424
Chapter 12
during subsequent contraction. In addition, drugs that block
β
-adrenergic receptors are used to lower the arterial pressure
in people with hypertension. They reduce myocardial work and
cardiac output by inhibiting the effect of sympathetic nerves on
heart rate and contractility. Drugs that prevent or reverse clot-
ting within hours of its occurrence are also extremely impor-
tant in the treatment (and prevention) of heart attacks. Use of
these drugs, including aspirin, will be described in Section F
of this chapter. Finally, a variety of drugs now in common use
lower plasma cholesterol by infl uencing one or more metabolic
pathways for cholesterol (Chapter 16). For example, one group
of drugs, sometimes referred to as “statins,” interferes with a
critical enzyme involved in the liver’s synthesis of cholesterol.
There are several surgical treatments for coronary
artery disease after cardiac angiography (described earlier in
this chapter) identifi es the area of narrowing or occlusion.
Coronary balloon angioplasty
involves threading a catheter
with a balloon at its tip into the occluded artery and then
expanding the balloon (see Figure 12–66c). This procedure
enlarges the lumen by stretching the vessel and breaking up
abnormal tissue deposits. It is generally accompanied by per-
manent placing of
coronary stents
in the narrowed or occluded
coronary vessel (see Figure 12–66d). Stents are tubes made of
a stainless steel lattice that provide a scaffold within a vessel
to open it and keep it open. Researchers are currently test-
ing stents made of a hardened, biodegradeable polymer that
is absorbed after six months to one year. Another surgical
treatment is
coronary bypass,
in which a new vessel is attached
across an area of occluded coronary artery. The new vessel is
often a vein taken from elsewhere in the patient’s body.
Atherosclerosis does not attack only the coronary vessels.
Many arteries of the body are subject to this same occluding
process, and wherever the atherosclerosis becomes severe, the
resulting symptoms refl ect the decrease in blood fl ow to the
specifi c area. For example, occlusion of a cerebral artery due
to atherosclerosis and its associated blood clotting can cause a
stroke. (Recall that rupture of a cerebral vessel, as in hyperten-
sion, is another cause of stroke.) People with atherosclerotic
cerebral vessels may also suffer reversible neurologic defi cits,
known as
transient ischemic attacks
(
TIAs
),
lasting minutes
to hours, without actually experiencing a stroke at the time.
Finally, note that both myocardial infarcts and strokes
due to occlusion may result when a fragment of blood clot or
fatty deposit breaks off and then lodges elsewhere, completely
blocking a smaller vessel. The fragment is called an
embolus,
and the process is
embolism
.
See Chapter 19 for a case study
that highlights the dangers of an embolism.
SECTION E SUMMARY
Hemorrhage and Other Causes of Hypotension
I. The physiological responses to hemorrhage are summarized in
Figures 12–52, 12–56, 12–58, and 12–59.
II. Hypotension can be caused by loss of body fl uids, by cardiac
malfunction, by strong emotion, and by liberation of
vasodilator chemicals.
III. Shock is any situation in which blood fl ow to the tissues is low
enough to cause damage to them.
The Upright Posture
I. In the upright posture, gravity acting upon unbroken columns
of blood reduces venous return by increasing vascular pressures
in the veins and capillaries in the limbs.
a. The increased venous pressure distends the veins, causing
venous pooling, and the increased capillary pressure causes
increased fi ltration out of the capillaries.
b. These effects are minimized by contraction of the skeletal
muscles in the legs.
Exercise
I. The cardiovascular changes that occur in endurance-type
exercise are illustrated in Figures 12–61 and 12–62.
II. The changes are due to active hyperemia in the exercising
skeletal muscles and heart, to increased sympathetic
outfl ow to the heart, arterioles, and veins, and to decreased
parasympathetic outfl ow to the heart.
III. The increase in cardiac output depends not only on the
autonomic infl uences on the heart, but on factors that help
increase venous return.
IV. Training can increase a person’s maximal oxygen consumption
by increasing maximal stroke volume and thus cardiac output.
Hypertension
I. Hypertension is usually due to increased total peripheral
resistance resulting from increased arteriolar vasoconstriction.
II. More than 95 percent of cases of hypertension are called
primary hypertension,
meaning the cause of the increased
arteriolar vasoconstriction is unknown.
Heart Failure
I. Heart failure can occur as a result of diastolic or systolic
dysfunction; in both cases, cardiac output becomes inadequate.
II. This leads to fl uid retention by the kidneys and formation of
edema because of increased capillary pressure.
III. Pulmonary edema can occur when the left ventricle fails.
Coronary Artery Disease and Heart Attacks
I. Insuffi cient coronary blood fl ow can cause damage to the heart.
II. Sudden death from a heart attack is usually due to ventricular
brillation.
III. The major cause of reduced coronary blood fl ow is
atherosclerosis, an occlusive disease of the arteries.
IV. People may suffer intermittent attacks of angina pectoris
without actually suffering a heart attack at the time of the pain.
V. Atherosclerosis can also cause strokes and symptoms of
inadequate blood fl ow in other areas.
VI. Coronary artery disease incidence is reduced by exercise, good
nutrition, and avoiding smoking.
VII. Treatments for coronary artery disease include drugs that
dilate blood vessels, reduce blood pressure, and prevent blood
clotting. Balloon angioplasty and coronary bypass are surgical
treatments.
SECTION E KEY TERMS
autotransfusion
413
erythropoiesis
414
erythropoietin
414
maximal oxygen consumption
(
V
˙
O
2
max)
417
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