Ventilation–perfusion (V/Q) matching
Also known as: V/Q ratio, V/Q mismatch, shunt, West zones, hypoxic pulmonary vasoconstriction, pulmonary circulation
Efficient gas exchange requires each alveolus to receive ventilation in proportion to its blood flow; the overall V/Q ratio is ≈0.8, varying from ≈3 at the apex to ≈0.6 at the base.
Overview
Alveolar ventilation (≈4.2 L/min) divided by pulmonary blood flow (≈5 L/min) gives an overall V/Q of ≈0.8. In the upright lung, gravity increases both ventilation and perfusion towards the base, but perfusion more steeply, so V/Q is high at the apex (≈3; PAO2 ≈130 mmHg, PACO2 ≈28 mmHg) and low at the base (≈0.6). West zones: zone 1 (PA > Pa > Pv) — no flow, normally absent but created by haemorrhage or positive-pressure ventilation; zone 2 (Pa > PA > Pv) — flow determined by the arterial–alveolar difference; zone 3 (Pa > Pv > PA) — continuous flow.
The spectrum runs from shunt (V/Q = 0: perfused but unventilated — pneumonia, atelectasis, ARDS) through normal units to dead space (V/Q = ∞: ventilated but unperfused — PE). Low-V/Q units cause hypoxaemia that improves with supplemental O2; true shunt does not. A normal physiological shunt of ≈2–5% of cardiac output comes from bronchial and Thebesian veins.
The pulmonary circulation is a low-pressure, low-resistance system (≈25/8 mmHg, mean ≈15 mmHg; resistance ≈1/10 of systemic) that accepts the whole cardiac output and can recruit and distend capillaries during exercise. Uniquely, alveolar hypoxia constricts pulmonary arterioles — hypoxic pulmonary vasoconstriction — diverting blood to better-ventilated regions (and, in the fetus, away from the lungs).
- Location
Throughout both lungs.
Function
- Matches regional blood flow to ventilation, optimising arterial oxygenation.
- Physiology
Pulmonary arterial pressure ≈25/8 mmHg (mean ≈15); pulmonary capillary wedge pressure ≈8–12 mmHg; pulmonary vascular resistance ≈1–2 Wood units.
Clinical significance
V/Q scanning
Matched defects suggest parenchymal disease; perfusion defects with normal ventilation (mismatched) suggest pulmonary embolism. CT pulmonary angiography is now preferred in most adults.
Pulmonary hypertension and cor pulmonale
Generalised hypoxic vasoconstriction (COPD, altitude, sleep apnoea) and vascular remodelling raise pulmonary artery pressure (mean > 20 mmHg defines pulmonary hypertension), leading to right ventricular hypertrophy and failure.
Positioning in unilateral lung disease
Lying with the good lung dependent ("good lung down") directs more perfusion to the better-ventilated lung and improves oxygenation; prone positioning improves V/Q matching in ARDS.
Compiled from standard anatomy and physiology references — see the Respiratory System sources. For education only; not medical advice.