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 constrict 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.