure receptors):

  • Aortic sinus → CN X (Vagus nerve) → Nucleus Tractus Solitarius (NTS, medulla).

  • Carotid sinus → CN IX (Glossopharyngeal nerve) → NTS.

• Low BP → ↓ vessel stretch → ↓ baroreceptor firing → NTS detects low BP.

Neural Compensation Pathways

1. Cardiac Accelerator Center (Sympathetic)

• Location: Medulla → spinal cord (T1–T5).

• Effect:

  • Norepinephrine → β1 receptors on SA node & AV node → ↑ HR.

  • Norepinephrine → β1 receptors on myocardium → ↑ contractility → ↑ SV.

• Net: ↑ HR + ↑ SV = ↑ CO = ↑ BP.

2. Vasomotor Center (Sympathetic)

• Fibers → arterioles (tunica media).

• Norepinephrine → α1 receptors → vasoconstriction → ↑ TPR.

• Net: ↑ TPR = ↑ BP.

3. Cardiac Inhibitory Center (Parasympathetic, Vagus)

• Normally slows HR.

• Inhibited during hypotension → prevents HR decrease.

Hormonal Compensation Pathways

Adrenal Medulla (Sympathetic)

• Releases 80% epinephrine, 20% norepinephrine.

• Effects mimic direct SNS: ↑ HR, ↑ contractility, ↑ vasoconstriction.

Renin–Angiotensin–Aldosterone System (RAAS)

1. Trigger: Low BP detected by JG cells (kidney).

   • Also stimulated by β1 activation (SNS).

   • JG cells release Renin.

2. Renin converts Angiotensinogen (liver) → Angiotensin I.

3. ACE (lungs) converts Angiotensin I → Angiotensin II.

4. Angiotensin II actions:

   • Adrenal cortex (zona glomerulosa): → Aldosterone release.

   • Hypothalamus: → ADH release + thirst stimulation.

   • Direct vasoconstriction.

Aldosterone

• Site: Distal tubule/collecting duct.

• Effects:

  • ↑ Na⁺ reabsorption (via new Na⁺ channels + Na⁺/K⁺ ATPase).

  • Water follows Na⁺ (requires ADH).

  • ↑ K⁺ secretion, ↑ H⁺ excretion.

• Net: ↑ Blood volume → ↑ SV → ↑ CO → ↑ BP.

Antidiuretic Hormone (ADH / Vasopressin)

• Site: Collecting ducts (V2 receptors).

• Mechanism: ↑ cAMP → aquaporin-2 insertion → ↑ water reabsorption.

• Net: ↑ Blood volume → ↑ EDV → ↑ SV → ↑ CO → ↑ BP.

Thirst Mechanism

• Angiotensin II → hypothalamic thirst center → ↑ fluid intake.

• Net: ↑ Blood volume → ↑ BP.

Summary of Compensation Responses

1. Neural (fast, seconds–minutes):

   • ↑ HR (β1), ↑ contractility (β1), vasoconstriction (α1).

2. Hormonal (slower, minutes–hours):

   • RAAS: Ang II, aldosterone, ADH, thirst.

3. Overall Goal:

   • ↑ HR, ↑ SV, ↑ TPR, ↑ blood volume = ↑ BP back to normal.