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Effect of spaceflight on the subcutaneous venoarteriolar reflex in the human lower leg

¹Department of Medicine, Center for Molecular Medicine, Karolinska Institute, Stockholm, Sweden; and ²Department of Biomedical Sciences, Faculty of Health Sciences, University of Copenhagen, Denmark

Submitted 16 August 2006 ; accepted in final form 16 June 2007

Whenever the legs are lowered in humans, a venoarteriolar reflex is activated by the hydrostatic distension of the venules. Through local axon reflexes, the adjacent arterioles are contracted to decrease blood flow and prevent formation of edema. Because the venoarteriolar reflex is activated by gravity, we tested the hypothesis that long-term weightlessness would attenuate it. The reduction in subcutaneous blood flow was measured by the ¹³³Xe washout technique just proximal to the ankle joint in dependent lower legs of eight supine astronauts, where the knee joint was passively bent by 90°. The measurements were conducted before spaceflight and 3–6 h on landing following 4–6.5 mo in space. Activation of the venoarteriolar reflex reduced subcutaneous blood flow by 37 ± 9% (P = 0.016) before flight and by 64 ± 8% (P < 0.001) following landing with no statistical significant difference between the two reductions (P = 0.062). Therefore, our results show that the venoarteriolar reflex is not attenuated by weightlessness and therefore does not need the everyday stimulus of gravity to maintain efficiency.

cardiovascular reflex; vasoconstriction; weightlessness