Since 1) the original piece was posted on the older version of Le Physiologiste and, 2) I will discuss a little bit more about the regulation of brain blood flow in the upcoming months, I have decided to repost “Brain blood flow: the basics”.
An elevation in brain activity is usually associated with an increase in brain metabolism manifested by an increase in blood flow. There is a precise coupling between brain blood flow and glucose consumption. On the other hand, while a precise coupling between oxygen delivery and uptake is taking place in skeletal muscle for example, oxygen delivery to the brain exceeds its demand.
In order to meet the cerebral metabolic demand, blood flow to the brain is controlled by constant adjustments in perfusion pressure (which roughly represents the pressure difference between cerebral arteries and veins) and cerebrovascular resistance.
The cerebrovascular resistance in itself is influenced by variations in arterial pressure, i.e. it decreases when arterial pressure is reduced and it increases when arterial pressures elevates. Consequently, brain blood flow remains relatively constant and the underlying regulatory changes are referred to as cerebral autoregulation.
In fact, maintenance of cerebral perfusion during physiological challenges, such as standing and exercise, is secured by mechanisms of slow and fast actions. The mechanism known as “static cerebral autoregulation” considers the net change in brain blood flow following the manipulation of cerebral perfusion pressure under steady state. It represents the ability of the cerebral vasculature to maintain brain blood flow relatively stable within a range of mean arterial pressure of 60-150 mmHg (see figure below; MCAv is Middle Cerebral Artery Blood Flow Velocity; CVR is Cerebrovascular Resistance). However, that concept has been recently challenged and will be further discussed in a different post.
Although brain blood flow is maintained relatively constant by static cerebral autoregulation, sudden changes in arterial pressure are transmitted directly to the brain circulation under usual circumstances. Still, brain blood flow tends to return to its baseline value within a brief period of time. This short-term control represents “dynamic cerebral autoregulation”.
Several physiological stimuli, other than arterial pressure, influence local brain blood flow. The metabolic activity of the brain, arterial gas tensions (carbon dioxide and oxygen) and cardiac output are among the important determinants of brain blood flow. However, the influence of sympathetic nervous activity on the regulation of brain blood flow remains debated to date.The figure below presents the changes in brain blood flow according to different stimuli.