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PULMONARY CIRCULATION & VENTILATION-PERFUSION RATIO

Seminar Summary - GK Pal Physiology

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1. WHAT IS PULMONARY CIRCULATION?

• Blood ejected by the right heart into the lungs equals the same amount ejected by the left heart into systemic circulation (same cardiac output, ~5 L/min at rest)
• But pulmonary circulation is a low-pressure, low-resistance system compared to systemic circulation
• Pulmonary vascular resistance = 1/10th of systemic vascular resistance
• Mean pulmonary arterial pressure = 15 mm Hg (systemic = 100 mm Hg)

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2. FUNCTIONAL ORGANIZATION

• Pulmonary arteries branch parallel to airways - each airway has a matching artery
• Pulmonary capillaries are large in diameter with multiple anastomoses
• Each alveolus is surrounded by a capillary basket - very efficient gas exchange
• Blood in pulmonary vessels = ~40% of total lung weight
• Distribution of ~500 mL blood: Arteries 150 mL | Veins 270 mL | Capillaries 80 mL

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3. FUNCTIONS OF PULMONARY CIRCULATION

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4. SPECIAL FEATURES OF PULMONARY CIRCULATION

Remember: Low pressure + Thin walls + High compliance = Easy flow

1. Low pressure (mean PA pressure = 15 mm Hg) - low resistance system
2. Thin walls - less smooth muscle, more compliant than systemic arteries
3. High compliance - can accommodate large volumes of blood (e.g., on changing posture from standing to lying)
4. Pulmonary arterioles - thin-walled, less ability to constrict vs systemic arterioles
5. Pulmonary capillaries - form a lattice in the alveolar wall (like a thin sheet of flowing blood); not a tubular network; collapse if alveolar pressure > capillary pressure
6. Wedge pressure - measured by Swan-Ganz catheter; reflects left atrial pressure - clinically important

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5. PULMONARY VASCULAR RESISTANCE (PVR)

What Affects PVR?

A. Lung Volumes

B. Hormones

• Vasoconstrictors: Serotonin, norepinephrine, histamine, thromboxane A₂, leukotrienes
• Vasodilators: Adenosine, acetylcholine, prostacyclin (PGI₂), bradykinin, nitric oxide

C. Oxygen Tension

• Low O₂ (hypoxia) → pulmonary vasoconstriction (unique - opposite to systemic!)
• Hypoxia directly contracts pulmonary vascular smooth muscle
• Mechanism: inhibits K⁺ channels → depolarization → voltage-gated Ca²⁺ channels open → vasoconstriction

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6. PULMONARY BLOOD FLOW & GRAVITY (The 3 Zones)

Blood flow increases from apex to base in upright posture due to gravity

           APEX
    ┌──────────────┐
    │   ZONE 1     │  PA > Pa > Pv  → Dead space (no flow if PA > Pa)
    │  (Upper)     │  Ventilation ↑↑, Perfusion ~0 → V/Q HIGH (~3)
    ├──────────────┤
    │   ZONE 2     │  Pa > PA > Pv  → Flow = Pa - PA (waterfall effect)
    │  (Middle)    │  Normal V/Q
    ├──────────────┤
    │   ZONE 3     │  Pa > Pv > PA  → Flow = Pa - Pv (normal)
    │  (Lower)     │  Perfusion ↑↑, Ventilation moderate → V/Q LOW (~0.6)
    └──────────────┘
           BASE

• Zone 1 (Apex): Capillaries may collapse; increases alveolar dead space; Zone 1 is usually very small or absent normally
• Zone 2 (Middle): Waterfall effect - blood collects in veins through constriction; flow determined by Pa - PA difference
• Zone 3 (Base): Best blood flow; flow determined by Pa - Pv; capillary distension → lower resistance

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7. WHY LOW PVR IS PHYSIOLOGICALLY IMPORTANT

1. Capillary recruitment - collapsed capillaries in upper lung open up as pressure rises → decreases overall resistance
2. Capillary distension - capillaries stretch → more surface area for gas exchange

• Blood moves slowly through capillaries → adequate time for O₂/CO₂ exchange
• Increased capillary surface area → better diffusion
• Prevents pulmonary edema (keeps capillary pressure low)

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8. FILTRATION ACROSS PULMONARY CAPILLARIES

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9. PULMONARY EDEMA

Causes:

1. ↑ Capillary hydrostatic pressure - most common cause (e.g., left heart failure, mitral stenosis)
2. ↑ Alveolar surface tension - decreased surfactant (e.g., ARDS)
3. ↓ Oncotic pressure - hypoproteinemia (starvation)
4. ↑ Capillary permeability - pulmonary vascular injury, oxidant damage, inflammatory reactions, neurogenic shock (e.g., head injury)

Effects of Pulmonary Edema:

• Decreases gas exchange → hypoxemia and hypercapnia
• Obstructs small airways → ↑ airway resistance
• ↓ Lung compliance → increased work of breathing

Treatment:

1. Diuretics - reduce blood volume
2. Digitalis - improves left ventricular function
3. Vasodilators - reduce systemic blood vessel load

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10. VENTILATION-PERFUSION (V̇A/Q̇) RATIO

Normal Values

• Alveolar ventilation (V̇A) = 4 L/min; Pulmonary blood flow (Q̇) = 5 L/min
• Normal V̇A/Q̇ ratio = 0.8
• At base: V̇A/Q̇ = 0.6 (more perfusion than ventilation)
• At apex: V̇A/Q̇ = ~3 (more ventilation than perfusion)

Why Does V/Q Ratio Vary?

Importance of V̇A/Q̇ Ratio

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11. EFFECTS OF CHANGING V̇A/Q̇ RATIO

Decreased V̇A/Q̇ (Bronchiolar obstruction)

• Ventilation ↓, perfusion normal
• Alveolar PO₂ ↓↓, PCO₂ ↑
• Blood leaving alveolus is poorly oxygenated → venous admixture

Increased V̇A/Q̇ (Vascular obstruction)

• Perfusion ↓, ventilation normal
• Alveolar PO₂ ↑↑, PCO₂ ↓↓
• Creates alveolar dead space (ventilation wasted)

Clinical Importance

• TB occurs more at the apex (high PO₂ - good environment for Mycobacterium)
• Decreased V̇A/Q̇ at base explains why some diseases localise there

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12. VENOUS ADMIXTURE & SHUNTS

Types of Shunts

• In bronchial disease: shunt can ↑ to 20% of cardiac output
• In congenital right-to-left shunts: can ↑ to 50% → severe hypoxia

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13. BRONCHIAL CIRCULATION

• Supplies blood to walls of conducting airways (not the alveoli)
• Receives only 1% of cardiac output (can increase to 10% in chronic bronchitis)
• Bronchial arterial pressure ≈ aortic pressure (much higher than pulmonary artery)
• Unique: Only the bronchial circulation can undergo angiogenesis (new vessel formation)
• Clinically important: when pulmonary circulation is blocked (e.g., by embolus), bronchial vessels form collaterals and keep lung parenchyma alive

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QUICK REVISION - KEY POINTS FOR EXAM

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CHAPTER SUMMARY (From GK Pal)

1. The resting sympathetic tone in pulmonary circulation is less - this keeps vascular resistance low and allows entire cardiac output to pass through easily
2. V/Q ratio is more at the apex and less at the base of the lung

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Source: GK Pal Physiology, Chapter 106 - Pulmonary Circulation and Ventilation-Perfusion Ratio (pp. 978-984)