The Aging Body
Aging is a process of gradual and spontaneous change, resulting in maturation through childhood, puberty, and young adulthood and then decline of many bodily functions through middle and late age. Aging is a continuous process that begins at birth and continues throughout all stages of life. It involves both the positive component of development and the negative component of decline.
There is no specific age at which a person becomes "elderly." Traditionally, age 65 has been so designated because it is the age at which people in industrialized societies generally leave the work force, though this is changing.
Aging can be classified into three categories: senescence, normal aging, and successful aging. Senescence describes the many changes that occur with age as a result of biologic, psychologic, and environmental factors as well as lifestyle choices. It is often difficult to separate the aspects of senescence that are purely biologic, and thus to be expected and difficult to avoid, from those that are nonbiologic, and thus preventable.
Normal aging refers to the common complex of diseases and impairments that affect many older people. It encompasses a wide spectrum, because people age very differently, including how they develop diseases and impairments.
Successful (healthy) aging refers to a process by which aging is not accompanied by debilitating disease and disability. People who age successfully may maintain an active healthy life until death and are said to have died of old age. In addition, people who age successfully do not experience many of the unwanted features of aging. For example, they may avoid extensive tooth loss, which used to be (and still is in some societies) common among older people. There is evidence that a greater percentage of people in the United States are aging successfully. For example, the percentage of older people residing in nursing homes has decreased, although the percentage of people over age 65 and, of those people, the proportion over age 85 have both increased. Furthermore, the percentage of people aged 75 to 84 who report disabilities has decreased, as has the percentage of people over age 65 with debilitating disease. Although there may be alternative explanations for these changes in health status, one viable explanation is an increase in the proportion of people who are aging successfully.
Disease Versus Aging
With age, many of the body's functions decline. The decline that occurs in normal aging, however, is not usually considered the same as disease, although the distinction is not always clear. As people age, blood sugar levels increase more after eating carbohydrates than they do in younger people; however, the very large increases that occur in diabetes are not considered normal aging (see Section 13, Chapter 165). Mental decline that includes more difficulty learning new languages and increased forgetfulness is nearly universal with advanced age and is considered normal aging; however, the serious mental decline of dementia, with its extreme loss of short-term memory, ability to learn, and understanding of the environment, although common in late life, is considered a disease (see Section 6, Chapter 83). Alzheimer's disease is an abnormal process distinct from normal aging, a conclusion supported by analysis of brain tissue at autopsy.
Longevity
The average life expectancy of Americans has been increasing dramatically over the past century. A male child born in 1900 could expect to live only 46 years; a female child, 48 years. Today, however, a male child can expect to live more than 73 years; a female child, nearly 80 years. Although many of these gains in longevity can be attributed to the significant decrease in childhood mortality, life expectancy at every age beyond 40 has also increased dramatically. For example, a 70-year-old man can now expect to live until age 83, and a 70-year-old woman to age 85.
Despite the increase in average life expectancy, the maximum life span--the oldest age to which people can live--has changed little since records have been kept. Despite the best genetic makeup and medical care, no one seems to live much beyond 125 years, although some experts suggest that this number may be slowly increasing. Currently, a person has a 1 in 2 billion chance of living to the age of 120.
Several factors influence longevity. One is heredity, which primarily influences whether an individual will contract a disease. For example, inheriting an increased risk of developing high cholesterol levels is likely to result in a shorter life, whereas inheriting genes that protect against heart disease and cancer can help to ensure a longer life. Another important influence on longevity is lifestyle; avoiding smoking, avoiding drug and alcohol abuse, maintaining a healthy weight and diet, and exercising appropriately help people avoid disease. Exposure to environmental toxins can shorten life expectancy even among people with the most robust genetic makeup. Medical care contributes to increased survival by preventing or treating diseases after they are contracted, especially when the diseases (for example, infectious diseases, cancer) are cured.
Bodily Changes
The human body changes in many noticeable ways with age. Often, the first signs of aging involve the musculoskeletal system. By age 35, peak performance, even among superior athletes, begins to decline. Sensory organs also begin to change early in mid-life. A common development, for example, is presbyopia, in which the eye can no longer focus easily on close objects. By age 40 or so, many people find it difficult to read without using reading glasses or changing to bifocals. Another, if later, common development is presbycusis, or age-associated hearing loss, which initially affects the ability to hear the highest pitches and, gradually, the lower pitches as well. Therefore, older people may find that violin music no longer sounds as exciting as it did when they were younger. Also, because most of the closed consonants of speech are high tones (sounds such as k, t, s, p, and ch), older people may think that others are mumbling.
In most people, the proportion of body fat increases by more than 30% by late age. The distribution of fat also changes: There is less fat under the skin and more in the abdominal area. Thus, skin becomes thinner, wrinkled (although sun exposure and smoking have a greater influence on wrinkling), and more fragile, and the shape of the torso changes.
Not surprisingly, most internal functions also decline with age. These functions generally peak shortly before age 30 and then begin a gradual but continuous decline. Even with this decline, however, most functions remain adequate throughout life, because most organs have considerably more functional capacity than the body needs (functional reserve). For example, even if half the liver is destroyed, more than enough liver tissue remains to maintain normal function. Disease, rather than normal aging, usually accounts for loss of function in old age. Even so, the decline in function means that older people are more likely to experience side effects from drugs, changes in the environment, toxins, and illness.
Although the decline in function of many organs has little effect on how people live, the decline in some organs can greatly affect health and well-being. For example, although the amount of blood that the heart can pump at rest is not greatly reduced in old age, the heart cannot pump as much when pushed to its maximum. This means that older athletes will not be able to perform as well as younger athletes. Changes in kidney function can dramatically affect how well older people are able to eliminate certain drugs from their body (see Section 2, Chapter 14).
Determining which changes are purely age-associated and which are the result of how a person has lived is often difficult. A sedentary lifestyle, poor diet, cigarette smoking, and alcohol and drug abuse can damage many organs over time, often more so than aging alone. People who have been exposed to toxins may experience a more significant or more rapid decline in the function of some organs, especially the kidneys, lungs, and liver. People who worked in loud environments are likely to lose more of their hearing. Some decline in function can be prevented by adopting a healthier lifestyle. For example, stopping smoking at any age, even in one's 80s, helps improve lung function, decrease the chance of developing lung cancer, and decrease the risk of heart disease. Weight-bearing exercise helps maintain muscle and bone strength regardless of age and helps prevent falls and debilitation.
 See the table Examples of How the Body Changes With Age.
Implications of Illness
Gerontology is the study of aging. Geriatrics is the branch of medicine that specializes in the care of older people. A number of disorders occur almost exclusively in older people and are sometimes called geriatric syndromes or geriatric diseases. Other disorders that affect people of all ages may cause different symptoms or complications in older people. For example, an underactive thyroid gland usually causes younger people to gain weight and feel sluggish, whereas in older people it may initially or predominantly cause a state of confusion. An overactive thyroid usually causes younger people to become agitated and lose weight; in older people, it may cause them to become sleepy, withdrawn, depressed, and confused. Depression usually causes younger adults to become tearful, withdrawn, and noticeably unhappy. In older people, depression sometimes causes confusion, loss of memory, and apathy without a sense of sadness. The confusion that results from these conditions is often mistaken for dementia in older people.
Acute illnesses that were once likely to result in death for older people, such as heart attacks, hip fractures, and pneumonia, are now often treatable and controllable. In addition, a chronic illness no longer necessarily means disability. Many people with diabetes, kidney problems, heart disease, and other chronic illnesses now find that they can remain functional, active, and independent.
Social factors play a major role in the health care of older people. Older people who maintain social contact, whether it be with a spouse, with friends, or through outside interests, have been shown to have fewer medical problems. For example, older people who are married or who live with a roommate tend to be in better health than those who live alone. Older people who do not live alone also have lower rates of hospitalization and nursing home admissions than those who do live alone.
Education also plays a role in the health of older people. Higher levels of education are associated with earlier detection of disease and better health outcomes, even when disease is not detected early.
Economic factors affect the way in which older people access health care. Poverty is more common among older people than among the general population. Despite the implementation of such programs as Medicare, Medicaid, and Social Security, some older people do not have adequate health insurance and find it difficult to pay for uncovered aspects of medical care, including drugs. Consequently, otherwise treatable diseases go untreated or are treated at a later stage.
Older people often have more than one disease at a time, each of which may have an effect on the other. For example, depression may make dementia worse, and an infection may worsen diabetes. Additionally, many older people tend to conceal minor problems and do not seek medical care until the problems become major.
Psychologic factors coexist with and may complicate disease in older people. An older person may become depressed if his illness leads to temporary or permanent loss of independence or as he sees his aging friends and loved ones die. For these reasons, geriatricians often recommend multidisciplinary care. With this type of care, a team of medical personnel, which may consist of doctors, nurses, social workers, therapists, pharmacists, and psychologists, plan and implement care--including social services--under the leadership of a primary doctor.
See the table Disorders That Mainly Affect Older People.
Diseases of Accelerated Aging
Progeroid Syndromes: The progeroid syndromes are conditions that produce premature aging and a shortened life expectancy. The most striking feature of these rare disorders is extremely accelerated aging. Affected children develop all of the external signs of old age, including baldness, hunched posture, and dry, wrinkled skin. Unlike normal aging, however, progeroid syndromes also include such features as lack of ovarian or testicular activity (including sterility and absence of menstrual periods) and unusually short stature. Thus, progeria is not an exact model of accelerated aging.
Two forms of progeroid syndromes are Hutchinson-Gilford syndrome (commonly referred to as progeria), which begins in early childhood, and Werner's syndrome, which begins in adolescence or early adult life. Both syndromes are hereditary, but the exact genetic basis of Hutchinson-Gilford syndrome remains undetermined. However, recent advances have been made in understanding the genetic basis of Werner's syndrome. Hutchinson-Gilford syndrome produces scleroderma, baldness, and other conditions normally associated with aging (for example, diseases of the heart, kidneys, and lungs). Similarly, Werner's syndrome produces the skin changes of scleroderma and baldness and a high rate of atherosclerosis. The central nervous system, and therefore daily activities dependent on brain function, is largely spared in both Hutchinson-Gilford syndrome and Werner's syndrome, unless an affected person has a stroke. Other features of Werner's syndrome include premature cataracts, muscle wasting, and a high rate of cancer (including some types that are rare in unaffected people).
Down Syndrome (see Section 23, Chapter 266): More common than the progeroid syndromes, Down syndrome also produces in younger adults conditions typical of old age, including glucose intolerance, blood vessel disease, a high rate of cancer, hair loss, degenerative bone disease, and premature death. In contrast to Werner's syndrome, Down syndrome greatly impairs the central nervous system, usually producing retardation and, later in life, brain changes characteristic of Alzheimer's dementia (see Section 6, Chapter 83).
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