CHAPTER 5 • VASCULAR FUNCTION
95
which a small artery becomes an arteriole.
Many investigators speak of different branch-
ing orders of arterial vessels within a tissue or
organ. Most would agree that arterioles have
only a few layers of vascular smooth muscle
and are, in general, <200 |J,m in diameter.
Together, the small arteries and arteri-
oles represent the primary resistance ves-
sels
that regulate
arterial
blood pressure
and blood flow within organs.
Resistance
vessels are highly innervated by autonomic
nerves
(particularly
sympathetic
adrener-
gic), and they constrict or dilate in response
to changes in nerve activity. The resistance
vessels are richly endowed with receptors
that bind circulating hormones (e.g., cat-
echolamines,
angiotensin
II),
which
can
alter vessel diameter (see Chapters 3 and 6).
They also respond to various substances (e.g.,
adenosine, potassium ion, and nitric oxide)
produced by the tissue surrounding the vessel
or by the vascular endothelium.
EXCHANGE VESSELS
As
arterioles become smaller in diameter
(<10 |J,m), they lose their smooth muscle.
Vessels that have no smooth muscle and are
composed of only endothelial cells and a
basement membrane are termed capillaries.
Although they are the smallest vessels within
the circulation, they have the greatest cross—
sectional area because they are so numerous.
Because the total blood flow of all capillaries
in the body is the same as the flow within the
aorta leaving the heart, and because the capil-
lary cross—sectional area is about 1000 times
greater than the aorta, the mean velocity of
blood flowing within capillaries (~0.05 cm/s)
is about 1000—
fold less than the velocity in the
aorta (~50 cm/s). The reason for this is that
flow (F) is the product of mean velocity (V)
times cross-sectional area (A) (F = V . A).
When this expression is rearranged, we find
that the mean velocity is inversely propor-
tional to cross-sectional area (V = F/A).
Capillaries have the greatest surface area
for exchange. Oxygen, carbon dioxide, water,
electrolytes,
proteins,
metabolic substrates
and by-products, and circulating hormones
are exchanged across the capillary endothe-
lium between the plasma and the surrounding
tissue interstitium (see Chapter 8). Capillaries,
therefore, are the primary exchange vessels
within the body.
When capillaries join together, they form
small, postcapillary venules, which are still
devoid of smooth muscle. They, like capil-
laries, serve as exchange vessels for fluid
and macromolecules because of their high
permeability.
CAPACITANCE VESSELS
As small postcapillary venules converge and
form larger venules, smooth muscle reap-
pears. These vessels, like the resistance ves-
sels, are capable of dilating and constricting.
Changes in venular diameter regulate capil-
lary pressure and venous blood volume. Ven-
ules converge to form larger veins. Together,
venules and veins are the primary capacitance
vessels of the body, that is, the site where most
of the blood volume is found and regional
blood volume is regulated. Constriction of
veins decreases venous blood volume and
increases venous pressure, which can alter
cardiac output by affecting right atrial pres-
sure and ventricular preload. The final venous
vessels are the inferior and superior vena
cavae, which carry the blood back to the right
atrium of the heart.
Distribution of Pressures
and Volumes
Mean blood pressure is highest in the aorta
(about 95 mm Hg in a normal adult) and pro-
gressively decreases as the blood flows further
away from the heart (Fig. 5.2). The reason why
pressure falls as blood flows through vessels is
because energy is lost as heat owing to friction
within the moving blood (related to blood vis-
cosity) and between the blood and the vessel
wall. Between any two points along the length
of an artery, for example, the drop in pressure
(AP) is related to the flow (F) and resistance to
flow (R) as shown in Equation 5—1.
Eq. 5-1
A
P = F
R
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