Chapter 18
The Cardiovascular System: The Heart
687
18
for each heartbeat, reducing stroke volume. Te nodes of the
heart’s conduction system may also become fibrosed, which
increases the incidence of arrhythmias and other conduction
problems.
Atherosclerosis.
Te insidious progress of atherosclerosis
begins in childhood, but inactivity, smoking, and stress ac-
celerate it. Te most serious consequences to the heart are
hypertensive heart disease and coronary artery occlusion,
both of which increase the risk of heart attack and stroke.
Although the aging process itself leads to changes in blood
vessel walls that promote atherosclerosis, many investigators
feel that diet, not aging, is the single most important con-
tributor to cardiovascular disease. We can lower our risk by
consuming less animal fat, cholesterol, and salt.
Check Your Understanding
17.
Name the two components of the fetal heart that allow
blood to bypass the lungs.
18.
In the past decade, many people over 70 have competed
in the Ironman World Championships in Hawaii. In what
way might age-related changes of the heart limit the
performance of these athletes?
For answers, see Appendix H.
Te heart is an exquisitely engineered double pump that
operates with precision to propel blood into the large arter-
ies leaving its chambers. However, continuous circulation of
blood also depends critically on the pressure dynamics in the
blood vessels. Chapter 19 considers the structure and func-
tion of these vessels and relates this information to the work
of the heart to provide a complete picture of cardiovascular
functioning.
In people who exercise regularly and vigorously, the heart
gradually adapts to the increased demand by enlarging and be-
coming more efficient and more powerful. Aerobic exercise also
helps clear fatty deposits from blood vessel walls throughout
the body, retarding atherosclerosis and coronary heart disease.
Barring some chronic illnesses, this beneficial cardiac response
to exercise persists into ripe old age.
Te key word on benefiting from exercise is
regularity
. Reg-
ular exercise gradually enhances myocardial endurance and
strength. For example, 30 minutes a day of moderately vigorous
activity (brisk walking, biking, or yard work) offers significant
health benefits to most adults. However, intermittent vigorous
exercise, enjoyed by weekend athletes, may push an uncondi-
tioned heart beyond its ability to respond to the unexpected
demands and bring on a myocardial infarction.
Because of the incredible amount of work the heart does over
the course of a lifetime, certain structural changes are inevitable.
Age-related changes affecting the heart include the following:
Valve flaps thicken and become sclerotic (stiff).
Tis change
occurs particularly where the stress of blood flow is great-
est (mitral valve). For this reason, heart murmurs are more
common in elderly people.
Cardiac reserve declines.
Although the passing years seem
to cause little change in resting heart rate, the aged heart is
less able to respond to both sudden and prolonged stressors
that demand increased output. In addition, the maximum
HR declines as sympathetic control of the heart becomes less
efficient. Tese changes are less of a problem in physically
active seniors.
Cardiac muscle becomes fibrosed (scarred).
As a person ages,
more and more cardiac cells die and are replaced with fibrous
tissue. As a result, the heart stiffens and fills less efficiently
Occurs in
about 1 in every
500 births
Occurs in
about 1 in every
1500 births
Occurs in
about 1 in every
2000 births
Narrowed
aorta
Ventricular septal defect.
The superior part of the inter-
ventricular septum fails to form,
allowing blood to mix between
the two ventricles. More blood is
shunted from left to right because
the left ventricle is stronger.
(a)
Coarctation of the aorta.
A part of the aorta is narrowed,
increasing the workload of
the left ventricle.
(b)
Tetralogy of Fallot.
Multiple defects
(
tetra
= four): (1) Pulmonary trunk too narrow
and pulmonary valve stenosed, resulting in
(2) hypertrophied right ventricle; (3)
ventricular septal defect; (4) aorta opens from
both ventricles.
(c)
Figure 18.25
Three examples of congenital heart defects.
Tan areas indicate the
locations of the defects.
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