Cerebral blood flow
Cerebral blood flow, or CBF, is the blood supply to the brain in a given time. In an adult, CBF is 750 mls/min or 15% of the cardiac output. On a weight basis, this is 50 to 54 milllitres/100grams/minute.
CBF is equal to the cerebral perfusion pressure (CPP) divided by the cerebrovascular resistance (CVR):
- CBF = CPP / CVR
Control of CBF is considered in terms of the factors affecting CPP and the factors affecting CVR.
Control of the CVR is by four major mechanisms:
- Metabolic control (or 'metabolic autoregulation')
- Pressure autoregulation
- Chemical control (by arterial pCO2 and pO2)
- Neural control
CBF is tightly regulated to meet the brain's metabolic demands, and on the average must be maintained at a flow of 50 milliliters of blood per 100 grams of brain tissue per minute in adult humans. It is important to maintain CBF within narrow limits because too much blood can raise intracranial pressure (ICP), which can compress and damage delicate brain tissue, and too little blood causes ischemia, or inadequate blood supply. Ischemia results if blood flow to the brain is below 18 to 20 ml per 100 g per minute, and tissue death occurs if flow dips below 8 to 10 ml per 100 g per minute. In brain tissue, a biochemical cascade known as the ischemic cascade is triggered when the tissue becomes ischemic, potentially resulting in damage to and death of brain cells. Medical professionals must take steps to maintain proper CBF in patients with conditions like shock, stroke, and traumatic brain injury. Cerebral blood flow in excess of 55 to 60 ml per 100 g per minute, called hyperemia, is more than the brain needs and can contribute to an increase in intracranial pressure.
Cerebral blood flow is determined by a number of factors, such as viscosity of blood, how dilated blood vessels are, and the net pressure of the flow of blood into the brain, known as cerebral perfusion pressure, which is determined by the body's blood pressure and intracranial pressure. Cerebral blood vessels are able to change the flow of blood through them by altering their diameters in a process called autoregulation; they constrict when systemic blood pressure is raised and dilate when it is lowered Arterioles also constrict and dilate in response to different chemical concentrations. For example, they dilate in response to higher levels of carbon dioxide in the blood.
Functional magnetic resonance imaging and positron emission tomography are neuroimaging techniques that can both be used to measure CBF. These techniques are also used to measure regional CBF (rCBF) within a specific brain region.
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