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ANATOMY AND PHYSIOLOGY
The blood flow to the brain is supplied by the two internal carotid arteries anteriorly and the vertebral arteries posteriorly. The internal carotid artery, branches into several arteries: ophthalmic, posterior communicating, anterior cerebral, and middle cerebral. Most of the arterial blood within the internal carotid arteries is distributed by way of the anterior and middle cerebral arteries. The anterior
cerebral arteries supply the medial surface of the cerebrum and the anterior half of the thalamus, the corpus striatum, part of the corpus callosum, and the internal capsule. The posterior cerebral arteries supply the remaining occipital and inferior regions of the temporal lobes. The distal branches of the internal carotid and vertebral arteries communicate at the base of
the brain through the circle of Willis; this anastomosis of arteries can provide continued circulation if blood flow through one of the main vessels is disrupted. Without collateral input, cessation of blood flow in cerebral arteries may result in neural damage, because metabolic needs of electrically active cells of the brain can no longer be met.
The two vertebral arteries arise from the subclavian artery and enter the foramina in the transverse spinal processes at the level of the sixth cervical vertebra and continue upward through the foramina of the upper six vertebrae; they wind behind the atlas and enter the skull through the foramen magnum and unit to form the basilar artery. Branches of the basilar and vertebral supply the medulla, pons,
cerebellum, midbrain, and caudal part of the diencephalons. Because the vertebral arteries supply the basic life support reflexes that are located in these areas of the brain, interruption of blood flow in the carotid arteries may result in severe coma, although not necessarily death.
The cerebral blood is drained by two sets of veins that empty into dural venous sinuses: the deep (great) cerebral venous system and the superficial venous system. The deep system is well protected, in contrast to the superficial cerebral veins that travel through the pia meter on the surface of the cerebral cortex. These vessels connect directly to the sagittal sinuses within the falx cerebri by way of bridging
veins. They travel through the CSF-filled CSFsubarachnoid space and penetrate the arachnoid and then the dura to reach the dural venous sinuses. This system of sinuses returns blood to the heart primarily by way of the internal jugular veins. Alternate routes for venous flow also exist; for example, venous blood may exit through the emissary veins that pass through the skull and through veins that traverse various foramina to empty into extracranial veins.
The intracranial venous system has no valves. The direction of flow depends on gravity of the relative pressure in the venous sinuses compared with that of the extracranial veins. Increases in intrathoracic pressure, as can occur with coughing or performance of a Valsalva maneuver, produce a rise in central venous pressure that is reflected back into the internal jugular veins and to the dural sinuses. This briefly raises the ICP.