Blood Vessels Are the Body's Plumbing
LEARNING OBJECTIVES
1.
Compare
the structure and function of the
different types of blood vessels.
2.
Describe
how capillary exchange works.
3.
Explain
how venous blood returns to the heart.
4.
Outline
the pulmonary and hepatic portal
circulations.
hile the job of the heart is to pump the blood
and generate pressure for blood flow, the
blood vessels transport and distribute blood
to and from the various tissues. From the
heart to the tissues, various types of blood vessels form a
W
series of branching tubes where single large vessels div-
vide into numerous smaller ones (
). From
the cells to the heart, small narrow blood vessels combine
into larger ones. The types of vessels include arteries, ar-
terioles, capillaries, venules, and veins.
O verview o f system ic b lo o d v essels • Figure 11.1 0
There are five types of blood vessels that carry blood away from the heart to the tissues
and return it to the heart. The systemic blood vessels transport blood to tissues, but the
distribution of blood between the heart and the various types of blood vessels is uneven.
Arteries branch off
into smaller
arterioles.
Arterioles
are smaller, almost microscopic, vessels that
branch off the small arteries into tissues. Their small
diameter and smooth muscles are important in regulat-
ing blood pressure and blood flow to the capillaries.
Smooth muscle fiber (cell)
Endothelium
Arteriole
Capillary
Arteries carry oxygen-rich blood away from the
heart. In the arterial tree, one large artery, the
aorta, branches off into many small arteries of the
head, arms, and legs. These arteries, in turn,
branch off into smaller ones.
Capillaries
are
thin-walled, microscopic
vessels that surround
every cell. Across their
thin walls, gases,
nutrients, and wastes are
exchanged between the
blood and cells.
Venule
Endothelium
To heart
Venules
, like arterioles, are small, almost microscopic
vessels that carry blood away from the capillaries toward the
heart. Their walls are thin near the capillaries and get thicker
near the veins. Compared to arterioles, venules have larger
diameters and their smooth muscle walls are thinner.
Pulmonary
vessels
9%
Veins carry oxygen-poor blood toward the heart.
The vein system is the opposite of the arterial
tree. Many smaller veins from the head, arms, and
legs merge into fewer larger ones, which then
merge into two single large veins that enter the
heart (superior and inferior venae cavae).
PLUS
NATIONAL
GEOGRAPHIC
Video
J
Systemic veins and venules
(blood reservoirs)
At any time, most of the
blood in the body is in the
blood vessels, mostly in
the veins and venules.
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