Human Physiology, 14th edition (2016)

Blood, Heart, and Circulation 433

Unlike the vessels of the arterial and venous systems, the

walls of capillaries are composed of just one cell layer—a sim-

ple squamous epithelium, or endothelium (see fig. 13.28 ). The

absence of smooth muscle and connective tissue layers permits

a more rapid exchange of materials between the blood and the

tissues.

Types of Capillaries

Different organs have different types of capillaries, distin-

guished by significant differences in structure. In terms of

their endothelial lining, these capillary types include those that

are continuous, those that are fenestrated, and those that are

discontinuous.

Continuous capillaries are those in which adjacent endothe-

lial cells are closely joined together. These are found in muscles,

lungs, adipose tissue, and the central nervous system. The lack of

intercellular channels in continuous capillaries in the CNS contrib-

utes to the blood-brain barrier (chapter 7, section 7.1). Continuous

capillaries in other organs have narrow intercellular channels (from

40 to 45 Å in width) that permit the passage of molecules other than

protein between the capillary blood and tissue fluid ( fig. 13.28 ).

Examination of endothelial cells with an electron microscope

has revealed the presence of pinocytotic vesicles ( fig.  13.28 ),

which suggests that the intracellular transport of material may

occur across the capillary walls. This type of transport appears to

be the only mechanism of capillary exchange available within the

central nervous system and may account, in part, for the selective

nature of the blood-brain barrier.

Fenestrated capillaries occur in the kidneys, endocrine

glands, and intestines. These capillaries are characterized by wide

intercellular pores (800 to 1,000 Å) that are covered by a layer of

decreases the blood flow downstream in the capillaries. Con-
versely, vasodilation of arterioles (by relaxation of the smooth
muscle layer) decreases the resistance and thus increases the
flow through the arterioles to the capillaries. This topic is dis-
cussed in more detail in chapter 14, section 14.3. There is evi-
dence of gap junctions between the cells of the arteriole wall in
both the endothelial and smooth muscle layers. The vasocon-
strictor effect of norepinephrine and the vasodilator effect of
acetylcholine may be propagated for some distance along the
arteriole wall by transmissions of depolarization and hyperpo-
larizations, respectively, through gap junctions in the vascular
smooth muscle.

Capillaries

The arterial system branches extensively ( table 13.8 ) to deliver
blood to over 40 billion capillaries in the body. The number of
capillary branches is so great that scarcely any cell in the body
is more than 60 to 80  m m away from a blood capillary. The tiny
capillaries provide a total surface area of 1,000 square miles
for exchanges between blood and tissue fluid.
The amount of blood flowing through a particular capil-
lary bed depends primarily on the resistance to blood flow in
the small arteries and arterioles that supply blood to that capil-
lary bed. Vasoconstriction in these vessels thus decreases blood
flow to the capillary bed, whereas vasodilation increases blood
flow. The relatively high resistance in the small arteries and
arterioles in resting skeletal muscles, for example, reduces
capillary blood flow to only about 5% to 10% of its maximum
capacity. In some organs (such as the intestine), blood flow
may also be regulated by circular muscle bands called precap-
illary sphincters at the origin of the capillaries ( fig. 13.27 ).

Figure 13.27 The microcirculation. Metarterioles (arteriovenous anastomoses) provide a path of least resistance between
arterioles and venules. Precapillary sphincter muscles regulate the flow of blood through the capillaries.

Blood

flow Venule

Metarteriole (forming

arteriovenous shunt)

Precapillary

Arteriole sphincter

Blood

flow

Artery Vein

Capillaries