The cardiovascular and pulmonary systems supply the body with the oxygen needed for life. On Earth, the heart must work against gravity constantly to push oxygenated blood through the arteries. In space, the lack of gravitational force causes blood and other fluids to shift from the lower to the upper body. This headward fluid shift triggers many changes in the cardiovascular system. Long- and short-term studies of humans in space have documented increased heart rate, narrowed pulse pressure, reduced plasma volume, decreased heart chamber volume and facial edema. Animal studies indicate that the myocardium may degenerate during microgravity exposure. The return of astronauts to Earth is accompanied by decreased exercise capacity and orthostatic intolerance. These factors decrease performance during descent from orbit and increase risk during emergency egress from the spacecraft. The mechanisms behind these phenomena remain uncertain, though hypotheses include decreased intravascular volume, increased post-flight venous pooling due to structrual and reflex vascular changes, and alterations in overall cardiovascular reflex control.
In-flight exercise appears to be only partially effective in treating cardiovascular deconditioning in space. Other preventative methods under examination include lower body negative pressure (LBNP) treatment and saline loading before return to Earth. The main goals of cardiovascular research in space are to understand the acute and long-term cardiovascular and pulmonary adaptation to space and readaptation to a gravity environment, including the associated mechanisms, and to ensure adequate physiological countermeasures to cardiovascular deconditioning.
Unlike the cardiovascular system, no pulmonary system problems have been associated with weightlessness per se, and researchers have devoted less attention to its physiology in microgravity. However, lung function can be altered by changes in vascular pressure and volume, and scientists have reported a drop in PO2 microgravity.
Along with alterations due to changes in vascular pressures and volumes, the lung can be damaged by inhaled gases, vapors, and aerosols. Integrity and normalcy of the pulmonary system cannot be assumed simply because of lack of symptoms or overt clinical signs.