Expansion
Pulmonary arterioles constrict in hypoxia, diverting blood to better ventilated alveoli
Expansion
Alveolar hypoxia constricts the local pulmonary arterioles, diverting blood towards better ventilated regions. It responds mainly to alveolar oxygen tension, becoming significant below about 8 kPa.
Beneficial when local
- Improves V/Q matching in pneumonia, atelectasis or a mucus plug
- Essential during one lung ventilation in thoracic anaesthesia, reducing shunt through the collapsed lung
Harmful when global
- At altitude or in chronic lung disease, the entire pulmonary bed constricts, raising pulmonary artery pressure and causing right ventricular hypertrophy and cor pulmonale
- Contributes to persistent pulmonary hypertension of the newborn
It is inhibited by volatile anaesthetic agents, calcium channel blockers, nitrates and other vasodilators, which is why these can worsen oxygenation during one lung ventilation.
Note the contrast with the systemic circulation, where hypoxia causes vasodilatation; the pulmonary response is unique and serves matching rather than local supply.