Cerebral circulation is the movement of blood through a network of cerebral arteries and veins supplying the brain. The rate of cerebral blood flow in an adult human is typically 750 milliliters per minute, or about 15% of cardiac output. Arteries deliver oxygenated blood, glucose and other nutrients to the brain. Veins carry "used or spent" blood back to the heart, to remove carbon dioxide, lactic acid, and other metabolic products. Because the brain would quickly suffer damage from any stoppage in blood supply, the cerebral circulatory system has safeguards including autoregulation of the blood vessels. The failure of these safeguards may result in a stroke. The volume of blood in circulation is called the cerebral blood flow. Sudden intense accelerations change the gravitational forces perceived by bodies and can severely impair cerebral circulation and normal functions to the point of becoming serious life-threatening conditions. The following description is based on idealized human cerebral circulation. The pattern of circulation and its nomenclature vary between organisms. Anatomy Blood supply Blood supply to the brain is normally divided into anterior and posterior segments, relating to the different arteries that supply the brain. The two main pairs of arteries are the Internal carotid arteries (supply the anterior brain) and vertebral arteries (supplying the brainstem and posterior brain).. The anterior and posterior cerebral circulations are interconnected via bilateral posterior communicating arteries. They are part of the Circle of Willis, which provides backup circulation to the brain. In case one of the supply arteries is occluded, the Circle of Willis provides interconnections between the anterior and the posterior cerebral circulation along the floor of the cerebral vault, providing blood to tissues that would otherwise become ischemic. Anterior cerebral circulation The anterior cerebral circulation is the blood supply to the anterior portion of the brain including eyes. It is supplied by the following arteries: Internal carotid arteries: These large arteries are the medial branches of the common carotid arteries which enter the skull, as opposed to the external carotid branches which supply the facial tissues; the internal carotid artery branches into the anterior cerebral artery and continues to form the middle cerebral artery. Anterior cerebral artery (ACA) Anterior communicating artery: Connects both anterior cerebral arteries, within and along the floor of the cerebral vault. Middle cerebral artery (MCA) Posterior cerebral circulation The posterior cerebral circulation is the blood supply to the posterior portion of the brain,