Essentials of Anatomy and Physiology

and nutrients and to remove waste products. There
are arterial anastomoses, for example, between some
of the coronary arteries that supply blood to the
myocardium.
A venous anastomosis helps ensure that blood will
be able to return to the heart in order to be pumped
again. Venous anastomoses are most numerous among
the veins of the legs, where the possibility of obstruc-
tion increases as a person gets older (see Box 13–2:
Disorders of Veins).

CAPILLARIES

Capillariescarry blood from arterioles to venules.
Their walls are only one cell in thickness; capillaries

are actually the extension of the endothelium, the sim-

ple squamous lining, of arteries and veins (see Fig.

13–1). Some tissues do not have capillaries; these are

the epidermis, cartilage, and the lens and cornea of the

eye.

Most tissues, however, have extensive capillary net-

works. The quantity or volume of capillary networks

in an organ reflects the metabolic activity of the organ.

The functioning of the kidneys, for example, depends

upon a good blood supply. Fig. 18–2 shows a vascular

cast of a kidney; you can see how dense the vessels are,

most of which are capillaries. In contrast, a tendon

such as the Achilles tendon at the heel or the patellar

tendon at the knee would have far fewer vessels,

because fibrous connective tissue is far less metaboli-

cally active.

The Vascular System 293

Tunica externa

External elastic

lamina

Tunica

media

Internal elastic

lamina

Endothelium (lining)

Artery

Arteriole

Endothelial

cells

Smooth muscle

Precapillary

sphincter

Capillary

Blood flow

Venule

Vein

Valve

Tunica

intima

Tunica

externa

Tunica

media

Figure 13–1. Structure of
an artery, arteriole, capillary
network, venule, and vein.
See text for description.
QUESTION:What tissue is the
tunica media made of, and
how is this layer different in an
artery and in a vein?