The lung is exquisitely sensitive to gravity, and so it is of interest
to know how its function is altered in the weightlessness of space. S
tudies on National Aeronautics and Space Administration (NASA) Spacela
bs during the last 4 years have provided the first comprehensive data
on the extensive changes in pulmonary function that occur in sustained
microgravity. Measurements of pulmonary function were made on astrona
uts during space shuttle flights lasting 9 and 14 days and were compar
ed with extensive ground-based measurements before and after the fligh
ts. Compared with preflight measurements, cardiac output increased by
18% during space flight, and stroke volume increased by 46%. Paradoxic
ally, the increase in stroke volume occurred in the face of reductions
in central venous pressure and circulating blood volume. Diffusing ca
pacity increased by 28%, and the increase in the diffusing capacity of
the alveolar membrane was unexpectedly large based on findings in nor
mal gravity. The change in the alveolar membrane may reflect the effec
ts of uniform filling of the pulmonary capillary bed. Distributions of
blood flow and ventilation throughout the lung were more uniform in s
pace, but some unevenness remained, indicating the importance of nongr
avitational factors. A surprising finding was that airway closing volu
me was approximately the same in microgravity and in normal gravity, e
mphasizing the importance of mechanical properties of the airways in d
etermining whether they close. Residual volume was unexpectedly reduce
d by 18% in microgravity, possibly because of uniform alveolar expansi
on. The findings indicate that pulmonary function is greatly altered i
n microgravity, but none of the changes observed so far will apparentl
y limit long-term space flight. In addition, the data help to clarify
how gravity affects pulmonary function in the normal gravity environme
nt on Earth.