Compliance changes in femoral veins of rabbits after 21 days of simulated weightlessness.

: Increased venous compliance in lower limbs may be contributed to postflight orthostatic intolerance; however, direct animal studies to address the changes of venous compliance to microgravity have been rare. The purpose of this study was to determine compliance changes in femoral veins of rabbits after 21 days of head-down rest. Head-down rest -20 dgrees rabbit model was used to simulate weightlessness. 24 healthy male New Zealand Rabbits were randomly divided into 21 days of head-down rest group (HDR), horizontal immobilization group (HIG) and Ctrl group (Ctrl), with 8 in each. We constructed pressure-volume relationships from femoral veins in vivo for all groups after simulation by changing the venous internal volume and measuring the corresponding pressure. Microstructure of femoral vein wall in 3 groups was observed. Compared among the groups, the corresponding intravenous pressure of Ctrl was the highest when intravenous volume was expanded and HDR was the lowest. The parameter 3 , and P 2 in quadratic equations of femoral venous P-V relationship of HDR group were significantly higher than these values of HIG group and Ctrl group. The structure of femoral vein wall of HDR rabbits changed significantly, outlines of some endothelium cells (EC) became short and columnar or cubic, some of EC fell off and smooth muscle layer became thinner. These results indicate that, the femoral venous compliance increased after weightlessness simulation. This may partially underlie the mechanism of orthostatic hypotension seen in astronauts during an orthostatic stress after exposure to microgravity.