Check Your Understanding
Name the type of artery that matches each description:
major role in dampening the pulsatile pressure of heart
contractions; vasodilation or constriction determines blood
ﬂow to individual capillary beds; have the thickest tunica
media relative to their lumen size.
Look at Figure 19.4 on p. 698 and assume that the capillary
bed depicted is in your calf muscle. Which condition—(a) or
(b)—would the bed be in if you were doing calf raises at the
What is the function of venous valves? What forms the
In the systemic circuit, which contains more blood—arteries
or veins—or is it the same?
For answers, see Appendix H.
Physiology of Circulation
Have you ever climbed a mountain? Well, get ready to climb a
hypothetical mountain as you learn about circulatory dynamics.
Like scaling a mountain, tackling blood pressure regulation and
other topics of cardiovascular physiology is challenging while
you’re doing it, and exhilarating when you succeed. Let’s begin
To sustain life, blood must be kept circulating. By now, you
are aware that the heart is the pump, the arteries are pressure
reservoirs and conduits, the arterioles are resistance vessels that
control distribution, the capillaries are exchange sites, and the
and subsequent rupture of atherosclerotic
is a marker of
systemic inﬂammation that is measured
to predict the likelihood of future heart
attacks and strokes.
Prevention and Treatment
Some risk factors are under our control.
We can avoid smoking, lose weight,
exercise regularly to increase blood
levels of high-density lipoprotein (HDL,
the “good” lipoprotein that removes
cholesterol from vessel walls and carries it
to the liver), and eat a healthy diet low in
saturated and trans fats.
But for many of us, these measures
are not enough and a pharmaceutical
approach is needed. At ﬁrst it was hoped
that cholesterol-lowering drugs called
statins would act as cardiovascular Dr
in effect washing fatty plaques off the
walls. Statins do lower LDL, but decrease
plaque size by only a small amount.
A signiﬁcant part of their action,
though, is their unexpected side beneﬁt—
anti-inﬂammatory activity, which appears
to help stabilize existing plaques and keep
them from rupturing.
The humble aspirin can also play a
role. The American Heart Association
recommends that people at high risk
for heart attack or stroke take one baby
aspirin (81 mg) daily to prevent clot
formation when plaques do rupture.
Larger plaques that partially block
arteries are treated in much the same way
we would treat a blocked sewer pipe—dig
it up and replace it or call Roto-Rooter to
drill through the obstruction. In
, veins removed from
the legs or small arteries removed from
the thoracic cavity are implanted in the
heart to restore myocardial circulation.
, a catheter with
a balloon tightly packed into its tip is
threaded through the vessels. When the
catheter reaches the obstruction, the
balloon is inﬂated to compress the fatty
mass against the vessel wall.
Angioplasty temporarily clears the path,
(new blockages) often
, short metal-mesh tubes
that look like a ziti noodle, are inserted
into the newly dilated vessels in order to
hold the vessel open. Stents that slowly
release drugs that inhibit smooth muscle
proliferation help reduce restenosis, but
these often become clogged as well.
Treating the area with bursts of radiation
) can help.
When an atheroma ruptures and
induces clot formation,
can help. A genetically
engineered form of the naturally occurring
tissue plasminogen activator
injected directly into the blocked vessel.
tPA restores blood ﬂow quickly and puts
an early end to many heart attacks and
strokes in progress.
Of course, it’s best to prevent
atherosclerosis from progressing in the
ﬁrst place by changing our lifestyles.
Americans like their burgers and butter.
But if heart disease can be prevented by
reversing atherosclerosis, many people
with diseased arteries may be willing to
trade lifelong habits for a healthy old age!
A CLOSER LOOK
veins are conduits and blood reservoirs. Now for the dynamics
of this system.
Introduction to Blood Flow,
Blood Pressure, and Resistance
Deﬁne blood ﬂow, blood pressure, and resistance, and
explain the relationships between these factors.
First we need to deﬁne three physiologically important terms—
blood ﬂow, blood pressure, and resistance—and examine how
these factors relate to the physiology of blood circulation.
Deﬁnition of Terms
is the volume of blood ﬂowing through a vessel, an or-
gan, or the entire circulation in a given period (ml/min). If we con-
sider the entire vascular system, blood ﬂow is equivalent to cardiac
output (CO), and under resting conditions, it is relatively constant.
At any given moment, however, blood ﬂow through
body organs may vary widely according to their immediate needs.
Blood Pressure (BP)
Blood pressure (BP)
, the force per unit area exerted on a ves-
sel wall by the contained blood, is expressed in millimeters of
mercury (mm Hg). For example, a blood pressure of 120 mm
Hg is equal to the pressure exerted by a column of mercury
120 mm high.