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All blood vessels contain an inner layer of epithelial cells called the
endothelium (FIGURE 15.1), which is continuous with the endocardial
lining of the heart. Th e endothelium is in direct contact with the
blood that fl ows through the lumen (interior opening) of the vessel.
Until recently, endothelial cells were regarded as little more than a
passive barrier between the blood and the remainder of the vessel
wall. It is now known that endothelial cells are active participants in
a variety of vessel-related activities, including physically infl uencing
blood fl ow, secreting locally acting chemical mediators that infl uence
the contractile state of the vessel’s overlying smooth muscle,
and assisting with capillary permeability. Surrounding the endothelium
of a blood vessel is a thin layer of extracellular material called
the basement membrane, which provides support to the endothelial
cells. With the exception of capillaries, all blood vessels contain layers
of smooth muscle and connective tissue that surround the basement
membrane, and these layers vary in thickness in diff erent types of
blood vessels (FIGURE 15.1).
The primary role of the smooth muscle of a blood vessel is
to regulate the diameter of the lumen. Sympathetic fibers of the
autonomic nervous system innervate vascular smooth muscle. An
increase in sympathetic stimulation typically causes the smooth
muscle to contract, squeezing the vessel wall and narrowing the
lumen. Such a decrease in the diameter of the lumen of a blood
vessel is called vasoconstriction. By contrast, when sympathetic
stimulation decreases, or in the presence of certain chemicals
(such as nitric oxide and H+ ions), smooth muscle fibers relax. The
resulting increase in lumen diameter is called vasodilation. The
smooth muscle of most blood vessels is at least partially contracted
at all times. The ability of a blood vessel's smooth muscle to maintain
a state of partial contraction is referred to as vascular tone. As
you will learn in more detail shortly, the rate of blood flow through
different parts of the body is regulated by the extent of smooth muscle
contraction in the walls of particular vessels. Furthermore, the
extent of smooth muscle contraction in particular vessel types is
crucial to the regulation of blood pressure.
Two types of connective tissue may be present in the walls of
blood vessels: elastic connective tissue and fibrous connective tissue.
Elastic connective tissue contains elastic fibers, which allow
blood vessels to stretch in response to incoming blood and then
to return to their original shape after being stretched. Fibrous connective
tissue contains collagen fibers, which provide blood vessels
with significant tensile strength that can sustain the pressure
that blood exerts against the vessel walls. Blood vessels vary with
respect to the amounts of elastic and fibrous connective tissues
that they contain.