Selected contribution: Redistribution of pulmonary perfusion during weightlessness and increased gravity

To compare the relative contributions of gravity and vascular structure to the distribution of pulmonary blood flow, we flew with pigs on the National Aeronautics and Space Administration KC-135 aircraft. A series of parabola s created alternating weightlessness and 1.8-G conditions. Fluorescent micr ospheres of varying colors were injected into the pulmonary circulation to mark regional blood flow during different postural and gravitational condit ions. The lungs were subsequently removed, air dried, and sectioned into si milar to 2 cm(3) pieces. Flow to each piece was determined for the differen t conditions. Perfusion heterogeneity did not change significantly during w eightlessness compared with normal and increased gravitational forces. Regi onal blood flow to each lung piece changed little despite alterations in po sture and gravitational forces. With the use of multiple stepwise linear re gression, the contributions of gravity and vascular structure to regional p erfusion were separated. We conclude that both gravity and the geometry of the pulmonary vascular tree influence regional pulmonary blood flow. Howeve r, the structure of the vascular tree is the primary determinant of regiona l perfusion in these animals.