The role of blood vessels in the circulation of blood

has been known since 1628, when William Harvey, an
English anatomist, demonstrated that blood in veins
always flowed toward the heart. Before that time, it
was believed that blood was static or stationary, some
of it within the vessels but the rest sort of in puddles
throughout the body. Harvey showed that blood
indeed does move, and only in the blood vessels
(though he did not know of the existence of capillar-
ies). In the centuries that followed, the active (rather
than merely passive) roles of the vascular system were
discovered, and all contribute to homeostasis.
The vascular system consists of the arteries, capil-
laries, and veins through which the heart pumps blood
throughout the body. As you will see, the major “busi-
ness” of the vascular system, which is the exchange of
materials between the blood and tissues, takes place in
the capillaries. The arteries and veins, however, are
just as important, transporting blood between the cap-
illaries and the heart.
Another important topic of this chapter will be
blood pressure (BP), which is the force the blood
exerts against the walls of the vessels. Normal blood
pressure is essential for circulation and for some of the
material exchanges that take place in capillaries.

ARTERIES

Arteriescarry blood from the heart to capillaries;
smaller arteries are called arterioles. If we look at an
artery in cross-section, we find three layers (or tunics)
of tissues, each with different functions (Fig. 13–1).
The innermost layer, the tunica intima, is the only
part of a vessel that is in contact with blood. It is made
of simple squamous epithelium called endothelium.
This lining is the same type of tissue that forms the
endocardium, the lining of the chambers of the heart.
As you might guess, its function is also the same: Its
extreme smoothness prevents abnormal blood clot-
ting. The endothelium of vessels, however, also pro-
duces nitric oxide (NO), which is a vasodilator. The
tunica media, or middle layer, is made of smooth
muscle and elastic connective tissue. Both of these tis-
sues are involved in the maintenance of normal blood
pressure, especially diastolic blood pressure when the
heart is relaxed. The smooth muscle is the tissue
affected by the vasodilator NO; relaxation of this mus-

cle tissue brings about dilation of the vessel. Smooth

muscle also has a nerve supply; sympathetic nerve

impulses bring about vasoconstriction. Fibrous con-

nective tissue forms the outer layer, the tunica

externa. This tissue is very strong, which is important

to prevent the rupture or bursting of the larger arter-

ies that carry blood under high pressure (see Box 13–1:

Disorders of Arteries).

The outer and middle layers of large arteries are

quite thick. In the smallest arterioles, only individual

smooth muscle cells encircle the tunica intima. As

mentioned, the smooth muscle layer enables arteries

to constrict or dilate. Such changes in diameter are

regulated by the medulla and autonomic nervous sys-

tem, and will be discussed in a later section on blood

pressure.

VEINS

Veinscarry blood from capillaries back to the heart;

the smaller veins are called venules. The same three

tissue layers are present in veins as in the walls of

arteries, but there are some differences when com-

pared to the arterial layers. The inner layer of veins is

smooth endothelium, but at intervals this lining is

folded to form valves(see Fig. 13–1). Valves prevent

backflow of blood and are most numerous in veins of

the legs, where blood must often return to the heart

against the force of gravity.

The middle layer of veins is a thin layer of smooth

muscle. It is thin because veins do not regulate blood

pressure and blood flow into capillaries as arteries do.

Veins can constrict extensively, however, and this

function becomes very important in certain situations

such as severe hemorrhage. The outer layer of veins is

also thin; not as much fibrous connective tissue is nec-

essary because blood pressure in veins is very low.

ANASTOMOSES

An anastomosisis a connection, or joining, of vessels,

that is, artery to artery or vein to vein. The general

purpose of these connections is to provide alternate

pathways for the flow of blood if one vessel becomes

obstructed.

An arterial anastomosis helps ensure that blood

will get to the capillaries of an organ to deliver oxygen

292 The Vascular System