Human Physiology, 14th edition (2016)

432 Chapter 13

their diameters. Unlike the larger elastic arteries, therefore,

the diameter of the smaller muscular arteries changes only

slightly as the pressure of the blood rises and falls during the

heart’s pumping activity. Because arterioles and small muscu-

lar arteries have narrow lumina, they provide the greatest resis-

tance to blood flow through the arterial system.

Small muscular arteries that are 100  m m or less in diam-

eter branch to form smaller arterioles (20 to 30  m m in diam-

eter). In some tissues, blood from the arterioles can enter the

venules directly through arteriovenous anastomoses. In most

cases, however, blood from arterioles passes into capillaries

( fig.  13.27 ). Capillaries are the narrowest of blood vessels (7

to 10  m m in diameter). They serve as the “business end” of the

circulatory system, where gases and nutrients are exchanged

between the blood and the tissues.

Resistance to blood flow is increased by vasoconstriction of

arterioles (by contraction of their smooth muscle layer), which

of the cardiac cycle—when the heart is resting and not provid-
ing a driving pressure.
The small arteries and arterioles are less elastic than the
larger arteries and have a thicker layer of smooth muscle for

Figure 13.26 The structure of blood vessels. Notice the relative thickness and composition of the tunicas (layers) in
comparable arteries and veins.

Venous Circuit

Tunica Large vein

externa

Arterial Circuit

Large artery

Medium-sized vein Medium-sized artery

Venule Arteriole

Tunica externa

Endothelium

Elastic layer

Tunica

interna

Tunica externa

Endothelium

Lumen

Precapillary

sphincter

Endothelial cells

Basement membrane

Capillary pores

Valve

Valve

Tunica interna

Tunica media

Endothelium

Tunica externa

Tunica externa

Tunica media

Tunica interna

Tunica Tunica media

media

Tunica

interna

Endothelium

Lumen

Inferior

vena cava Aorta

Fenestrated

capillary

Continuous

capillary

CLINICAL APPLICATION

An aneurism is a balloon-like swelling in an artery or in a

weakened ventricular wall. It most commonly occurs in the

aorta—either as a thoracic aortic aneurism or an abdomi-

nal aortic aneurism, but can occur in cerebral and other

arteries. A dissected aorta is a tear in the wall of the aortic

aneurism, which may be detected and corrected before it

completely bursts. Aneurisms may result from congenital

causes and atherosclerosis (section 13.7), but conditions

such as hypertension and diabetes can increase the risk.