Читать книгу Respiratory Medicine - Stephen J. Bourke - Страница 27

The pulmonary circulation operates under much lower pressure than the systemic circulation. At rest, the driving pressure is only on the order of 15 mmHg. In the upright posture, therefore, there is barely enough pressure to fill the upper parts of the system and the apices of the lung receive very little perfusion at all from the pulmonary circulation. The relative over perfusion of the bases mirrors the pattern seen with ventilation (which is fortunate, if our aim is to bring blood and air into contact), but the disparity is even greater in the case of perfusion. Thus, at the bases of the lungs, perfusion exceeds ventilation, while, at the apices, ventilation exceeds perfusion.

The distribution of perfusion is also heavily influenced by another factor: hypoxia. By a mechanism we do not fully understand, low oxygen levels in a region of the lung have a direct vasoconstrictor effect on the pulmonary artery supplying that region. This has the beneficial effect of diverting blood away from the areas of lung that are poorly ventilated towards the well‐ventilated areas. This ‘automatic’ ventilation/perfusion (V/Q) matching system aims to maximise the contact between air and blood and is critically important to gas exchange.