Metabolism and Cerebral Blood Flow

Abstract

5.1 The basic principles of brain metabolism and blood flow 5.1.1 CEREBRAL HEMODYNAMICS (a) Anatomical and physiological considerations Cerebral blood flow is influenced and regulated by a number of factors, including arterial blood pressure, intracranial pressure, venous outflow, blood viscosity, P a CO 2 and P a O 2 , collateral flow, vasoreactivity and the status of cerebral autoregulation. The regulation of vascular resistance lies mainly in the arterioles and precapillary segments (Auer and Loew, 1983). It is important to note that cerebral metabolism is the major determinant of regional blood flow. Anatomy Each carotid artery contributes approximately 40% to the total cerebral perfusion, the remaining 20% coming from the two vertebral arteries, which fuse to form the basilar artery. Autopsy studies, however, show that there are many variations in the vascular anatomy and a 'normal circle of Willis' was seen in only 52% of brains (Alpers, Berry and Paddison, 1959). Collateral channels occur on the pial surface or within the brain itself. These channels cross the arterial boundary zones (sometimes termed 'watershed' zones) and the severity of focal ischemia after systemic hypotension or occlusion of a major vessel is determined by these collaterals. The major fraction of venous blood draining from the brain is collected in the transverse sinuses which, together with the inferior petrosal sinuses, form the internal jugular veins. The cerebral venous drainage territories are not sharply delineated and overlap. It is estimated that approximately 70% of blood in each internal jugular vein originates in the ipsilateral and 30% in the contralateral hemisphere. Approximately 3% of blood in the internal jugular vein is from extracranial sources, mainly via the superior petrosal sinus (Jennett, Miller and Harper, 1976). The distribution of capillaries is functionally organized throughout the central nervous system. The capillary density may provide an anatomical indicator of oxidative metabolism. Brain areas with high basal levels of glucose metabolism contain a high density of capillaries. Some 90% of cerebral capillaries are continuously perfused and 10% are subject to recruitment in response to metabolic and other stimuli (Sokoloff et al., 1977). A close relationship exists between aerobic metabolism and the microvasculature.