Blood flow to the brain (cerebral blood flow) is essential to maintain consciousness and many vital functions. Unlike certain other parts of the body, a few seconds without blood flow will not be detrimental to functioning but in the brain, a lack of blood flow for even just 10 seconds will lead to unconsciousness. The brain accounts for only 2.5% of the body weight but receives about 15% of the resting cardiac output – this is the amount of oxygenated blood pushed out of the heart every minute at rest. It equates to between 750 milliliters to 1 liter of blood per minute.
Blood Vessels of the Brain
Arteries of the Brain
The blood supply to the brain is via the internal carotid arteries and vertebral arteries. The internal carotid artery and its branches make up the anterior circulation of the brain via the anterior and middle cerebral arteries whereas the vertebrobasilar arteries make up the posterior circulation of the brain via the posterior cerebral arteries.
The internal carotid artery arises from the common carotid artery in the neck, enters the cranial cavity through the carotid canal in the temporal bone and gives rise to two terminal branches – anterior and middle cerebral arteries. The anterior cerebral artery supplies the medial and superior surfaces of the brain as well as the frontal pole. The middle cerebral artery supplies the laterals surface of the brain and the temporal lobe. An anterior communicating artery connects the anterior cerebral arteries from each side.
The vertebral artery is the first branch of the subclavian artery. It ascends up the neck by weaving through the transverse foramina of the cervical vertebra (first six neck vertebra). At the level of C1, the vertebral arteries on either side pierce the meninges and then merge to form the basilar artery. It then ends by dividing into the posterior cerebral arteries which supply the inferior surface of the brain and the occipital lobes. The posterior cerebral arteries join the internal carotid arteries by the posterior communicating arteries.
Circle of Willis
The circle of Willis, the common name for the cerebral arterial circle, is an important point when the four arteries (two internal carotid arteries and two vertebral arteries) communicate with each other. Located the base of the brain, this vascular circle is made up by the anterior communicating, anterior cerebral, internal carotid, posterior communication and posterior cerebral arteries. Branches from this circle supply various parts of the brain.
Veins of the Brain
Blood draining from the various veins of the brain ultimately end up in the internal jugular vein via the dural venous sinuses. Deoxygenated blood from the superolateral surfaces of the brain (top and sides) drains via the superior cerebral veins drain into the superior sagittal sinus. This superior cerebral veins along with the inferior cerebral veins and drains blood from the cerebellum into the transverse sinus. Blood from the inferior (bottom), posteroinferior (back and bottom) and deep parts of the cerebrum drain into the straight, transverse and petrosal sinuses via the inferior and superficial cerebral veins. The single large midline vein, known as the great cerebral vein (vein of Galen) is formed by the joining of the two internal cerebral veins. This then drains into the straight sinus.
Regulation of Cerebral Blood Flow
The main three factors for controlling blood flow to the brain includes :
- carbon dioxide concentration
- oxygen concentration
- hydrogen ion concentration
The brain is a a very “oxygen-hungry” organ utilizing one-sixth of the cardiac output although it accounts for less than 3% of the body weight. When the carbon dioxide levels build up, it combines with water to form carbonic acid and the hydrogen ions due to subsequent dissociation. This leads to vasodilation of the cerebral arteries to increase blood flow to the brain. However, an increase in acidity within the tissue spaces of the brain (hydrogen ions) can also elicit a similar effect even though the carbon dioxide levels are normal.
A drop in oxygen levels within the blood will also trigger vasodilation, even if the carbon dioxide or hydrogen ion concentration is normal. This may be seen where the demand for more oxygen by the brain, like during increased activity, triggers the appropriate mechanism to increase the flow of oxygenated blood.