Physiology of the cerebral circulation

The brain has ongoing, substantial energy requirements but minimal stores of energy-generating substrates. As a result, it is completely dependent on a continuous, uninterrupted supply of substrate (oxygen, glucose). Fortunately, under physiological conditions (normal intercranial haemodynamics, normal cardiorespiratory function), this requirement is provided more than adequately by the 15% of the normal cardiac output which perfuses the brain. However, even physiologically nothing is absolutely static: there are the everyday fluctuations in posture, autonomic nervous activity, wakefulness, concentration, mental/physical activity and so on which have an impact on the cerebral circulation. Once again, however, the mechanisms intrinsic to the cerebral circulation can adapt appropriately. The chapter describes the physical, and physiological, contributions which can influence, and modulate, the cerebral circulation in the normal, healthy individual. In addition, and where appropriate, the underlying mechanisms are discussed briefly.