ABSTRACT
This chapter defines the cerebral circulation during gravitational stress when the individual is subject to significant falls in arterial pressures. Adequate cerebral blood flow is essential if consciousness is to be preserved in the upright posture and during increased gravitational stress. In addition to the pressures that influence perfusion, the cerebral vasculature has an inherent ability to preserve blood flow within a limited range of arterial pressure. Positive acceleration is a term used in aviation physiology that refers to an acceleration in the headwards direction with the resultant inertial force in the head to foot direction leading to hypotension. Both cerebrovascular and neurological adaptations must be sought in understanding the preservation of consciousness during increased gravitational stress. Cerebral blood flow may be assisted by reduced intracranial volumes of blood and cerebrospinal fluid and, possibly, by a boost to the siphon effect.
