Cerebral blood flow, or CBF, is the circulation of blood through the brain. Blood flows into the brain from the vertebral and carotid arteries in the neck, and leaves mainly through the internal jugular veins, which are also situated in the neck. Inside the brain, cerebral circulation takes place inside the cerebral blood vessels, with the total flow being regulated to remain at a constant level. At the same time the blood supply to different parts of the brain varies according to the amount of activity there.
A number of different factors are involved in regulating the total cerebral blood flow. The pressure of the blood in arteries and veins and the thickness, or viscosity, of the blood all have an effect on the overall flow. In addition, the small arteries in the brain can constrict or dilate, and the overall pressure inside the skull can change. All of these elements interact to keep the total CBF at a constant level. The pressure of the flow of blood through the brain is sometimes referred to as the cerebral perfusion pressure.
Carbon dioxide levels in the blood have been shown to affect cerebral blood flow. When the pressure of carbon dioxide becomes low, as can happen when people hyperventilate, the cerebral blood flow decreases. This is because low carbon dioxide levels cause the cerebral blood vessels to constrict, so less blood is available for brain tissues and the person feels dizzy and light-headed.
Variations in blood flow in different regions of the brain are linked to metabolic activity there, with active areas, in which tissues are taking up more glucose, being the places with a higher blood flow. The exact mechanism by which this takes place is still not fully understood. Perfusion scanning is a technique using technology such as MRI, or magnetic resonance imaging, to produce images of the brain showing the increased blood flow in such areas. These images can be used to demonstrate which parts of the brain are involved in carrying out certain activities, and they can also help in understanding diseases such as Alzheimer’s.
Using imaging techniques, altered cerebral blood flow has been found in association with a range of different conditions, including hypothyroidism, where the thyroid gland does not produce enough thyroid hormone. Here, decreased blood supply has been found in certain areas of the brain, including those concerned with memory and attention, which could give rise to some of the symptoms of the disease. Hypothyroidism can be treated by taking thyroid hormone tablets to replace the missing natural hormones.