50
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
Media
Smooth muscle
Collagen
Elastin
Adventitia
Collagen
Fibroblasts
Vasa vasorum
Nerves
Intima
Endothelium
■ FIGURE 3.7 Blood vessel components. Blood vessels, except capillaries and small postcapillary venules,
are composed of three layers: intima, media, and adventitia. Capillaries and small postcapillary venules do
not have media and adventitia. The prim ary components are given for each layer.
of thin endothelial cells, which are separated
from the media by a basal lamina. In larger
vessels, a region of connective tissue also
exists between the endothelial cells and the
basal lamina. The media contains smooth
muscle cells, imbedded in a matrix of col-
lagen,
elastin,
and
various
glycoproteins.
Depending on the size of the vessel, there
may be several layers of smooth muscle cells,
some arranged circumferentially and others
arranged helically along the longitudinal axis
of the vessel. The smooth muscles cells are
organized so that their contraction reduces
the vessel diameter. The ratio of smooth mus-
cle, collagen, and elastin, each of which has
different elastic properties, determines the
overall mechanical properties of the vessel.
For example, the aorta has a large amount of
elastin, which enables it to passively expand
and contract as blood is pumped into it from
the heart. This mechanism enables the aorta
to dampen the arterial pulse pressure (see
Chapter 5). In contrast, smaller arteries and
arterioles have a relatively large amount of
smooth muscle, which is required for these
vessels to contract and thereby regulate arte-
rial blood pressure and organ blood flow. The
outermost layer, or adventitia, is separated
from the media by the external elastic lamina.
The adventitia contains collagen, fibroblasts,
blood vessels (vasa vasorum found in large
vessels), lymphatics, and autonomic nerves
(primarily
sympathetic
adrenergic).
The
smallest vessels, capillaries, are composed of
endothelial cells and a basal lamina; they are
devoid of smooth muscle.
Vascular Smooth Muscle Cells
CELLULAR STRUCTURE OF VASCULAR
SMOOTH MUSCLE
Vascular smooth muscle cells are typically 5 to
10 |J,m in diameter and vary from 50 to 300 |J,m
in length. Numerous small invaginations (cav-
eolae) found in the cell membrane significantly
increase the surface area of the cell (Fig. 3.8).
The sarcoplasmic reticulum is poorly devel-
oped compared with the sarcoplasmic reticu-
lum found in cardiac myocytes. Contractile
proteins (actin and myosin) are present; how-
ever, the actin and myosin in smooth muscle are
not organized into distinct bands of repeating
units as they are in cardiac and skeletal mus-
cle. Instead, bands of actin filaments are joined
together and anchored by dense bodies within
the cell or dense bands on the inner surface
of the sarcolemma, which function like Z-lines
in cardiac myocytes. Each myosin filament is
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