To reduce the risk of post-flight orthostatic intolerance for astronauts, a better understanding of the response of the cardiovascular system to changes in hydrostatic pressure is essential. The objective of this study is to assess the regulatory mechanisms of the arterial and venous system in response to acute changes in hydrostatic pressure.

The experimental protocol applied to twelve volunteers consisted of: 3x3min 70-degree head-up tilt (HUT) and 3x3min 45-mmHg Lower Body Negative Pressure (LBNP). Blood Pressure, 4-segment electrical impedance (thorax, splanchnic, upper leg, and lower leg-Z_leg) and Femoral artery blood flow (FABF) was assessed.

FABF responses were characterized by a half-time decay constant, τ, of 6.10±0.34s and a volume increase of 27.2±3.4mL. Z_leg after tilting and LBNP onset decreases faster with HUT (τ=6.9±0.7s LBNP and 2.7±0.7s HUT, p<.0001), whereas upon tilting-back and removal of LBNP no differences were obtained (τ=3.3±0.7s LBNP and 2.1±0.7s HUT).

We can assume that the extra FABF volume and dynamics is mainly related to a vasoconstriction sympathetic reaction independent of the stress conditions. The faster decrease in Z_leg for HUT indicates a faster fluid shift in the lower leg than with LBNP, whereas the similar time course upon tilt-back and removal of LBNP may mainly be related to the compliance of the tissues in the lower limb, and thus independent of the stress conditions. With this study, we have shown that LBNP onset induces a delayed blood fluid shift compared to HUT whereas fluid emptying on removal coincided with lower limb tissue properties.