Blood Pressure Regulation

The Core Equation

1. Cardiac Output

• Stroke volume is primarily determined by filling pressure (preload), which depends on blood volume and sodium homeostasis.
• Heart rate and myocardial contractility are controlled by the α- and β-adrenergic systems. β1-adrenergic stimulation increases both heart rate and contractility; α1-adrenergic stimulation raises vascular tone.

2. Peripheral (Vascular) Resistance

3. Sodium Homeostasis and Blood Volume

4. The Renin-Angiotensin-Aldosterone System (RAAS)

1. Renin is released from renal juxtaglomerular cells (surrounding the afferent arterioles) in response to:
   • Low blood pressure/flow in afferent arterioles
   • Elevated circulating catecholamines
   • Low sodium in the distal convoluted tubule (e.g., when GFR falls)
2. Renin cleaves angiotensinogen (made by liver) → Angiotensin I
3. ACE (found mainly in pulmonary endothelium) converts Angiotensin I → Angiotensin II
4. Angiotensin II raises BP by three mechanisms:
   • Direct vasoconstriction (raises peripheral resistance)
   • Stimulates aldosterone secretion from the adrenal cortex
   • Directly increases tubular sodium resorption in the kidney
5. Aldosterone acts on the distal tubule ENaC channels → resorbs Na⁺ and water → increases blood volume → raises BP

• Robbins Cotran Pathologic Basis of Disease, p. 462-463

5. Counter-Regulatory: Natriuretic Peptides

• ANP (Atrial Natriuretic Peptide): released constitutively from atrial myocardium
• BNP (B-type Natriuretic Peptide): released from ventricular myocardium in volume overload states

• Causing systemic vasodilation
• Inhibiting sodium resorption in distal renal tubules → natriuresis and diuresis

6. Short-Term Neural Control: The Baroreceptor Reflex

• Baroreceptors in the carotid sinus and aortic arch detect wall stretch (i.e., arterial pressure).
• A rise in BP → increased baroreceptor discharge → inhibits sympathetic outflow and enhances vagal (parasympathetic) tone → heart rate slows, vessels dilate → BP falls.
• A fall in BP → decreased baroreceptor discharge → allows sympathetic vasoconstriction and tachycardia → BP rises.
• Parasympathetic activity mediates rapid baroreceptor responses; sympathetic activity is more important for long-term pressure control.
• The reflex begins to fail when arterial BP falls below ~50 mmHg (e.g., severe shock).

7. Other Local Renal Factors

Summary: Integrated Control

• Robbins & Kumar Basic Pathology, p. 309
• Robbins Cotran Pathologic Basis of Disease, p. 462
• Katzung's Basic and Clinical Pharmacology, 16th Ed., p. 268

REGULATION OF BLOOD PRESSURE

A. SHORT-TERM (NEURAL) REGULATION

1. Vasomotor Centre (VMC)

• Located in the reticular formation of the medulla oblongata and lower pons
• Functionally divided into:
  • Vasoconstrictor area (C1, rostral VLM): tonically active; sends sympathetic impulses via the spinal cord → releases norepinephrine → acts on α1 receptors of arteriolar smooth muscle → vasoconstriction
  • Vasodilator area: inhibits the vasoconstrictor area → net result is vasodilation
  • Cardioinhibitory area (dorsal motor nucleus of vagus): sends parasympathetic impulses → decreases heart rate

2. Baroreceptor Reflex (Most important short-term regulator)

• High-pressure mechanoreceptors (stretch receptors) located in the:
  • Carotid sinus (CN IX, Hering's nerve)
  • Aortic arch (CN X, vagus)

• ↓ baroreceptor discharge → VMC vasoconstrictor area disinhibited → sympathetic surge → tachycardia, vasoconstriction, ↑ contractility → BP restored

• Operates as a negative feedback loop
• Resets at a new set-point during sustained hypertension (baroreceptor adaptation)
• Reflex fails when arterial BP < 50 mmHg (e.g., severe hemorrhagic shock)
• Parasympathetic tone mediates the rapid component; sympathetic tone is more important for sustained control

3. Peripheral Chemoreceptor Reflex

• Afferents → medulla → vasoconstriction (positive drive on VMC) + bradycardia (cardioinhibitory center activated)
• Primarily a respiratory reflex, but has significant BP effects, especially during hypoxia/hypotension
• During hemorrhage: BP drop → reduced carotid/aortic body perfusion → local hypoxia → chemoreceptor activation → vasoconstriction (supports BP)

4. CNS Ischemic Response (Cushing Reflex)

• Triggered when cerebral blood flow becomes severely compromised (BP < 60 mmHg or raised ICP)
• Direct CO₂ accumulation in the VMC → massive sympathetic discharge
• Produces extreme vasoconstriction and hypertension
• Acts as an emergency last-resort mechanism ("last ditch" response)
• Accompanied by bradycardia (vagal reflex to the hypertension) → classic Cushing's triad: hypertension + bradycardia + irregular breathing

5. Other Short-Term Reflexes

B. INTERMEDIATE-TERM REGULATION

1. Renin-Angiotensin-Aldosterone System (RAAS) - Rapid arm

• Trigger: ↓ BP in afferent arterioles, ↑ catecholamines, ↓ NaCl at macula densa
• Renin (juxtaglomerular cells) → cleaves angiotensinogen → Angiotensin I
• ACE (pulmonary endothelium) → Angiotensin II
• Angiotensin II effects (within minutes):
  1. Direct vasoconstriction → ↑ PVR
  2. Stimulates aldosterone release (adrenal cortex) → Na⁺/water retention
  3. Directly increases proximal tubular Na⁺ resorption
  4. Stimulates ADH (vasopressin) release from posterior pituitary

2. Capillary Fluid Shift Mechanism

• When BP drops, capillary hydrostatic pressure falls
• Oncotic pressure exceeds hydrostatic pressure → net absorption of interstitial fluid into capillaries
• Can shift up to 0.5–1 L of fluid from interstitium into the circulation within 30–60 minutes
• Helps restore blood volume and cardiac output
• Explains the hemodilution seen after acute hemorrhage

3. Stress Relaxation (Reverse Stress Relaxation)

• When blood volume drops, vascular smooth muscle relaxes → vessels accommodate the reduced volume at lower pressure
• When blood volume is restored, stress relaxation reverses → vascular tone returns → supports BP
• Occurs over minutes to hours

4. Adrenal Medullary Response

• Sympathetic activation → adrenal medulla → epinephrine + norepinephrine release into circulation
• Epinephrine: ↑ HR, ↑ contractility (β1), vasodilation in skeletal muscle (β2)
• Norepinephrine: predominantly vasoconstriction (α1)
• Amplifies and sustains the acute neural response

5. Vasopressin (ADH)

• Released from posterior pituitary in response to ↓ BP (via atrial baroreceptors) and ↑ plasma osmolality
• Causes arteriolar vasoconstriction (V1 receptors)
• Causes water retention in collecting ducts (V2 receptors) → ↑ blood volume
• Important during severe hemorrhage

C. LONG-TERM REGULATION

1. Pressure Natriuresis and Pressure Diuresis (The fundamental mechanism)

• Core principle: When arterial BP rises → kidneys excrete more salt and water (natriuresis + diuresis) → ↓ blood volume → ↓ CO → BP falls back to normal
• When BP falls → kidneys retain Na⁺ and water → ↑ blood volume → ↑ CO → BP rises

• At BP 50 mmHg → urine output ≈ zero
• At BP 100 mmHg → urine output is normal
• At BP 200 mmHg → urine output is 4–6× normal

2. Renin-Angiotensin-Aldosterone System (RAAS) - Chronic arm

• Aldosterone acts on ENaC channels in the distal nephron → Na⁺ and water resorption → ↑ blood volume → ↑ BP
• RAAS amplifies the renal pressure-natriuresis curve
• Chronic RAAS suppression (by ACE inhibitors, ARBs) shifts the natriuresis curve, reducing long-term set-point BP

3. Atrial Natriuretic Peptide (ANP) and B-type Natriuretic Peptide (BNP)

• Released by atrial myocardium (ANP) and ventricular myocardium (BNP) in response to volume overload / ↑ atrial stretch
• Counter-regulatory to RAAS:
  • Systemic vasodilation (↓ PVR)
  • Inhibit ENaC in distal tubules → natriuresis and diuresis → ↓ blood volume → ↓ BP
• Elevated levels signal hypervolemia (e.g., congestive heart failure)

4. Aldosterone Escape and Salt Sensitivity

• Normally, chronic increases in salt intake up to 6× normal cause minimal change in BP because the kidneys upregulate natriuresis efficiently
• In salt-sensitive individuals (impaired nephrons, excess RAAS activity), this buffering is lost → BP rises with salt intake

Summary Table

Key exam point: The renal-body fluid system is the only mechanism with infinite gain - it never resets or adapts. Long-term BP is therefore ultimately determined by the kidneys. All other mechanisms are temporary buffers.

REGULATION OF BLOOD PRESSURE

BP = Cardiac Output (CO) × Peripheral Vascular Resistance (PVR) CO = Heart Rate × Stroke Volume

I. NEURAL (SHORT-TERM) REGULATION

A. Vasomotor Centre (VMC)

• Located in the reticular formation of the medulla oblongata and lower pons
• Functionally divided into three areas:

• VMC receives input from all reflexes below and sends integrated efferent output to heart and vessels

B. Baroreceptor Reflex (Most important short-term mechanism)

• Carotid sinus → afferents via CN IX (Hering's nerve)
• Aortic arch → afferents via CN X (vagus)

↑ BP → vessel wall stretch
    → ↑ baroreceptor firing
    → NTS in medulla
    → activates vasodilator area + cardioinhibitory centre
    → ↓ sympathetic outflow + ↑ parasympathetic outflow
    → ↓ HR, ↓ contractility, vasodilation
    → BP returns to normal

↓ BP → ↓ baroreceptor firing
    → vasoconstrictor area disinhibited
    → ↑ sympathetic outflow
    → ↑ HR, ↑ contractility, vasoconstriction
    → BP restored

• Operates as a negative feedback loop
• Parasympathetic tone mediates the rapid component
• Sympathetic tone is more important for sustained pressure support
• Resets at a new set-point in chronic hypertension (baroreceptor adaptation)
• Fails when arterial BP < 50 mmHg (e.g., severe hemorrhagic shock)

C. Peripheral Chemoreceptor Reflex

• Afferents → NTS → medullary VMC
• Vasoconstriction (positive drive on vasoconstrictor area)
• Bradycardia (cardioinhibitory centre activated simultaneously)
• Primarily a respiratory reflex; contributes to BP support during hypoxia and hemorrhage

D. CNS Ischemic Response (Cushing Reflex)

• Triggered when cerebral perfusion fails (BP < 60 mmHg or severely raised ICP)
• CO₂ accumulates directly in the VMC → massive sympathetic discharge
• Produces extreme vasoconstriction → sharp rise in BP
• Accompanied by reflex bradycardia (vagal response to the hypertension)
• Classic triad: Hypertension + Bradycardia + Irregular respiration
• Acts as a last-resort emergency mechanism; has no regulatory role in normal physiology

E. Other Neural Reflexes (Brief)

II. INTERMEDIATE-TERM REGULATION

A. Renin-Angiotensin-Aldosterone System (RAAS) — Acute Arm

Juxtaglomerular cells → Renin release
    → Renin cleaves Angiotensinogen (liver) → Angiotensin I
    → ACE (pulmonary endothelium) → Angiotensin II

1. Direct arteriolar vasoconstriction → ↑ PVR → ↑ BP
2. Stimulates aldosterone from adrenal cortex → Na⁺/water retention
3. Directly ↑ proximal tubular Na⁺ resorption
4. Stimulates ADH (vasopressin) release from posterior pituitary

B. Vasopressin (ADH)

• Released from posterior pituitary in response to ↓ BP (low-pressure atrial receptors) and ↑ plasma osmolality
• V1 receptors: arteriolar vasoconstriction → ↑ PVR
• V2 receptors: water reabsorption in collecting ducts → ↑ blood volume
• Especially important in severe haemorrhage

C. Adrenal Medullary Response

• Sustained sympathetic activation → adrenal medulla → epinephrine + norepinephrine into circulation
• Epinephrine: ↑ HR, ↑ contractility (β1); vasodilation in skeletal muscle (β2)
• Norepinephrine: predominant vasoconstriction (α1)
• Amplifies and prolongs the acute neural response over 30–60 minutes

D. Transcapillary (Capillary) Fluid Shift

• When BP drops → capillary hydrostatic pressure falls
• Oncotic pressure now exceeds hydrostatic pressure
• Net absorption of interstitial fluid into capillaries (up to 0.5–1 L in 30–60 min)
• Restores plasma volume and cardiac output
• Explains haemodilution seen after acute haemorrhage

E. Stress Relaxation (Vascular Stress Relaxation)

• When blood volume drops, vascular smooth muscle relaxes → vessels accommodate reduced volume at a lower wall tension → buffers the fall in BP
• When volume is restored, reverse stress relaxation occurs → vascular tone returns gradually
• Operates over minutes to hours; particularly important in veins (capacitance vessels)

III. LONG-TERM REGULATION

A. Pressure Natriuresis and Pressure Diuresis (The fundamental long-term mechanism)

↑ BP
    → ↑ renal perfusion pressure
    → ↑ Na⁺ and water excretion (natriuresis + diuresis)
    → ↓ ECF and blood volume
    → ↓ venous return → ↓ CO
    → BP returns to normal set-point

↓ BP → ↓ renal output → ↑ blood volume → ↑ CO → BP rises

B. RAAS — Chronic Arm (Aldosterone)

• Aldosterone acts on ENaC channels in the distal nephron
• Increases Na⁺ (and water) resorption → ↑ blood volume → ↑ CO → ↑ BP
• RAAS amplifies the pressure–natriuresis relationship
• Chronic blockade (ACE inhibitors, ARBs, spironolactone) lowers the long-term BP set-point

C. Atrial and B-type Natriuretic Peptides (ANP/BNP) (Counter-regulatory)

• ANP: released constitutively from atrial myocardium
• BNP: released from ventricular myocardium in volume overload states

1. Systemic vasodilation (↓ PVR)
2. Inhibit ENaC in distal tubules → natriuresis and diuresis → ↓ blood volume → ↓ BP
3. Inhibit renin and aldosterone secretion directly

• Elevated ANP/BNP = hypervolaemia (hallmark of congestive heart failure)

D. Salt Sensitivity and Kidney Mass

• In healthy individuals, salt intake varying from 1/6 to 6× normal produces minimal change in BP because pressure natriuresis buffers the load
• Salt-sensitive hypertension occurs when the natriuresis curve is blunted:
  • Loss of functional nephrons (diabetes, CKD, ageing)
  • Excess RAAS activity (primary aldosteronism)
  • Genetic variants in renal sodium transporters

SUMMARY TABLE

Exam high-yield point: The renal pressure–natriuresis system is the only mechanism with infinite gain and is therefore the ultimate determinant of long-term blood pressure. All short- and intermediate-term mechanisms are buffers that buy time until the renal system reaches equilibrium.