The Blood Supply of the Brain

 Food and oxygen are carried to the brain by many blood vessels. These vessels are found on the surface of the brain and deep within the brain. The blood vessels (and nerves) enter the brain through holes in the skull called foramina (red arrows in the picture on the right).

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 necessary for the brain to function properly. The blood also removes materials from the brain.

Blood is supplied to the entire brain by 2 pairs of arteries: the internal carotid arteries and vertebral arteries. As you can see in the figure below, the right and left vertebral arteries come together at the base of the brain to form a single basilar artery. The basilar artery joins the blood supply of the internal carotid arteries in a ring at the base of the brain. This ring of arteries is called the circle of Willis. The circle of Willis provides a safety mechanism...if one of the arteries gets blocked, the "circle" will still provide the brain with blood.

Base of the brain

Only some of the vessels that exist in a real brain have been labeled.

Brain Attack = Stroke

You may know someone, a parent or grandparent, who has had a "stroke," also called a "brain attack." What exactly is a stroke? A stroke occurs when the blood supply to the brain is stopped. If this happens for enough time, neurons will start to die because they will not get enough oxygen. Paralysis or aphasia (loss of speech) are possible consequences of a stroke.

There are two major causes of a stroke:

  • Blockage of a blood vessel (in the brain or neck) caused by:
    • a blood clot in the brain or neck (this is called a thrombosis)
    • a blood clot from somewhere else that has moved and now blocks a blood vessel in the brain or neck (this is called an embolism)
    • constriction or narrowing of an artery in the head or neck (this is called a stenosis)
  • Bleeding of a blood vessel (this is called hemorrhagic stroke)

There are several warning signs that occur with a brain attack:
Reprinted with permission from The National Institute of Neurological Disorders and Stroke

  • Sudden weakness or numbness of the face, arm, or leg on one side of the body.
  • Sudden dimness or loss of vision, particularly in one eye.
  • Sudden difficulty speaking or trouble understanding speech.
  • Sudden severe headache with no known cause.
  • Unexplained dizziness, unsteadiness, or sudden falls, especially with any of the other signs.

There are several conditions linked to stroke:
Reprinted with permission from The National Institute of Neurological Disorders and Stroke

  • High blood pressure - Eat a balanced diet, maintain a healthy weight, and exercise to reduce blood pressure. Drugs are also available.
  • Cigarette smoking - Don't start smoking and if you do smoke, quit!
  • Heart disease - Your doctor will treat your heart disease and may also prescribe medication to help prevent the formation of clots.
  • Diabetes - Treatment can delay complications that increase the risk of stroke.
  • Transient ischemic attacks - These are brief episodes of stroke's warning signs and can be treated with drugs or surgery.

Did you know?
  • Each year there are 600,000 people in the United States who suffer a stroke. Stroke is the THIRD leading cause of death in the US and kills about 160,000 Americans each year. (Statistic from American Stroke Association)

  • The word "carotid" (carotid artery) comes from the Greek word karotis meaning "deep sleep." This is because it has been known for a long time that pressure to the carotid arteries causes animals to become sleepy.

  • In humans, the brain uses 15-20% of the body's oxygen supply. In the African elephant nose fish, the brain uses 60% of the oxygen supply! (Source: Nilsson, G.E., Brain and body oxygen requirements of Gnathonemus perterssi, a fish with an exceptionally large brain. J. Experi. Biol., 199:603-607, 1996.)

More about stroke:

BACK TO: Exploring the Nervous System Table of Contents

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