Page 399 - Atlas of Histology with Functional Correlations

trunk and aorta with their major branches, the brachiocephalic, common

carotid, subclavian, vertebral, pulmonary, and common iliac arteries. The walls
of elastic vessels are composed of elastic connective tissue fibers interspersed
with circularly arranged smooth muscle fibers that provide great resilience and
flexibility during blood flow.

The large elastic arteries branch and become medium-sized muscular

arteries, the most numerous vessels in the body. In contrast to elastic arteries,
the walls of muscular arteries contain greater amounts of smooth muscle fibers.

Arterioles are the smallest branches of the arterial system. Their walls
consist of one to five layers of smooth muscle fibers. Arterioles deliver blood to

the smallest blood vessels, the capillaries; capillaries connect arterioles with the
smallest veins or venules.

Structural Plan of Arteries

The wall of a typical artery contains three concentric layers, or tunics (Fig.
10.1). The innermost layer facing the lumen is the tunica intima. This layer
consists of a simple squamous epithelium, called endothelium in the vascular
system, and a thin underlying layer of subendothelial connective tissue. The
middle layer is the tunica media, composed primarily of smooth muscle fibers.

Interspersed among the smooth muscle fibers are elastic and reticular fibers. In
the muscular and elastic arteries, smooth muscle fibers produce the elastic
fibers, some collagen fibers, and extracellular elements. The collagen fibers

provide tensile strength to the arterial walls, whereas the elastic fibers allow for
the distention and recoil of the vessel walls during heart contraction and blood
ejection. The outermost layer is the tunica adventitia, composed primarily of
longitudinally oriented collagen fibers and elastic connective tissue fibers;
adventitia consists primarily of collagen type I fibers.

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