Balancing ventilation and perfusion
The lung uses hypoxic vasoconstriction to balance ventilation and perfusion. When a lung region becomes hypoxic, small arteries feeding that region sense hypoxia in alveolar gas and con- strict in response. Constriction redirects blood away from the hypoxic region to betteroxygenated lung areas. When a V/Q mismatch or imbalance occurs, as from lung disease, hypoxemia
may result. Causes of extreme V/Q mismatching include:
• shunting, in which alveolar perfusion is normal but ventilation is absent
• dead space portions of the respiratory tract where air doesn’t participate in gas exchange
• nonperfused lung areas.