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Tracking the changes that are part of normal aging of the cardiovascular system is difficult because of the high incidence of disease affecting the heart and blood vessels, which increases exponentially with age. For example, 60% of men over age sixty have at least one narrowed coronary artery; the same is true for women over age eighty. Signs of cardiovascular disease may appear long before symptoms arise. Autopsies of soldiers killed in the Korean and Vietnam wars, for example, reveal significant plaque buildup in the arterial walls of otherwise healthy young men. Some degree of cholesterol deposition in blood vessels may be part of normal aging in many types of animals, but the accumulation is great enough to lead to overt disease in humans. This is because we can extend our lives in ways that other animals cannot, and hidden disease conditions can become problems if one lives long enough!

Assessing cardiac output over a lifetime vividly illustrates how cardiovascular disease prevalence can interfere with studying the changes associated with normal aging. Recall that cardiac output is the ability of the heart to meet the body's oxygen requirements and is calculated as the heart rate in beats per minute multiplied by the stroke volume in milliliters per beat. For many years, studies indicated that cardiac output declines with age, but when researchers began to screen participants for hidden heart disease with treadmill stress tests, then evaluated only individuals with completely healthy car diovascular systems, they discovered that cardiac output at rest is maintained as a person ages. It does decline during exercise for some people, however.

The heart normally remains the same size or may even shrink a little with age, but cardiovascular disease may cause enlargement. It is normal for the proportion of the heart that is cardiac muscle to decline with age, because cardiac muscle cells do not divide—that is, they are not replaced. The number and size of cardiac muscle cells fall, and lipofuscin pigments become especially prominent in this cell type. Fibrous connective tissue and adipose tissue fill in the spaces left by the waning population of cardiac muscle cells, thickening the endocardium. Adipose cells may also accumulate in the ventricle walls and the septum between them. As a result, the left ventricular wall may be up to 25% thicker at age eighty than it was at age thirty.

The heart valves tend to thicken and become more rigid after age sixty, but these changes may actually begin as early as the third decade. The valves may calcify— some calcification of the aortic valve in particular after the seventh decade is very common and may be considered a normal part of aging.

Just as the heart need not falter with age, so does the cardiac conduction system remain functional despite change. The sinoatrial and atrioventricular nodes and the atrioventricular bundle become more elastic. However, these changes may alter the ECG pattern.

Systolic blood pressure increases with age; a blood pressure reading of 140/90 is not considered abnormal in an older person. In about 40% of the elderly, the systolic pressure exceeds 160. The increase seems to be due to the decreasing diameters and elasticity of arteries, an effect that is dampened somewhat by regular exercise. Resting heart rate declines with age, from 145 or more beats per minute in a fetus, to 140 beats per minute in a newborn, then levels out in an adult to about 70 (range of 60-99) beats per minute.

In the vascular system, changes that are part of aging are most apparent in the arteries. The tunica interna thickens. Dividing smooth muscle cells in the tunica media may push up the endothelium in places, and over time, the lumens of the larger arteries narrow. Rigidity increases as collagen, calcium, and fat are deposited as elastin production declines. Arterial elasticity at age seventy is only about half of what it was at age twenty. The arterioles have diminished ability to contract in response to cold temperatures and to dilate in response to heat, contributing to the loss of temperature control that is common among the elderly. The extent of change in arteries may reflect how much stress they are under— that is, not all arteries "age" at the same rate.

Veins may accumulate collagen and calcify but, in general, do not change as much with age as do arteries. Thickened patches may appear in the inner layer, and fibers in the valves, but venous diameters are large

Superficial temporal v

Anterior facial v. Internal jugular v

Right brachiocephalic v

Axillary v Cephalic v

Brachial v.

Basilic v

Median cubital v.

Renal v. Radial v Ulnar v

Common iliac v. External iliac v.

Figure 15.60

Major vessels of the venous system (v. stands for vein)

Figure 15.60

Major vessels of the venous system (v. stands for vein)

Great Saphenous Vein Function

Great saphenous v.

External jugular v. Subclavian v.

Superior vena cava Azygos v. Hepatic v.

Inferior vena cava

Ascending lumbar v. Gonadal v. Internal iliac v.

Great saphenous v.

enough that these changes have little impact on function. The venous supply to many areas is so redundant that alternate vessels can often take over for damaged ones. The number of capillaries declines with age. The once-sleek endothelium changes as the cells become less uniform in size and shape. The endothelial inner linings of blood vessels are important to health because these cells release nitric oxide, which signals the vessels to dilate to increase blood flow, which counters atherosclerosis and thrombosis.

At least one study demonstrates that exercise can help maintain a "young" vascular system. The study compared the vascular endothelial linings of athletic and sedentary individuals of various ages and found that the status of the vessels of the exercising elderly were very similar to those of either athletic or sedentary people in their twenties. This finding is consistent with results of the Honolulu Heart Program, which found that walking 1.5 miles each day correlates to lowered heart disease risk in older people.

Overall, aging-related changes affect many components of the cardiovascular system. But in the absence of disease, the system is so fine-tuned and redundant that effective oxygen delivery can continue well into the later decades of life.

H Explain why the heart may enlarge with age.

Describe what happens to resting heart rate with age.

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