| Research Abstract |
To examine the role of hydrostatic pressure difference in acute hemodynamic change during microgravity, cerebral perfusion pressure, capillary microcirculation of the iris, local blood flow of the cerebral cortex and the temporal muscle were measured in anesthetized rats. Microgravity was produced by free drop or parabolic flight. Rats were placed in 30° head-up whole body-tilt or horizontal flat position. In the flat group, none of the measured variables was significantly affected by microgravity, whereas in the head-up group, cerebral perfusion pressure and blood flow in the iris capillary, the temporal muscle, and the cerebral cortex were increased. Thus disappearance of the hydrostatic pressure difference induces cephalad fluid shift, and then increases blood flow of the head. On the other hand, hypergravity induced hypotension, if the baroreflex and vestibulosympathetic reflex did not operate. If the only vestibulosympathetic reflex operated, hypergravity induced an increase in renal sympathetic nerve activity and a huge pressor response, which were attenuated by the baroreflex. Based on these experimental data, block diagram of the control system during gravitational stress was depicted and the transfer function of the system was estimated. In this system, gravitational stress is sensed by the vestibular system, and projects to the medial vestibular nucleus and paraventricular hypothalamic nucleus, and then increases sympathetic nerve activity and elicits pressor response. The increase in AP is sensed by baroreceptor and the increase is suppressed. Thus the control system for protecting the AP from gravitational stress is composed of two subsystems, i.e., the baroreflex feedback control system and the vestibular feedforward control system. The estimated transfer functions of vestibular system and baroreflex were 16 and 8, respectively.
|
