Although the brain is only about 2% of the total body weight in humans, it receives 15-20% of the body's blood supply. Because brain cells will die if the supply of blood which carries oxygen is stopped, the brain has top priority for the blood. Even if other organs need blood, the body attempts to supply the brain with a constant flow of blood. The blood brings many materials including oxygen, carbohydrates, amino acids, fats, hormones and vitamins necessary for the brain to function properly. The blood also removes materials including carbone dioxide, ammonia, lactate and hormones from the brain. This function is performed by many blood vessels.
The head’s blood supply comes mainly from the external and internal carotid arteries.
The internal carotid artery travels up from the aortic arch just outside the heart. It travels into the brain to provide oxygenated blood to the eyes, the front of the brain, and portions of the scalp.
The external carotid artery helps supply part of the brain through its many branches, and it also gives blood to the thyroid gland in the neck.
These arteries go through the neck and reach the head.
The vertebral arteries arising from the subclavian arteries and continuing to the basilar artery is another source of the blood vessels to the brain.
In the brain, a circle consisting to of two carotid arteries and the basilar artery form the circle of Willis. This supplies blood in the center of the brain and branches to the cerebrum, pons, medulla oblongata, cerebellum, and the beginning of the spinal cord.
Deoxygenated blood leaves the brain and the head and goes back to the heart through veins accompanied with the arteries.
The cranial venous sinuses also remove blood from the head. Unlike normal veins, these are large channels that drain blood.
