We investigated the integrated cardiovascular responses of 15 human su
bjects to the acute gravitational changes (micro- and hypergravity por
tions) of parabolic flight. Measurements were made with subjects quiet
ly seated and while subjects performed controlled Valsalva maneuvers.
During quiet, seated, parabolic flight, mean arterial pressure increas
ed during the transition into microgravity but decreased as microgravi
ty was sustained. The decrease in mean arterial pressure was accompani
ed by immediate reflexive increases in heart rate but by absent (or la
ter-than-expected) reflexive increases in total vascular resistance. M
ean arterial pressure responses in Valsalva phases II1, III, and IV we
re accentuated in hypergravity relative to microgravity (P < 0.01, P <
0.01, and P < 0.05, respectively), but accentuations differed qualita
tively and quantitatively from those induced by a supine-to-seated pos
tural change in 1 G. This study is the first systematic evaluation of
temporal and Valsalva-related changes in cardiovascular parameters dur
ing parabolic flight. Results suggest that arterial baroreflex control
of vascular resistance may be modified by alterations of cardiopulmon
ary, vestibular, and/or other receptor activity.