4. 腦血管解剖學.ppt

脑血管解剖学;The brain is a highly vascular organ, its profuse blood supply characterized by a densely branching arterial network. It has a high metabolic rate that reflects the energy requirements of constant neural activity. It receives about 15% of the cardiac output and utilizes 25% of the total oxygen consumption of the body. Blood flow to the brain averages 750 ml/min and is maintained at a relatively constant level. The necessity for constant cerebral blood flow becomes crystal clear when one takes into account that neurons are totally intolerant of ischemia. Blood flowing through coronary arteries accounts for 4~5% of cardiac output. One hundred grams of myocardial tissue consumes 8-10 ml oxygen from the blood every minutes. The heart absorbs 70-75% of oxygen of the blood when it flows through the heart. ;Major branches of cerebral arteries that lie in the subarachnoid space over the surface of the brain have a smooth muscle media and a distinct elastic lamina. However, as arteries enter the subpial space and penetrate the brain, they lose their elastic laminae, and consequently the cerebral cortex and white matter typically contain only arterioles, venules and capillaries. The exceptions are the large penetrating vessels in the basal ganglia, where many arteries retain their elastic laminae and thick smooth muscle media. Enlarged perivascular spaces form around these large arteries in ageing individuals. ;Arterioles and venules in the cortex and white matter can be distinguished from each other because arterioles are surrounded by a smooth muscle coat, and the veins and venules have larger lumina and thinner walls. Cerebral capillaries are the site of the blood–brain barrier. They are lined by endothelial cells which are joined by tight junctions. The endothelial cytoplasm contains a few pinocytotic vesicles. The cells are surrounded