| Abstract: | Early in spaceflight, anapparently paradoxical condition occurs in which, despite an externallyvisible headward fluid shift, measured central venous pressure is lowerbut stroke volume and cardiac output are higher, and heart rate isunchanged from reference measurements made before flight. This paperpresents a set of studies in which a simple three-compartment,steady-state model of cardiovascular function is used, providinginsight into the contributions made by the major mechanisms that couldbe responsible for these events. On the basis of these studies, weconclude that, during weightless spaceflight, the chest relaxes with aconcomitant shape change that increases the volume of the closed chestcavity. This leads to a decrease in intrapleural pressure, ultimately causing a shift of blood into the vessels of the chest, increasing thetransmural filling pressure of the heart, and decreasing the centralvenous pressure. The increase in the transmural filling pressure of theheart is responsible, through a Starling-type mechanism, for theobserved increases in heart size, left ventricular end-diastolicvolume, stroke volume, and cardiac output. |
