576
UNIT FOUR
Sometimes metarterioles connect directly to venules,
and blood entering them can bypass the capillaries. These
connections between arteriole and venous pathways, shown
in
f
gure 15.27
, are called
arteriovenous shunts.
PRACTICE
30
Describe the wall of an artery.
31
What is the function of the smooth muscle in the arterial wall?
32
How is the structure of an arteriole diF
erent from that of an
artery?
Capillaries
Capillaries
(kap
ı˘-ler
e
¯z) are the smallest diameter blood
vessels. They connect the smallest arterioles and the small-
est venules. Capillaries are extensions of the inner linings of
arterioles in that their walls are endothelium—a single layer
of squamous epithelial cells
(f
g. 15.28
a
)
. These thin walls
form the semipermeable layer through which substances in
the blood are exchanged for substances in the tissue fl uid
surrounding body cells.
Capillary Permeability
The openings or intercellular channels in the capillary walls
are thin slits where endothelial cells overlap. The sizes of
these openings, and consequently the permeability of the
capillary wall, vary from tissue to tissue. For example, the
openings are relatively small in the capillaries of smooth,
skeletal, and cardiac muscle, whereas those in capillaries
associated with endocrine glands, the kidneys, and the lin-
ing of the small intestine are larger.
Capillaries with the largest openings include those of
the liver, spleen, and red bone marrow. These capillaries are
discontinuous, and the distances between their cells appear
as small cavities (sinusoids) in the organ. Discontinuous
capillaries allow large proteins and even intact cells to pass
through as they enter or leave the circulation (±
g. 15.28
b
and
c
). From Science to Technology 5.1 (p. 145) discusses
the blood-brain barrier, the protective tight capillaries in
the brain. The barrier is not present in the pituitary and
pineal glands and parts of the hypothalamus.
Capillary Arrangement
The higher a tissue’s rate of metabolism, the denser its
capillary networks. Muscle and nerve tissues, which use
abundant oxygen and nutrients, are richly supplied with cap-
illaries; cartilaginous tissues, the epidermis, and the cornea,
where metabolism is slow, lack capillaries.
If the capillaries of an adult were unwound and spread end to end,
they would cover from 25,000 to 60,000 miles.
example, endothelium releases the gas nitric oxide, which
relaxes the smooth muscle of the vessel.
The middle layer, tunica media, makes up the bulk of the
arterial wall. It includes smooth muscle ± bers, which encir-
cle the tube, and a thick layer of elastic connective tissue.
The connective tissue gives the vessel a tough elasticity that
enables it to withstand the force of blood pressure and, at the
same time, to stretch and accommodate the sudden increase
in blood volume that accompanies ventricular contraction.
The outer layer, tunica externa (adventitia), is thin and
chiefl
y consists of connective tissue with irregular elastic and
collagenous ±
bers. This layer attaches the artery to the sur-
rounding tissues. It also contains minute vessels (vasa vaso-
rum) that give rise to capillaries and provide blood to the
more external cells of the artery wall.
The sympathetic branches of the autonomic nervous sys-
tem innervate smooth muscle in artery and arteriole walls.
Vasomotor f
bers
stimulate the smooth muscle cells to con-
tract, reducing the diameter of the vessel. This is called
vaso-
constriction
(vas
o-kon-strik
-shun). If vasomotor impulses
are inhibited, the muscle ± bers relax, and the diameter of
the vessel increases. This is called
vasodilation
(vas
o-di-
la
shun). Changes in the diameters of arteries and arterioles
greatly infl
uence blood fl
ow and blood pressure.
The walls of the larger arterioles have three layers simi-
lar to those of arteries (±
g. 15.25
c
), but the middle and outer
layers thin as the arterioles approach the capillaries. The wall
of a very small arteriole consists only of an endothelial lin-
ing and some smooth muscle ± bers, surrounded by a small
amount of connective tissue
(f
g. 15.26)
. Arterioles, which
are microscopic continuations of arteries, give off branches
called
metarterioles
that, in turn, join capillaries.
The arteriole and metarteriole walls are adapted for
vasoconstriction and vasodilation in that their muscle ± bers
respond to impulses from the autonomic nervous system by
contracting or relaxing. In this way, these vessels help con-
trol the fl
ow of blood into the capillaries.
Arteriole
Capillary
Smooth
muscle cell
Endothelium
Precapillary
sphincter
FIGURE 15.26
The smallest arterioles have only a few smooth
muscle ±
bers in their walls. Capillaries lack these ±
bers.
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