Pulmonary function and cardiopulmonary interactions at microgravity.

During short-lasting microgravity the weights of surrounding organs are eliminated, and the lungs attain a slightly reduced volume. Long-term microgravity may lead to deconditioning of respiratory muscles. The distribution of ventilation becomes more homogeneous but not completely so. Indirect estimates of the perfusion distribution point to a reduction of gross interregional differences but a maintained intraregional inhomogeneity. The gas/blood interface becomes more effective, as shown by an improved diffusion capacity. Thus, data on lung function in man at microgravity confirm that gravity is an important determinant of the distributions of ventilation and perfusion in the lung. However, both for ventilation and perfusion, significant inhomogeneities of distribution persist also in the weightless state, showing that non-gravity-related factors also play important roles. It could be speculated that more homogeneous mechanical properties of the lung tissues surrounding the heart contribute to facilitate cardiac diastolic function in microgravity.