578
UNIT FOUR
Exchanges in the Capillaries
The vital function of exchanging gases, nutrients, and met-
abolic by-products between the blood and the tissue fl uid
surrounding body cells occurs in the capillaries. The bio-
chemicals exchanged move through the capillary walls by
diffusion, F
ltration, and osmosis.
RECONNECT
To Chapter 3, Movements Into and Out of the Cell,
pages 90–94.
Diffusion is the most important means of transfer. Blood
entering systemic capillaries carries high concentrations of
oxygen and nutrients, so these substances diffuse through
the capillary walls and enter the tissue fl
uid. Conversely, the
concentrations of carbon dioxide and other wastes are gen-
erally greater in the tissues, and such wastes tend to diffuse
into the capillary blood.
The paths these substances follow depend primarily on
their solubilities in lipids. Substances soluble in lipid, such as
oxygen, carbon dioxide, and fatty acids, can diffuse through
most areas of the cell membranes that make up the capillary
wall because the membranes are largely lipid. Lipid-insoluble
substances, such as water, sodium ions, and chloride ions,
diffuse through pores in the cell membranes and through
the slitlike openings between the endothelial cells that form
the capillary wall (see F g. 15.28). Plasma proteins generally
remain in the blood because they are not soluble in the lipid
of the endothelial cell membranes, and they are too large
to diffuse through the membrane pores or slitlike openings
between the endothelial cells of most capillaries.
In
filtration, hydrostatic pressure
forces molecules
through a membrane. In the capillaries, the blood pressure
generated when ventricle walls contract provides the force
for F
ltration.
Blood pressure also moves blood through the arteries
and arterioles. This pressure decreases as the distance from
the heart increases because of friction (peripheral resistance)
between the blood and the vessel walls. ±or this reason,
blood pressure is greater in the arteries than in the arterioles
and greater in the arterioles than in the capillaries. It is simi-
larly greater at the arteriolar end of a capillary than at the
venular end.
The walls of arteries and arterioles are too thick to allow
blood components to pass through. However, the hydrostatic
pressure of the blood pushes small molecules through capillary
walls by F ltration primarily at the arteriolar ends of capillaries,
whereas diffusion takes place along their entire lengths.
The presence of an impermeant solute on one side of a
cell membrane creates an osmotic pressure. Plasma proteins
trapped in the capillaries create an osmotic pressure that
draws water into the capillaries. The term
colloid osmotic
pressure
describes this osmotic effect due solely to the
plasma proteins.
The effect of capillary blood pressure, which favors F l-
tration, opposes the actions of the plasma colloid osmotic
The spatial patterns of capillaries also differ in various
body parts. ±or example, some capillaries pass directly from
arterioles to venules, but others lead to highly branched net-
works
(± g. 15.29)
. Such physical organization makes it pos-
sible for the blood to follow different pathways through a
tissue attuned to cellular requirements.
Blood fl
ow can vary among tissues as well. During exer-
cise, for example, blood is directed into the capillary net-
works of the skeletal muscles, where the cells require more
oxygen and nutrients. At the same time, the blood bypasses
some of the capillary networks in the tissues of the digestive
tract, where demand for blood is less critical. Conversely,
when a person is relaxing after a meal, blood can be shunted
from the inactive skeletal muscles into the capillary networks
of the digestive organs.
Regulation of Capillary Blood Flow
The distribution of blood in the various capillary pathways
is mainly regulated by the smooth muscles that encircle the
capillary entrances. As F gures 15.26 and 15.27 show, these
muscles form
precapillary sphincters,
which may close a cap-
illary by contracting or open it by relaxing. A precapillary
sphincter responds to the demands of the cells the capillary
supplies. When these cells have low concentrations of oxy-
gen and nutrients, the precapillary sphincter relaxes, and
blood fl
ow increases; when cellular requirements have been
met, the precapillary sphincter may contract again.
PRACTICE
33
Describe a capillary wall.
34
What is the function of a capillary?
35
What controls blood F
ow into capillaries?
FIGURE 15.29
Light micrograph of a capillary network (100
×
).
Arteriole
Capillary
Venule
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