Cerebral blood flow

Cerebral blood flow (CBF) refers to the blood supply to the brain in a given period of time. In an adult, CBF is typically 750 millilitres per minute or 15% of the cardiac output. This equates to an average perfusion of 50 to 54 millilitres of blood per 100 grams of brain tissue per minute. CBF is tightly regulated to meet the brain's metabolic demands. Too much blood can raise intracranial pressure (ICP), which can compress and damage brain tissue. Too little blood can lead to ischemia, insufficient oxygen and glucose supply to meet metabolic demand.

Regulation of Cerebral Blood Flow

CBF is regulated by various mechanisms to ensure that the brain receives the necessary blood supply. These mechanisms include myogenic, metabolic, and neurogenic factors.

Myogenic Regulation

Myogenic regulation refers to the ability of cerebral vessels to constrict or dilate in response to changes in blood pressure. When blood pressure increases, vessels constrict to prevent an over-perfusion that could lead to a brain edema. Conversely, when blood pressure decreases, vessels dilate to maintain adequate perfusion.

Metabolic Regulation

Metabolic regulation, also known as autoregulation, involves changes in the diameter of cerebral blood vessels in response to changes in the concentration of certain metabolites such as carbon dioxide, oxygen, and hydrogen ions. An increase in these metabolites leads to vasodilation, increasing CBF. Conversely, a decrease in these metabolites leads to vasoconstriction, decreasing CBF.

Neurogenic Regulation

Neurogenic regulation involves the release of vasoactive substances from nerve endings that can cause either vasoconstriction or vasodilation. These substances include acetylcholine, norepinephrine, and nitric oxide.

Measurement of Cerebral Blood Flow

CBF can be measured using various techniques such as transcranial Doppler ultrasound, computed tomography (CT) perfusion, magnetic resonance imaging (MRI) perfusion, and positron emission tomography (PET).

Clinical Significance

Alterations in CBF can lead to various pathological conditions such as stroke, traumatic brain injury, and Alzheimer's disease. Understanding the mechanisms of CBF regulation and having the ability to accurately measure CBF are crucial for the diagnosis and treatment of these conditions.