Current views and future programs in cardiovascular physiology in space.

The volume shift of 2000 cm3 from the lower to the upper part of the human body during weightlessness gave rise to theoretical and practical questions which are addressed in this communication. The analysis revealed that the mobilized fluid reduced the interstitial fluid of the lower extremities by 40%. Applying the current ideas in the field of interstitial tissue physiology to these problems, one must conclude that the fluid displacement can only be brought about by a change of the interstitial tissue compliance. Based on the observations made by the astronauts and on our working hypothesis, a method was proposed to follow the fluid migration and to measure the tissue compliance in man. Results are reported from experiments under terrestrial conditions. They show that the tissue compliance indeed can be modulated. Applying the method in space can eventually help to clarify several concepts in terrestrial physiology.