Expansion
Low pressure, low resistance, high compliance, and it constricts in hypoxia
Expansion
| Feature | Pulmonary | Systemic |
|---|---|---|
| Mean pressure | About 15 mmHg | About 95 mmHg |
| Resistance | Very low | High |
| Compliance | High | Lower |
| Response to hypoxia | Vasoconstriction | Vasodilatation |
| Flow | Same as systemic | Same as pulmonary |
Both circulations carry the same cardiac output, so the entire pressure difference reflects resistance.
Hypoxic pulmonary vasoconstriction diverts blood away from poorly ventilated alveoli and so improves ventilation-perfusion matching. It is useful locally, but when hypoxia is global, as at altitude or in chronic lung disease, it raises pulmonary artery pressure and causes cor pulmonale.
Pulmonary resistance falls further as cardiac output rises, through recruitment of closed capillaries and distension of open ones. This is why pulmonary pressure barely changes during exercise despite a fourfold rise in flow.