The cerebral blood flow is carried by the paired internal carotid arteries and the basilar artery. These vessels provide input to the anastomotic circle of Willis at the base of the brain, which gives rise to the paired anterior, middle and posterior cerebral arteries. Variability in the development of anterior and posterior communicating arteries is associated with variability in functional adequacy of the circle. The circle of Willis is anatomically incomplete in 50% of cases.
The brain comprises 2% of the body mass and accounts for 15% of the resting metabolic rate. The cerebral blood flow comprises 15% of the total resting cardiac output. This is equal to 50-55ml/100g brain tissue per minute. Grey matter blood flow equals 70-80ml/100g per minute. White matter blood flow equals 15-20 ml/100 g per minute. Grey matter blood flow relative to white matter blood flow = 3-4:1. A fall in cerebral blood flow to less than 10ml/100 g per minute is associated with irreversible neuronal damage secondary to failure of the Na-K ATPase pump mechanism.
The Fick principle for measuring cerebral blood flow involves the use of a metabolically inert, freely diffusible tracer with a known partition coefficient between the blood and the brain. Providing that full saturation is achieved, the brain concentration of the tracer can be estimated from the cerebral venous blood.
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