This chapter reviews the normal structure and function of the vascular
component of the cardiovascular system and then considers the pathophysiology
of three common conditions frequently seen by practicing physicians:
atherosclerosis, hypertension, and shock.
The blood vessels are a closed system of conduits that carry blood
from the heart to the tissues and back to the heart. All of the
blood flows through the lungs, but the systemic circulation is made
up of many different circuits in parallel (Figure
11–1). This permits wide variation in regional systemic
blood flow without changing the total systemic flow.
Diagram of the circulation in the adult.
(Redrawn, with permission, from Ganong WF. Review
of Medical Physiology, 22nd ed. McGraw-Hill, 2005.)
The characteristics of the various types of blood vessels in humans
are summarized in Figure 11–2. Note
that as the diameter of the vessels decreases, their number in the
body increases so that the total cross-sectional area increases.
Characteristics of systemic blood vessels. Cross sections
of the vessels are not drawn to scale because of the huge range
in size from aorta and vena cava to capillaries.
(Redrawn from Burton AC. Relation of structure
to function of the tissues of the wall of blood vessels. Physiol
All the blood vessels are lined by a single layer of endothelial
cells. Collectively, the endothelial cells constitute a remarkable
organ that secretes substances that affect the diameter of the vessels
and provide for their growth, their repair when injured, and the
formation of new vessels that carry blood to growing tissues.
The aorta, the large arteries, and the arterioles are made up
of an outer layer of connective tissue, the adventitia; a
middle layer of smooth muscle, the media; and an inner
layer, the intima, containing the layer of endothelial
cells and some subendothelial connective tissue. The walls of the
aorta and the large arteries contain abundant elastic tissue, much
of it concentrated in the internal elastic lamina, a
prominent band between the intima and the media, and another band,
the external elastic lamina, between the media and
the adventitia (Figure 11–3). The
vessels are stretched by the force of cardiac ejection during systole,
and the elastic tissue permits them to recoil during diastole. This
maintains diastolic pressure and aids the forward motion of the
blood. The walls of the arterioles contain less elastic tissue than
the arteries but proportionately more smooth muscle (Figure
11–2). The muscle is extensively innervated by noradrenergic
nerve fibers, which are constrictor in function. In some instances,
there is a cholinergic innervation, which is vasodilator in function.