Cardiovascular Physiology
395
capillaries. Slow forward fl
ow through the capillaries maxi-
mizes the time available for substances to exchange between
the blood and interstitial fl
uid. The velocity of fl ow then
progressively increases in the venules and veins because the
cross-sectional area decreases. To reemphasize, blood veloc-
ity is not dependent on proximity to the heart, but rather on
total cross-sectional area of the vessel type.
The cross-sectional area of the capillaries accounts for
another important feature of capillaries: Because each capil-
lary is very narrow, it offers considerable resistance to fl ow,
but the huge total number of capillaries provides such a large
cross-sectional area that the total resistance of
all
the capillar-
ies is much lower than that of the arterioles.
Diffusion Across the Capillary Wall: Exchanges
of Nutrients and Metabolic End Products
The extremely slow forward movement of blood through the
capillaries maximizes the time for substance exchange across the
capillary wall. Three basic mechanisms allow substances to move
between the interstitial fl uid and the plasma: diffusion, vesicle
Intercellular clefts
Endothelial
cell
Enlargement
of capillary
Metarteriole
To veins
Capillaries
Precapillary
sphincters
Arteriole
Smooth
muscles
Venule
Figure 12–38
Diagram of microcirculation. Note the absence of smooth muscle in the capillaries.
Adapted from Chaffee and Lytle.
Figure 12–39
Relationship between total cross-sectional area and fl ow velocity. (a) The total cross-sectional area of the small tubes is three times greater than
that of the large tube. Accordingly, fl ow velocity is one-third as great in the small tubes. (b) Cross-sectional area and velocity in the systemic
circulation.
Distance
moved
in 1 min
Distance
moved
in 1 min
Balls
expelled
in 1 min
(a)
Aorta
Arteries
and arterioles
Venules
and
veins
C
a
pill
a
rie
s
Total
cro
ss
-
s
ectional
area (cm
2
)
Mean linear
velocity
(cm/
s
)
0
3000
2000
1000
0
30
20
10
(b)
Begin
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