Effect of Acceleration and Weightlessness on Lung Mechanics

Abstract

It has been long recognized that earth gravity (1G) plays an important role in determining the static mechanical properties of the respiratory system. Indeed, when changing from upright posture to recumbency, there is a substantial change in the functional residual capacity (FRC).1 As with relaxed respiratory muscles, the FRC is determined by the balance between the inward recoil of the lung and the outward recoil of the chest wall, the postural changes in FRC could be due to changes in the static pressure-volume (P-V) curve of either the lung or chest wall, or both. In normal man, however, the static P-V curve of the lung does not change appreciably with body posture.2 Accordingly, the effect of earth gravity appears to affect mainly the mechanical properties of the chest wall, more specifically the abdomen. In the upright position gravity acts in the inspiratory direction on the abdomen and in the expiratory direction on the rib cage; the effect of the abdomen is greater at small than at high lung volumes because at large volumes the height of the abdomen is smaller and its wall stiffer. In the supine position the gravitational effect changes little with volume and the action of gravity on both abdomen and rib cage is expiratory. Hence, the reduction in FRC between the upright and supine posture is due almost entirely to the abdomen.