Human Physiology, 14th edition (2016)

436 Chapter 13

13.7 ATHEROSCLEROSIS AND

CARDIAC ARRHYTHMIAS

Atherosclerosis is a disease process that can lead to

obstruction of coronary blood flow. As a result, the electri-

cal properties of the heart, and the heart’s ability to func-

tion as a pump, may be seriously compromised. Abnormal

cardiac rhythms, or arrhythmias, can be detected by the

abnormal electrocardiogram patterns they produce.

and decreasing the pressure in the thoracic cavity. The pressure
difference in the veins created by this inspiratory movement of
the diaphragm forces blood into the thoracic veins that return the
venous blood to the heart.

Figure 13.30 A demonstration of venous valves by
William Harvey. By blocking venous drainage with a tourniquet,
Harvey showed that the blood in the bulged vein was not
permitted to move away from the heart, thereby demonstrating
the action of venous valves. After William Harvey, On the Motion
of the Heart and Blood in Animals, 1628.

CLINICAL APPLICATION

Varicose veins are enlarged surface veins, generally in the

lower limbs, which occur when venous congestion stretches

the veins to the point that the venous valves no longer close

effectively. Genetic susceptibility, occupations that require

long periods of standing, obesity, age, and pregnancy (due

to compression of abdominal veins by the fetus) are risk fac-

tors. Walking can reduce venous congestion, as can com-

pression stockings and leg elevation; in bedridden patients,

flexing and extending the ankle joints activates the soleus

muscle pump to help move blood from the legs back to the

heart. Surgical treatments of varicose veins include sclero-

therapy (where chemicals are injected into the veins to scar

them), laser therapy (using lasers to destroy the veins), liga-

tion and stripping (tying off and removing the veins), and

other techniques.

Inadequate venous flow in a bedridden patient increases

the risk of deep vein thrombosis, a dangerous condition that

can lead to a venous thromboembolism (a traveling blood

clot). Walking around as soon as possible after a surgery

reduces the risk, as do the use of compression stockings and

devices that compress the leg. Anticoagulant drugs or throm-

bolytic agents (discussed in section 13.2) may sometimes be

necessary to prevent or treat a thromboembolism so that it

doesn’t result in a potentially fatal pulmonary embolism.

| CHECKPOINT

12a. Describe the basic structural pattern of arteries

and veins. Explain how arteries and veins differ in

structure and how these differences contribute to

their differences in function.

12b. Describe the functional significance of the skeletal

muscle pump and illustrate the action of venous

valves.

13. Explain the functions of capillaries and describe the
    structural differences between capillaries in different
    organs.

LEARNING OUTCOMES

After studying this section, you should be able to:

14. Explain the causes and dangers of atherosclerosis.


15. Explain the cause and significance of angina
    pectoris.


16. Describe how different arrhythmias affect the ECG.

Atherosclerosis

Atherosclerosis is the most common form of arteriosclerosis

(hardening of the arteries) and, through its contribution to heart

disease and stroke, is responsible for about 50% of the deaths

in the United States, Europe, and Japan. In atherosclerosis,

localized plaques, or atheromas, protrude into the lumen of

the artery and thus reduce blood flow. The atheromas addition-

ally serve as sites for thrombus (blood clot) formation, which

can further occlude the blood supply to an organ ( fig. 13.31 ).

It is currently believed that the process of atherosclero-

sis begins as a result of damage, or “insult,” to the endothe-

lium. Such insults are produced by smoking, hypertension

(high blood pressure), high blood cholesterol, and diabetes.

The first anatomically recognized change is the appearance

of fatty streaks, which are gray-white areas that protrude into

the lumen of arteries, particularly at arterial branch points.

These are aggregations of lipid-filled macrophages and lym-

phocytes within the tunica interna. In the intermediate stage,

the area contains layers of macrophages and smooth muscle