Vascular Biology, Structure and Function
23
Table
2.2.1
:
External diameters
and
taper factors.
0.342
0.0 18*0.007
In the broadest sense, the arterial wall (Fig. 2.2.2) consists of elastin,
collagen, and smooth muscle embedded in a mucopolysaccharide ground
substance.
A
cross section reveals the tunica intima, which is the
innermost layer consisting of
a
thin layer
(0.5-1
pm) of endothelial cells,
connective tissue, and basement membrane. The next layer is the thick
tunica media, separated from the intima by
a
prominent layer of elastic
tissue, the internal lamina. The media contains elastin, smooth muscle,
and collagen. The difference in their composition divides arteries into
elastic and muscular vessels.
The relative content of these in different vessels
is
shown in Fig.
2.2.3. All vessels, including the capillary, have endothelium.
The
capillary does not have smooth muscle content and has only
a
single
layer
of
endothelial cells. The outermost layer
is
the adventitia which is
made up mostly of stiff collagenous fibers.
Elastic laminae are concentrically distributed and attached by smooth
muscle cells and connective tissue. Longitudinally, we find that the
number of elastic laminae decreases with increasing distance from the
aorta, but the amount of smooth muscle increases and the relative wall
thickness increases. Thus, the wall thickness to radius ratio, or h/r is
increased. The net stiffness
is
also increased, accounting for the increase
in pulse wave velocity towards the periphery. The mechanical behavior
of peripheral vessels is largely influenced by the behavior
of
the smooth
muscle, particularly by its degree of activation.
