Cerebral blood flow forms the basis for supplying the nerve cells in the brain with oxygen as well as various nutrients. In a healthy person, it is estimated that about 15 percent of what is called cardiac output flows through the brain. The surrounding tissue is also supplied with blood, with a total of approximately 700 milliliters of blood per minute flowing through the brain.
What is cerebral blood flow?
Cerebral blood flow forms the basis for supplying the nerve cells in the brain with oxygen as well as various nutrients. Cerebral blood flow is a measure that indicates the blood supply to the brain during a fixed unit of time. Cerebral blood flow is often abbreviated CBF, derived from the English term cerebral blood flow. Although the brain accounts for only two percent of the total body weight of an adult, cerebral blood flow takes up about 15 percent of the cardiac output. Thus, it is about 700 to 750 milliliters per minute. In order to take into account the differences in size of different individuals, cerebral blood flow is usually expressed as the so-called flow volume of 100 grams of brain mass per minute. Total cerebral blood flow differs from so-called regional cerebral blood flow, with the abbreviation rCBF. This acts as a measure of blood supply and perfusion to specific areas of the brain. By determining the regional cerebral blood flow, statements can be made about sections of the brain with a stronger and weaker blood supply. Cerebral blood flow is expressed in the same unit as regional cerebral blood flow. It must be taken into account that the result values are usually highly dependent on the respective method of measurement. Cerebral blood flow can be calculated using a formula based on mean arterial pressure, cerebral vascular resistance, and intracranial pressure. The normal value for cerebral blood flow is about 45 to 55 milliliters per 100 grams per minute. At the same time, cerebral blood flow in the tissues of the brain exhibits considerable regional variation. For example, it is much lower within the white matter of the brain than in the so-called gray matter. Cerebral blood flow can be determined using various measurement methods that rely on imaging techniques. These are performed in vivo, for example MRI, PET, SPECT or transcranial Doppler sonography. However, these techniques primarily focus on measuring regional cerebral blood flow. This is because it is often more relevant from a clinical perspective than total cerebral blood flow. Identification of areas of the brain with inferior blood flow plays a significant role in the diagnosis of numerous diseases.
Function and task
Cerebral blood flow is essential for the supply of nutrients and oxygen to the brain and, in particular, to neurons. It is thus of fundamental importance for the entire organism and the brain’s ability to function. Cerebral blood flow is primarily regulated by the resistance of the cerebral vessels. This in turn is based on the mean arterial pressure. As part of the so-called Bayliss effect, the arterioles become narrower as systemic blood pressure rises. On the other hand, they dilate when blood pressure drops. In healthy individuals, the body is able to maintain cerebral blood flow within a certain size range. This mechanism is also called autoregulation. In addition, the arterioles adapt to the concentration of specific gases dissolved in the blood. If the level of carbon dioxide in the blood is increased, the brain vessels dilate. This improves blood flow to the brain. If the partial pressure of CO2 decreases, the vessels constrict. As a result, cerebral blood flow is reduced. On the other hand, the so-called oxygen partial pressure has only a slight effect on cerebral vascular resistance. Only when the concentration of oxygen falls below a certain value do the cerebral vessels dilate and blood flow increases. In addition, the sympathetic and parasympathetic nerves also influence vascular resistance. Basically, the capillary bed in the brain is composed of a tightly interwoven network of vessels that communicate with each other. In total, the capillaries in the human brain cover approximately 640 kilometers.Pressure differences between arterioles and venules regulate blood flow within capillaries.
Diseases and ailments
For the brain to perform its functions, cerebral blood flow is essential. Deviations in blood flow to the brain result in functional impairment and can quickly become life-threatening. Under ordinary conditions, electrical functions in the brain fail as soon as cerebral blood flow drops below a value of 18-20 ml/100 g/min. Even a momentary drop or interruption in cerebral blood flow results in fainting. If cerebral blood flow remains decreased for a prolonged period of time, irreversible damage to the nerves of the brain will result. This is because during this time, the brain is not adequately supplied with oxygen and nutrients. If the blood flow to the brain is too strong, the intracranial pressure may increase. This can cause damage to the brain tissue. If a person’s blood pressure is permanently elevated, the limit of the autoregulation mechanism shifts upward. In reversible cerebral vasoconstriction syndrome, there is a sudden reduction in blood flow. Affected individuals complain of annihilation headache and other neurologic impairment.
