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Mammalian cardiovascular morphology and the maintenance of cerebral function at high Gz.

The persistence of consciousness at low cerebral perfusion pressures during exposure to high sustained G force in a head-to-foot direction (+Gz), is the rule. Concomitant changes in cerebrospinal fluid and cerebral venous dynamics together with cerebral autoregulation have been proposed to explain this preservation of cerebral function at high Gz. The non-uniform distensibility of the mammalian aortic distributing system plays a fundamental role in maintaining optimal cardiac energy exchange. Morphological evidence is presented suggesting that such a non-uniformity probably extends cranially as well as caudally in man. One consequence of such a cranial extension would be the amplification of the arterial pressure waves to the brain, with the potential for significant pulsatile flow at relatively low mean arterial blood pressures. Such phenomena would result from fundamental design properties of the mammalian cardiovascular system, would not be detected by conventional invasive pressure monitoring, and would serve as a mechanism to maintain optimal cerebral perfusion under condition of increasing +Gz.