Pulmonary Circulation

Pulmonary Circulation - Lung's Low‑Pressure Lane

• Pathway: Right Ventricle → Pulmonary Artery (deoxygenated blood) → Pulmonary Capillaries (gas exchange) → Pulmonary Veins (oxygenated blood) → Left Atrium.
• Vessel Characteristics:
  • Pulmonary arteries: Thin-walled, larger diameter, less smooth muscle, high compliance compared to systemic arteries.
  • Pulmonary veins: Short, carry oxygenated blood back to the heart.
• Hemodynamics:
  • Low-pressure system: Mean Pulmonary Artery Pressure (mPAP) $ \approx $ 15 mmHg (systemic circulation mean arterial pressure $ \approx $ 90-100 mmHg).
  • Low-resistance system: Pulmonary Vascular Resistance (PVR) $ \approx \frac{1}{10} $ Systemic Vascular Resistance (SVR).
  • High flow: Accommodates entire cardiac output (CO).

⭐ During exercise, pulmonary vascular resistance (PVR) normally decreases due to recruitment (opening of previously closed capillaries) and distension (widening of already open capillaries), allowing increased blood flow with only a slight rise in pulmonary arterial pressure. This protects the right ventricle from overload and prevents pulmonary edema.

Pulmonary Circulation - Gentle Flow Dynamics

• Characterized by low pressure, low resistance, and high compliance, facilitating large blood volume passage.
• Key Pressures:
  • Mean Pulmonary Artery Pressure (PAP): ~15 mmHg (Normal range: 9-18 mmHg).
  • Pulmonary Capillary Wedge Pressure (PCWP): 2-12 mmHg; reflects Left Atrial Pressure (LAP).
  • Mean Pulmonary Capillary Pressure: ~7 mmHg, crucial for Starling forces & fluid exchange.
• Pulmonary Vascular Resistance (PVR):
  • Represents resistance to blood flow through pulmonary vasculature.
  • Formula: $PVR = (Mean PAP - PCWP) / CO$ (CO = Cardiac Output).
  • Normal: <2.5 Wood units (or <200 dynes.sec.cm⁻⁵).

⭐ Hypoxic Pulmonary Vasoconstriction (HPV) is a critical adaptive mechanism: alveolar hypoxia triggers localized pulmonary artery vasoconstriction, diverting blood from poorly ventilated to well-ventilated lung segments, thus optimizing ventilation/perfusion (V/Q) matching.

• Major Factors Affecting PVR:

Pulmonary Circulation - Breath & Blood Harmony

• Low pressure, high flow, low resistance; handles full Cardiac Output. Mean PAP ~15 mmHg.
• Regulation:
  • Hypoxic Pulmonary Vasoconstriction (HPV): Low alveolar $O_2$ → arteriolar constriction → diverts blood, optimizes V/Q.
  • Others: NO (dilation), ET-1 (constriction).
• V/Q Ratio (Ventilation/Perfusion): Normal ~0.8.
  • Apex: ↑V/Q (Zone 1).
  • Base: ↓V/Q (Zone 3).
• West's Zones: (PA=Alveolar, Pa=Arterial, Pv=Venous pressure)
  • Zone 1: PA > Pa > Pv (Min. flow)
  • Zone 2: Pa > PA > Pv (Intermittent flow)
  • Zone 3: Pa > Pv > PA (Continuous, largest)
• V/Q Mismatch & Effects:

⭐ Chronic hypoxia (e.g., high altitude, COPD) → sustained HPV → pulmonary HTN & RV hypertrophy.

Pulmonary Circulation - Traffic Control Lungs

• Low pressure (~25/10 mmHg, mean ~15 mmHg), low resistance (1/10th of systemic).
• Regulation:
  • Hypoxic Pulmonary Vasoconstriction (HPV): Key! Alveolar hypoxia ($P_{A}O_2$ < 70 mmHg) → local vasoconstriction.
    • Redirects blood to well-ventilated areas; optimizes $V/Q$ match.
  • Humoral: Vasoconstrictors (Endothelin, Thromboxane A2); Vasodilators (NO, Prostacyclin).
  • Neural: Minor role (Sympathetic α1-receptors: vasoconstriction; β2-receptors: vasodilation).
• Pulmonary Vascular Resistance (PVR):
  • $PVR = (Mean P_{PA} - P_{LA}) / Cardiac Output$
  • ↑ by: Hypoxia, acidosis, hypercapnia, ↑ lung volumes (extremes), serotonin.
  • ↓ by: $O_2$, NO, prostacyclin, β-agonists, ↓ lung volumes (from high).
• Shunts: Blood bypassing gas exchange.
  • Physiological: ~2-5% Cardiac Output (bronchial & Thebesian veins).
  • Pathological: ↑ in ARDS, pneumonia, atelectasis → hypoxemia.

⭐ HPV is significantly blunted by volatile anesthetics, sepsis, and certain vasodilators (e.g., nitroglycerin, CCBs).

High‑Yield Points - ⚡ Biggest Takeaways

• Pulmonary circulation is a low-pressure, low-resistance system, distinct from systemic circulation.
• Pulmonary artery carries deoxygenated blood to lungs; pulmonary veins carry oxygenated blood to the left atrium.
• Hypoxic pulmonary vasoconstriction (HPV) optimizes V/Q matching by diverting blood from hypoxic alveoli.
• PVR decreases with increased cardiac output (recruitment/distension) and at moderate lung volumes.
• Normal mPAP is ~15 mmHg; pulmonary hypertension is mPAP >20 mmHg.
• West's lung zones (1, 2, 3) describe gravity-dependent perfusion patterns.
• PCWP estimates left atrial pressure and left ventricular end-diastolic pressure (LVEDP).