Chapter 19
The Cardiovascular System: Blood Vessels
693
19
inspired experiments of William Harvey, an English physician.
Prior to that time, people thought, as proposed by the ancient
Greek physician Galen, that blood moved through the body like
an ocean tide, first moving out from the heart and then ebbing
back in the same vessels.
PART 1
Blood Vessel Structure
and Function
Te three major types of blood vessels are
arteries, capillaries
,
and
veins
. As the heart contracts, it forces blood into the large ar-
teries leaving the ventricles. Te blood then moves into succes-
sively smaller arteries, finally reaching their smallest branches,
the
arterioles
(ar-te
9
re-ōlz; “little arteries”), which feed into the
capillary beds of body organs and tissues. Blood drains from the
capillaries into
venules
(ven
9
ūlz), the smallest veins, and then on
into larger and larger veins that merge to form the large veins
that ultimately empty into the heart. Altogether, the blood ves-
sels in the adult human stretch for about 100,000 km (60,000
miles) through the internal body landscape!
Arteries
carry blood
away from
the heart, so they are said to
“branch,” “diverge,” or “fork” as they form smaller and smaller
divisions.
Veins
, by contrast, carry blood
toward
the heart and
so are said to “join,” “merge,” and “converge” into the succes-
sively larger vessels approaching the heart. In the systemic circu-
lation, arteries always carry oxygenated blood and veins always
carry oxygen-poor blood. Te opposite is true in the pulmonary
circulation, where the arteries, still defined as the vessels leading
away from the heart, carry oxygen-poor blood to the lungs, and
the veins carry oxygen-rich blood from the lungs to the heart.
Te special umbilical vessels of a fetus also differ in the roles of
veins and arteries.
Of all the blood vessels, only the capillaries have intimate
contact with tissue cells and directly serve cellular needs. Ex-
changes between the blood and tissue cells occur primarily
through the gossamer-thin capillary walls.
Structure of Blood Vessel Walls
Describe the three layers that typically form the wall of a
blood vessel, and state the function of each.
Define vasoconstriction and vasodilation.
Te walls of all blood vessels, except the very smallest, have three
distinct layers, or
tunics
(“coverings”), that surround a central
blood-containing space, the vessel
lumen
(Figure 19.1)
.
Te innermost tunic is the
tunica intima
(in
9
tĭ-mah). Te
name is easy to remember once you know that this tunic is in
intimate
contact with the blood in the lumen. Te tunica in-
tima contains the
endothelium
, the simple squamous epithe-
lium that lines the lumen of all vessels. Te endothelium is
continuous with the endocardial lining of the heart, and its flat
cells fit closely together, forming a slick surface that minimizes
friction as blood moves through the lumen. In vessels larger
than 1 mm in diameter, a
subendothelial layer
, consisting of a
basement membrane and loose connective tissue, supports the
endothelium.
Te middle tunic, the
tunica media
(me
9
de-ah), is mostly cir-
cularly arranged smooth muscle cells and sheets of elastin. Te
activity of the smooth muscle is regulated by sympathetic
vaso-
motor nerve fibers
of the autonomic nervous system and a whole
battery of chemicals. Depending on the body’s needs at any given
moment, regulation causes either
vasoconstriction
(lumen di-
ameter decreases as the smooth muscle contracts) or
vasodila-
tion
(lumen diameter increases as the smooth muscle relaxes).
Te activities of the tunica media are critical in regulating circu-
latory dynamics because small changes in vessel diameter greatly
influence blood flow and blood pressure. Generally, the tunica
media is the bulkiest layer in arteries, which bear the chief re-
sponsibility for maintaining blood pressure and circulation.
Te outermost layer of a blood vessel wall, the
tunica ex-
terna
(also called the
tunica adventitia
; ad
0
ven-tish
9
e-ah;
“coming from outside”), is composed largely of loosely woven
collagen fibers that protect and reinforce the vessel, and anchor
it to surrounding structures. Te tunica externa is infiltrated
with nerve fibers, lymphatic vessels, and, in larger veins, a net-
work of elastic fibers. In larger vessels, the tunica externa con-
tains a system of tiny blood vessels, the
vasa vasorum
(va
9
sah
va-sor
9
um)—literally, “vessels of the vessels”—that nourish the
more external tissues of the blood vessel wall. Te innermost
(luminal) portion of the vessel obtains nutrients directly from
blood in the lumen.
Te three vessel types vary in length, diameter, wall
thickness, and tissue makeup (see
Table 19.1
on p. 696).
Figure 19.2
summarizes how these vascular channels relate
to one another and to vessels of the lymphatic system. Te
lymphatic system recovers fluids that leak from the circula-
tion and is described in Chapter 20.
Check Your Understanding
1.
Which branch of the autonomic nervous system innervates
blood vessels? Which layer of the blood vessel wall do these
nerves innervate? What are the effectors (cells that carry out
the response)?
2.
When vascular smooth muscle contracts, what happens to
the diameter of the blood vessel? What is this called?
For answers, see Appendix H.
Arterial System
Compare and contrast the structure and function of the
three types of arteries.
In terms of relative size and function, arteries can be divided into
three groups—elastic arteries, muscular arteries, and arterioles.
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