the layer of endothelial cells that provides a smooth, low friction innermost
surface. This layer is in direct contact with the blood, and damage to it leads
to serious clinical problems. The tunica media is a relatively thick middle
layer composed of smooth muscle and elastic fibers, which are arranged both
lengthways and circumferentially. The amount of elastic material varies with
the vessel. Closer to the heart, the aorta and large branches contain larger
amounts because they have to accommodate the stroke volume. In contrast,
the pulmonary artery has rather thin walls. The smooth muscle is under
autonomic nervous control that regulates the diameter of the vessel. The
fibrous components limit the amount of stretch under a surge of pressure.
The tunica adventitia is the outer covering fibrous layer composed of tough
collagenous material.
Arteries branch repeatedly in order to supply all parts of the body. The diameter
of the aorta is about 25 mm, and that of a medium-sized artery about 4 mm.
BLOOD VESSELSCZhhVg6]bZY!BVjgZZc9Vlhdc!8]g^hHb^i]:YLddY (-*
Figure 14.7 (A) Photomicrograph showing the
cross sections of an artery and a vein. Schematics
illustrating the layers in the walls of (B) an artery
and (C) a vein.ArteryVeinEndotheliumSmooth muscleInternal elastic laminaExternal elastic lamina
Connective tissue/collagenEndotheliumSmooth muscleInternal elastic laminaExternal elastic lamina
Connective tissue/collagenBloodBloodA)EndotheliumSmooth muscleInternal
elastic laminaExternal
elastic laminaConnective
tissue/collagenB) C)