Capillary blood flow is therefore dependent on the arterial–alveolar pressure difference. P A therefore increases and the lung exerts increased extramural pressure on the pulmonary vasculature: alveolar pressure thus exceeds venous pressure, causing compression of the venous end of the pulmonary capillary. Alveoli are pulled open and become less compliant. As the lung is ascended, there is an increasing effect of the weight of the lung. West zone 3 is how normal, healthy lungs behave below the level of the hilum. Capillary blood flows continuously throughout the cardiac cycle – flow is dependent on the arterial–venous pressure difference, which is generated by the right ventricle. As the alveoli occupy a small volume, they exert minimal extramural pressure on the pulmonary vasculature. For this reason, the basal alveoli are not particularly distended and thus sit upon a more compliant part of the pressure–volume loop. The lungs are suspended superiorly in the chest from the large airways and therefore there is little weight acting upon the base of the lung.
This is because of the effects of gravity on alveolar volume. Both arterial and venous pressures are greater than alveolar pressure. In West zone 3, P a > P v > P A (a: arterial v: venous A: alveolar). The variation in alveolar perfusion in the three West zones ( Figure 16.1) is most easily explained by starting from West zone 3, at the base of the lung:
How do the changes in arterial, venous and alveolar pressures affect alveolar perfusion?
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