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The animation shows the normal direction of flow through the circle of Willis to the middle cerebral artery. The circle of Willis is a specialized arrangement of arteries that ensure constant perfusion of the cerebrum even in the event of a blockage of one of the arteries in the circle. Watch this animation to see how blood flows to the brain and passes through the circle of Willis before being distributed through the cerebrum. Figure 14.2.1 – Circle of Willis: The blood supply to the brain enters through the internal carotid arteries and the vertebral arteries, eventually giving rise to the circle of Willis. The left and right internal carotid arteries and branches of the basilar artery all become the circle of Willis, a confluence of arteries that can maintain perfusion of the brain even if narrowing or a blockage limits flow through one part ( Figure 14.2.1). The two vertebral arteries then merge into the basilar artery, which gives rise to branches to the brain stem and cerebellum.

Branches off the left and right vertebral arteries merge into the anterior spinal artery supplying the anterior aspect of the spinal cord, found along the anterior median fissure. The vertebral arteries enter the cranium through the foramen magnum of the occipital bone. A second set of vessels that supply the CNS are the vertebral arteries, which are protected as they pass through the neck region by the transverse foramina of the cervical vertebrae. The internal carotid artery enters the cranium through the carotid canal in the temporal bone. Heart rate increases-a reflex of the sympathetic division of the autonomic nervous system-and this raises blood pressure. The orthostatic reflex is a reaction to this change in body position, so that blood pressure is maintained against the increasing effect of gravity (orthostatic means “standing up”). The bases of the common carotids contain stretch receptors that immediately respond to the drop in blood pressure upon standing. The external carotid arteries supply blood to the tissues on the surface of the cranium. The next branches give rise to the common carotid arteries, which further branch into the internal carotid arteries. The very first branches off the aorta supply the heart with nutrients and oxygen. The major artery carrying recently oxygenated blood away from the heart is the aorta. There are multiple routes for blood to get into the CNS, with specializations to protect that blood supply and to maximize the ability of the brain to get an uninterrupted perfusion. Blood Supply to the BrainĪ lack of oxygen to the CNS can be devastating, and the cardiovascular system has specific regulatory reflexes to ensure that the blood supply is not interrupted. Beyond the supply of blood, the CNS filters that blood into cerebrospinal fluid (CSF), which is then circulated through the cavities of the brain and spinal cord called ventricles. This begins with a unique arrangement of blood vessels carrying fresh blood into the CNS. Because of this privilege, the CNS needs specialized structures for the maintenance of circulation. To protect this region from the toxins and pathogens that may be traveling through the blood stream, there is strict control over what can move out of the general systems and into the brain and spinal cord. The function of the tissue in the CNS is crucial to the survival of the organism, so the contents of the blood cannot simply pass into the central nervous tissue.

The CNS has a privileged blood supply, as suggested by the blood-brain barrier. The CNS is crucial to the operation of the body, and any compromise in the brain and spinal cord can lead to severe difficulties. Explain how a disruption in circulation would result in a stroke.Explain the production of cerebrospinal fluid and its flow through the ventricles.