Methods
Over the course of several flights of the space shuttle in which the Spacelab laboratory was carried, we performed measurements of pulmonary function in microgravity, and compared these with data collected on the ground in both the standing and supine postures. By using various single breath tests and multiple breath washouts, we were able to determine the degree of inhomogeneity of ventilation, perfusion and ventilation-perfusion ratio.

Results
The universal finding in these studies was that while gravity plays an important role in the inhomogeneity of pulmonary function, considerable unevenness persists in microgravity. The inhomogeneity of ventilation measured during single-breath vital capacity maneuvers was considerably reduced (Guy et al., 1994). However, multiple-breath washouts performed with near-normal tidal volumes were only slightly less homogeneous in microgravity compared to 1G (Prisk et al., 1995a). Subsequent studies using gases of different diffusivity suggest that the remaining non-gravitational inhomogeneity exists between acini or a few groups of acini (Prisk et al, 1998). Indices of the inhomogeneity of pulmonary perfusion suggested that while the removal of gravity abolished the top-to-bottom gradient in blood flow, considerable inhomogeneity persisted (Prisk et al, 1994). While there was some reduction in the inhomogeneity of V/Q consistent with the abolition of the gravitational gradient in blood flow, there was no reduction when considering the V/Q seen in the normal tidal volume range (Prisk et al., 1995b). Considering the reduced inhomogeneity in both ventilation and perfusion this result seems paradoxical. However, examination of the cardiogenic oscillations suggests that in microgravity, the normally present spatial correlation between areas of high ventilation and areas of high perfusion is reversed (Lauzon et al., 1998). Thus, V/Q remains unchanged in the face of more even distribution of both ventilation and perfusion.

Conclusions
The results from these studies suggest that while gravity clearly plays a major role in the unevenness of lung function in normal humans, factors intrinsic to the lung are responsible for a considerable degree of inhomogeneity in function.