Arterial Structure

Arteries are classified into three categories based on their size and histological features. The large or elastic arteries, such as the aorta and its major branches, have three layers: the tunica intima, the tunica media, and the tunica adventitia. In large arteries, the intima is a smooth layer of thin endothelial cells on a basement membrane, the media is the muscular layer that is rich in elastic tissue, and the adventitia is a layer of connective tissue with elastic fibers, nerve fibers, and the vasa vasorum. With aging, the elastic fibers of the media in large vessels become less resilient. This causes less expansion with blood pressure elevations and predisposes to tortuosity. The medium or muscular arteries also consist of three layers. The distinguishing features of muscular arteries are the internal elastic lamina that forms the outer limit of the intima and the external elastic membrane that forms the outer layer of the media and is more defined than in large arteries. The adventitial layer in muscular arteries contains more nerves than this layer in large vessels due to their role in autonomic control. Arteriosclerosis and atheromatosis are most characteristic of the large and medium-sized vessels. Medial calcific sclerosis can affect medium-sized arteries. Finally, small arteries are characterized by a progressive loss of the three layers such that arterioles have no identifiable layers. The extracranial cerebral arteries have the usual structure of elastic or muscular arteries. However, intracranial

arteries differ because they have no external elastic lamina and there is no vasa vasorum. y Blood-Brain Barrier

The microcirculation of the brain is unique because of the blood-brain barrier (BBB). This barrier to free metabolic exchange keeps a homeostatic environment around neurons. There are actually two parts to the BBB, including the BBB itself and the blood-CSF barrier. y The BBB consists of capillary endothelial cells that are joined at tight junctions devoid of fenestrae. There is very little pinocytosis occurring, and passage of substances depends on their affinity for lipids and the presence of carrier proteins. The blood-CSF barrier consists of the periventricular region, including the choroid plexus, median eminence, and the area postrema. In these locations, there is active pinocytosis that allows small molecules into the subependymal interstitial space. Cerebral edema occurs when there is an increase in water content in the brain. Cerebral infarctions, hemorrhages, and contusions cause vasogenic edema as a result of leakage of water and plasma directly into the central nervous system due to damaged capillary endothelial cells that have lost their BBB function.

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