Departments of Medicine and Radiology, University of California, San Diego, USA.
Compr Physiol. 2011 Jan;1(1):339-55. doi: 10.1002/cphy.c090007.
Efficient gas exchange in the lung depends on the matching of ventilation and perfusion. However, the human lung is a readily deformable structure and as a result gravitational stresses generate gradients in both ventilation and perfusion. Nevertheless, the lung is capable of withstanding considerable change in the applied gravitational load before pulmonary gas exchange becomes impaired. The postural changes that are part of the everyday existence for most bipedal species are well tolerated, as is the removal of gravity (weightlessness). Increases in the applied gravitational load result only in a large impairment in pulmonary gas exchange above approximately three times that on the ground, at which point the matching of ventilation to perfusion is so impaired that efficient gas exchange is no longer possible. Much of the tolerance of the lung to alterations in gravitation stress comes from the fact that ventilation and perfusion are inextricably coupled. Deformations in the lung that alter ventilation necessarily alter perfusion, thus maintaining a degree of matching and minimizing the disruption in ventilation to perfusion ratio and thus gas exchange.
肺部的气体交换效率取决于通气和灌注的匹配。然而，人体肺部是一个容易变形的结构，因此重力会导致通气和灌注的梯度分布。尽管如此，肺部在肺气体交换受到损害之前，能够承受施加的重力负荷的相当大的变化。对于大多数两足动物来说，日常生活中的姿势变化是可以很好地耐受的，就像去除重力（失重）一样。只有当施加的重力负荷增加到地面上的大约三倍以上时，才会导致肺气体交换严重受损，此时通气与灌注的匹配受到严重破坏，以致无法进行有效的气体交换。肺部对重力变化的耐受程度很大程度上来自于通气和灌注不可分割地结合这一事实。改变通气的肺部变形必然会改变灌注，从而保持一定程度的匹配，最大限度地减少通气到灌注的比例中断，从而维持气体交换。
