One of our most important countermeasures for cardiovascular deconditioning is the G-suit. Wearing this device, our astronauts returning from long space missions are much better able to cope with the relative novelty of moving about in a one G field. Without this simple garment most would find a relatively rapid heart rate and diminished blood pressure while standing erect leading to a fainting tendency.
During weightless conditions two things happen to make this response almost inevitable upon return to Earth. One is a relative decrease in circulating blood volume and the other is sluggishness of the vascular reflexes, which adjust the tone of our arteries and veins when upright. Both are due to the absence of hydrostatic pressure in the weightless environment.
Less circulating blood is required to meet the needs of the body when weight of the long columns of blood is eliminated. Similarly, when reflex adjustments of blood vessel tone no longer are required due to the absence of weight they suffer disuse changes and become sluggish or completely unresponsive. The G-suit partially compensates for the decreased blood volume and for diminished vascular tone.
Of interest to space scientists is the hydro suit concept of G protection currently under development for fighter pilots. This might be an even better G-suit for returning astronauts.
Duane Graveline MD MPH
Former USAF Flight Surgeon
Former NASA Astronaut
Retired Family Doctor