132 Chapter 7In the condition called telangiectasia, capillaries,
arterioles, or veins show up on the body surface as
“spider veins.” Most people have at least a few by
the time they reach age 30. Research this topic to
find out the most common causes of spider veins
and recommended treatments for them.THiNk oUTsiDE THE Book
Capillaries: Where substances Move
between blood and tissues
n Blood entering the systemic circulation flows fast in the
aorta, but has to slow in order for substances to move into
and out of the bloodstream.
n Link to Diffusion 3.10a vast network of capillaries brings blood
close to nearly all body cells
Your body comes equipped with as many as 40 billion
capillaries—each one so thin that it would take a hundred
of them to equal the thickness of a human hair. And at
least one of these tiny vessels is next to living cells in
nearly all body tissues.
In addition to forming a vast network of vessels (Figure
7.16A), this branching system also affects the speed at
which blood flows through it. Recall from Sections 7.5–7.6
that the flow is fastest in the aorta, quickly “loses steam” in
the more numerous arterioles, and slows to a relative crawl
in the narrow capillaries (Figure 7.16B). The flow of blood
speeds up again as blood moves into veins for the return
trip to the heart.
Why have such an extensive system of capillaries in
which blood slows to a snail’s pace? As you have read,
capillaries are where all the substances that enter and
leave cells are exchanged with the blood. Many of these
exchanges occur by diffusion—but diffusion is a slow
process that is not efficient over long distances. In a large,
multicellular organism such as a human, having billions
of narrow capillaries solves both these problems. There
is a capillary close to nearly every cell, and in each one
the blood is barely moving. As blood “creeps” along in
capillaries, there is time for the necessary exchanges of fluid
and solutes to take place. In fact, most solutes that enter and
leave the bloodstream diffuse across capillary walls.some substances pass through
“pores” in capillary walls
Some substances enter and leave capillaries by way of
slitlike areas between the cells of capillary walls (Figure
7.16C). These “pores” are filled with water. They are
passages for substances that cannot diffuse through the
lipid bilayer of the cells that make up the capillary wall,
but that can dissolve in water.
When the blood pressure inside a capillary is greater
than pressure from the extracellular fluid outside, water
and solutes may be forced out of the vessel—a type of
fluid movement called “bulk flow” (Figure 7.17). Various
factors affect this process, but on balance, a little more water
leaves capillaries than enters them. You may remember
that Section 7.1 mentioned a close association between
the cardiovascular system and the lymphatic system.ABBiophoto Associates/Science SourceScience Source- 7
F i g u r e 7.16 Capillaries deliver blood close to cells. A A resin
cast showing a dense network of capillaries. B Red blood cells
moving single file in capillaries. C How substances pass through
slitlike pores in the wall of a capillary.porescell of
capillary wallC© Cengage LearningCopyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).