Chapter 11. Blood Supply
The Blood Supply
The Blood Supply Medial View
The Blood Supply Lateral View
Blood transports oxygen and other nutrients necessary for the health of neurons, so a constant flow of blood to the brain must be maintained.
According to Love and Webb,1992, the brain uses approximately twenty percent of the body's blood and needs twenty-five percent of the body's oxygen supply to function optimally. Blood flow in a healthy person is 54 milliliters per 1000 grams of brain weight per minute. There are 740 milliliters of blood circulating in the brain every minute. 3.3 milliliters of oxygen are used per minute by every 1000 grams of brain tissue. This means that approximately 46 milliliters of oxygen are used by the entire brain in one minute. During sleep, blood flow to the brain is increased, but the rate of oxygen consumption remains the same.
The main artery of the body is called the aorta. It supplies blood to all parts of the body with the exception of the lungs. The aorta ascends from the heart and forms an arch, from which arise two subclavian arteries. Each subclavian has two main branches, the common carotid and the vertebral. Both of these carry blood to the brain.
Each common carotid divides into an external carotid artery, which supplies blood to the face and an internal carotid artery which supplies the brain with blood.
The external carotid is a fairly straight artery, so it is not prone to blockages due to the build up of cholesterol. Even if a blockage does occur, it would obviously not cause a stroke as this artery does not carry blood to the brain.
Each internal carotid artery ascends along one side of the neck. They pass behind the ear into the temporal lobe and enter the subarachnoid space. Then, they run posteriorly to the medial end of the fissure of Sylvius where they bifurcate into two main branches, the anterior cerebral artery and the middle cerebral artery.
As the internal carotids have many twists and turns, there are many places where plaque can build up, causing a blockage. Such blockages can be identified by sonogram (non-invasive), or by angiograms (invasive). Also, a sound called a bruit can sometimes be heard via stethoscope when a blockage exists.
The anterior cerebral artery goes above the optic chiasm to the medial surface of the cerebral hemispheres. It arches around the genu (horn) of the corpus callosum (FitzGerald, 1996). It supplies blood to the medial cortex, including the medial aspect of the motor strip and the sensory strip. This means that damage to the anterior cerebral artery can cause sensory and motor impairment in the lower body. For example, a patient who has had a stroke affecting this artery may be incontinent or have unilateral paralysis from the hips on down.
The anterior cerebral artery also delivers blood to some parts of the frontal lobe and corpus striatum. So a blockage in this artery can affect cognition and cause motoric problems due to damage to fibers in the internal capsule or to the basal ganglia.
The other main branch of the internal carotids is the middle cerebral artery. This large artery has tree-like branches that bring blood to the entire lateral aspect of each hemisphere. This means that this artery supplies blood to the cortical areas involved in speech, swallowing and language, including the lateral motor strip, lateral sensory strip, Broca's area, Wernicke's area, Heschl's gyrus, and the angular gyrus. In addition, it provides most of the blood supply to the corpus striatum.
If a patient has a blockage in the middle cerebral artery, it is probable that s/he will have aphasia. S/he will probably also have impaired cognition and corticohyposthesia, or numbness, on the opposite side of the body. Problems with hearing and the sense of smell may also result from damage to this artery because it supplies the lateral surface of the temporal lobe.
The central branches of the middle cerebral are the medial and lateral striata arteries. The striata supply the basal ganglia, internal capsule, and thalamus (FitzGerald, 1996). Because they are the main blood supply to the internal capsule, they are called by some the arteries of stroke. When something happens to these arteries, the bottleneck of fibers within the internal capsule can be damaged, causing many disabilities. The striata are very thin arteries and blood pressure within it high. For this reason, they are considered by many to be more vulnerable to hemorrhages than to blockages, although FitzGerald says that occlusion of one of these areteries is the major cause of of classical stroke where pyramidal tract damage results in contralateral hemiplegia.
Other arteries which arise from the internal carotid arteries include the anterior communicating artery and the posterior communicating arteries.
The anterior communicating artery joins the anterior cerebral arteries of each hemisphere together.
The posterior communicating arteries join the middle cerebral arteries to the posterior cerebral arteries, which are part of the basilar artery system.
Both of the vertebral arteries ascend through the spinal column and enter the brain through the magnum foramen. Once in the brain, they continue to ascend, traveling beside the brain stem. At the lower border of the pons the two vertebral arteries join together to form the basilar artery or vertebro-basilar artery.
The vertebral arteries and the basilar are straight arteries and therefore not as subject to blockages due to the build up of cholesterol as are the internal carotids.
The posterior inferior cerebellar not only supply the cerebellum but take blood to the lateral medulla. Anterior and posterior spinal arteries the ventral and dorsal medulla, respectively (FitzGerald 1996). The three arteries are branches of the vertebral.
The side of the pons and the cerebellum receive blood from the anterior inferior cerebellar artery and the superior cerebellar artery. These arteries are branches of the basilar. The anterior inferior cerebellar artery also has a branch, the labyrinthine artery, that supplies the inner ear. The basilar also gives off about twelve pontine arteries that supply the medial pons (FitzGerald, 1996).
At the superior border of the pons, the basilar artery divides to form the two posterior cerebral arteries.
Before the basilar artery divides, several other arteries arise from it. These include the anterior, inferior, and posterior cerebellar arteries as well as pontine branches. So, the cerebellum and pons are supplied by branches of the basilar.
The posterior cerebral arteries supply the part of the brain found in the posterior fossa of the skull, including the medial area of the occipital lobes and the inferior aspects of the temporal lobes. They also supply the midbrain and deliver blood to the thalamus and some other subcortical structures. Blockages in this artery can affect the sense of smell, and cause cranial nerve damage, as well as visual problems, including visual agnosia, hemianopsia and alexia.
The choroidal arteries, which arise both from the divisions of the internal carotid arteries and from the basilar system, supply blood to the choriod plexuses and also to the hippocampus. Blockages in these arteries can affect the production of cerebrospinal fluid and can also cause memory problems. Below is a graphic image of the circle of Willis. Open the link to visualize some of the vascular problems that can occur.
The Circle of Willis or the Circulus Arteriosus is the main arterial anastomatic trunk of the brain. According to Bhatnagar and Andy, 1995, anastomosis occurs when blood vessels bring blood to one spot from which it is then redistributed. The Circle of Willis is a point where the blood carried by the two internal carotids and the basilar system comes together and then is redistributed by the anterior, middle, and posterior cerebral arteries.
The anterior cerebral arteries of the two hemispheres are joined together by the anterior communicating artery. The middle cerebral arteries are linked to the posterior cerebral arteries by the posterior communicating arteries. This anastamosis or communication between arteries make collateral circulation which Love and Webb, 1995, define as "the flow of blood through an alternate route" (p. 40) possible. This is a safety mechanism, allowing brain areas to continue receiving adequate blood supply even when there is a blockage somewhere in an arterial system. The blood streams of the internal carotid system and the basilar system meet in the posterior communicating arteries. If there are no problems in either system, the pressure of the streams will be equal and they will not mix. However, if there is a blockage in one of them blood will flow from the intact artery to the damaged one, preventing a cerebral vascular accident.
As long as the Circle of Willis can maintain blood pressure at fifty percent of normal, no infarction or death of tissue will occur in an area where a blockage exists. If collateral circulation is good, no permanent effects may result from a blockage.
Sometimes, an adjustment time is required before collateral circulation can reach a level that supports normal functioning; the communicating arteries will enlarge as blood flow through them increases. In such cases, a transient ischemic attack may occur, meaning that parts of the brain are temporarily deprived of oxygen.
Some people lack one of the communicating arteries that form the Circle of Willis. In this case, if a blockage develops, collateral circulation will be impeded and the collateral blood supply will be compromised, causing brain damage to occur.
There are some watershed areas in the brain located at the ends of the vascular systems. Problems with blood supply are particularly likely to occur here, especially in those who have hardening of the arteries. Blockages in the water shed areas can cause transcortical aphasia.
Extraneural Factors Affecting the Blood Supply to the Brain
Low or High Blood Pressure
Abnormally low blood pressure can cause brain damage. This may occur as a result of surgical shock which involves blood pressure as low as 70 milliliters per kilogram of tissue.
Hypertension, or blood pressure that remains high regardless of activity level, can cause arteries to narrow over time.
Cerebrovascular Resistance
Cerebrovascular resistance makes collateral circulation more difficult. It can be caused by an arterial spasm (remember that arteries are lined with muscles). Another potential cause of resistance is increased viscosity of the blood, which can result from leukemia or from high levels of tri-glycerides in the blood, or other factors such as increased red blood cells, often seen in chronic obstructive pulmonary disease (COPD). Increased cerebrospinal fluid pressure can also lead to high levels of cerebrovascular resistance.
Atherosclerosis
This is hardening of the arteries which often occurs with old age but can also happen in young people.
An occluded artery may cause a stroke due to one of the extraneural factors listed above which can compromise the overall integrity of the cerebrovascular system. If an individual does not have one of these problems and has a sufficient number of communicating arteries, a blockage may not have a significant effect on blood supply throughout the brain.
Many substances present in the blood supply are unable to pass through the meninges into the cells of the central nervous system. The blood brain barrier includes two components, the blood/cerebrospinal fluid barrier and the arachnoid barrier layer.
Cerebrospinal fluid is a filtrate of blood by the choroid plexuses (capillary networks) of the ventricles which are formed by fusion of the pia mater and the ependyma (ventricular lining). In the course of this process, not all components of blood are allowed to enter the brain. According to Webster, 1999, only clear plasma passes through, leaving blood cells behind.
The arachnoid barrier layer is a part of the arachnoid meningeal layer. It is formed by tight junctions between the endothelial cells of cerebral capillaries in the arachnoid mater.
Glucose diffuses across the blood-brain barrier through a process that is like selective osmosis.