590
UNIT FOUR
The cardiovascular system adapts to exercise.
The conditioned athlete experiences increases in
heart pumping efficiency, blood volume, blood
hemoglobin concentration, and the number of
mitochondria in muscle F
bers. These adaptations
improve oxygen delivery to, and use by, muscle
tissue.
An athlete’s heart typically changes in response
to these increased demands and may enlarge 40%
or more. Myocardial mass increases, the ventricu-
lar cavities expand, and the ventricle walls thicken.
Stroke volume increases, and heart rate decreases,
as does blood pressure. To a physician unfamiliar
with a conditioned cardiovascular system, a trained
athlete may appear to be abnormal.
The cardiovascular system responds beauti-
fully to a slow, steady buildup in exercise frequency
and intensity. It does not react well to sudden
demands—such as when a person who never
exercises suddenly shovels snow or runs 3 miles.
±or exercise to benefit the cardiovascular
system, the heart rate must be elevated to 70%
W
e know that exercise is good for the
heart. Yet each year, a few individu-
als die of sudden cardiac arrest while
shoveling snow, running, or engaging in some
other strenuous activity. The explanation for this
apparent paradox is that exercise is good for the
heart—but only if it is a regular part of life.
Physiological responses to intense aerobic
exercise generally increase blood ²
ow, and there-
fore oxygen delivery, to active muscles. In mus-
cles, vasodilation opens more capillaries. At the
same time, vasoconstriction diminishes blood
flow where it is not immediately needed, such
as to the digestive tract. Blood flow, however,
is maintained in the brain and kidneys, which
require a steady stream of oxygen and nutrients
to function. Respiratory movements and skel-
etal muscle activity increase venous return to the
heart. As venous return to the heart increases,
ventricular walls stretch, stimulating them to
contract with greater force. Heart rate increases
as well.
to 85% of its “theoretical maximum” for at least
half an hour three times a week. You can calculate
your theoretical maximum by subtracting your
age from 220. If you are eighteen years old, your
theoretical maximum is 202 beats per minute.
One hundred forty-one to 172 beats per minute
is 70% to 85% of this value. Some good activities
for raising the heart rate are tennis, skating, ski-
ing, handball, vigorous dancing, hockey, basket-
ball, biking, and fast walking.
It is wise to consult a physician before starting
an exercise program. People over the age of thirty
are advised to have a stress test, which is an electro-
cardiogram taken while exercising. (The standard
electrocardiogram is taken at rest.) An arrhythmia
that appears only during exercise may indicate
heart disease that has not yet produced symptoms.
The American Heart Association suggests that
after a physical exam, a sedentary person wishing
to start an exercise program begin with 30 minutes
of activity (perhaps broken into two 15-minute ses-
sions at F
rst) at least F
ve times per week.
15.6
CLINICAL APPLICATION
Exercise and the Cardiovascular System
Pulmonary Circuit
Blood enters the pulmonary circuit as it leaves the right ven-
tricle through the pulmonary trunk. The pulmonary trunk
extends upward and posteriorly from the heart, and about 5
centimeters above its origin, it divides into the right and left
pulmonary arteries. These branches penetrate the right and
left lungs, respectively. In the lungs, they diverge into
lobar
branches
(three on the right side and two on the left) that
accompany the main divisions of the bronchi (airways) into the
lobes of the lungs. After repeated divisions, the lobar branches
give rise to arterioles that continue into the capillary networks
associated with the walls of the alveoli (air sacs)
(f g. 15.40)
.
The blood in the arteries and arterioles of the pulmonary
circuit is low in oxygen and high in carbon dioxide. Gases
are exchanged between the blood and the air as the blood
moves through the
alveolar capillaries,
discussed in chapter 19
(pp. 760–761).
The right ventricle contracts with less force than the left
ventricle, so the arterial pressure in the pulmonary circuit is
less than that in the systemic circuit. Therefore, the alveolar
capillary pressure is low.
The force that moves fl uid out of an alveolar capillary is
23 mm Hg; the force pulling fl uid into it is 22 mm Hg. Thus,
such a capillary has a net F ltration pressure of 1 mm Hg.
This pressure propels a slight, continuous fl ow of fl uid into
pressure can lead to peripheral edema because the resulting
higher capillary hydrostatic pressure favors movement of
fl uid into the tissues. Clinical Application 15.6 discusses the
effects of exercise on the heart and blood vessels.
PRACTICE
53
What is the function of the venous valves?
54
How do skeletal muscles a³
ect venous blood ²
ow?
55
How do respiratory movements a³
ect venous blood ²
ow?
56
What factors stimulate venoconstriction?
15.6
PATHS OF CIRCULATION
Recall from F
gure 15.1 that the blood vessels can be divided
into two major pathways. The
pulmonary circuit
(or circula-
tion) consists of vessels that carry blood from the heart to
the lungs and back to the heart. The
systemic circuit
(or cir-
culation) carries blood from the heart to all other parts of
the body and back again. The systemic circuit includes the
coronary circulation.
The pathways described in the following sections are
those of an adult. Chapter 23 (pp. 897–898) describes the
somewhat different fetal pathways.
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