Aging, irreversible biological changes that occur in all living things with the passage of time, eventually resulting in death. Although all organisms age, rates of aging vary considerably. Fruit flies, for example, are born, grow old, and die in 30 or 40 days, while field mice have a life span of about three years. Dolphins may live to age 25, elephants to age 50, and Galapagos tortoises to 100. These life spans pale in comparison to those of some species of giant sequoia trees, which live hundreds of years.
Among humans, the effects of aging vary from one individual to another. The average life expectancy for Americans is around 75 years, almost twice what it was in the early 1900s. Although some people never reach this age, and others are beset with illnesses if they do, more and more people are living healthy lives well into their 90s and older. The study of the different aging processes that occur among individuals and the factors that cause these changes is known as gerontology. Geriatrics is a medical specialty concerned with the prevention, diagnosis, and treatment of diseases in the elderly.
II. EFFECTS OF AGING ON THE HUMAN BODY
Several general changes take place in the human body as it ages: hearing and vision decline, muscle strength lessens, soft tissues such as skin and blood vessels become less flexible, and there is an overall decline in body tone.
Most of the body's organs perform less efficiently with advancing age. For example, the average amount of blood pumped by the heart drops from about 6.9 liters (7.3 quarts) per minute at age 20 to only 3.5 liters (3.7 quarts) pumped per minute at age 85. For this same age range, the average amount of blood flowing through the kidneys drops from approximately 0.6 liters (0.6 quarts) per minute to 0.3 liters (0.3 quarts). Not all people experience decreased organ function to the same degree—some individuals have healthier hearts and kidneys at age 85 than others do at age 50.
The immune system also changes with age. A healthy immune system protects the body against bacteria, viruses, and other harmful agents by producing disease-fighting proteins known as antibodies. A healthy immune system also prevents the growth of abnormal cells, which can become cancerous. With advancing age, the ability of the immune system to carry out these protective functions is diminished—the rate of antibody production may drop by as much as 80 percent between age 20 and age 85. This less-effective immune system explains why a bout of influenza, which may make a young adult sick for a few days, can be fatal for an elderly person. Thus, it is as important for an older person to be vaccinated against the flu and pneumonia as it is for young people to be vaccinated against childhood diseases.
Most of the glands of the endocrine system, the organs that secrete hormones regulating such functions as metabolism, temperature, and blood sugar levels, retain their ability to function into advanced age. However, these glands often become less sensitive to the triggers that direct hormone secretion. In the aging pancreas, for example, higher blood sugar levels are required to stimulate the release of insulin, a hormone that helps the muscles convert blood sugar to energy.
The ovaries and the testes, the endocrine glands that regulate many aspects of sexual reproduction, alter during the aging process. As a man ages, the testes produce less of the male sex hormone, testosterone. A woman's ovaries undergo marked changes from about age 45 to age 55 during a process known as menopause. The ovaries no longer release egg cells, and they no longer generate the hormones that stimulate monthly menstrual cycles. After women have gone through menopause, they are no longer capable of having children without the aid of reproductive technology. The physical changes associated with aging do not have a significant impact on sexual activity—most healthy people maintain an interest in sex all of their lives.
III. THE EFFECTS OF AGING ON THE MIND
One of the myths of aging is that intelligence diminishes with age. Early studies that used intelligence tests designed for children revealed that older people scored lower than young adults. However, these tests relied heavily on skills commonly used in school classrooms, such as arithmetic, and required the test to be completed within a specific time limit. Older people may require more time to answer questions, and more recent studies based on untimed tests and other measures of intellectual activity, such as problem-solving and concept formation, show that there is relatively little decline in mental ability in healthy people at least up to age 70.
The aging brain does undergo a progressive loss of neurons or nerve cells, but these losses represent only a small percentage of neurons in the brain. The speed of conduction of a nerve impulse declines with age, but it drops only about 15 percent over the age span from 30 to 85 years. Although intelligence is generally not affected by the aging process, studies show that some older people may find it difficult to deal with many stimuli at once. For example, an older individual requires more time to sort out all of the information when many highway signs come into view simultaneously. Traveling at 97 km/h (60 mph), an elderly driver may miss the information he or she needs or may act on the wrong information. But if older individuals recognize this limitation and adjust their behavior accordingly, they can continue driving safely well into old age.
Many older people experience problems with memory, and up to 10 percent of the elderly have memory problems significant enough to interfere with their ability to function independently. Memory problems were once considered an inevitable effect of the aging process, but researchers have determined that many of the brain-related changes often observed in elderly people, including memory loss, are actually a result of such diseases as Alzheimer’s disease and diseases associated with blood vessels and blood flow in the brain, such as stroke. Memory loss is sometimes treatable, and certain memory-aiding strategies have been found to help reverse the short-term memory loss experienced by many older people.
Another myth about aging is that people tend to grow sour and mean-spirited with age. Research shows that personalities really do not change much over time. A mean-spirited, grumpy old person was probably that way when he or she was 30. And, as humans age, most still like to do the things they did when they were young. For example, those who were athletic in their youth may continue to enjoy athletic activities as they age.
An older person's social environment, however, can have a marked impact on personality. The social isolation that often exists among older people can dramatically influence mental attitudes and behavior. In the United States, 33 percent of all older people live alone, most of them widowed women over the age of 85. About 5 percent of elderly Americans live in some type of long-term care facility, and almost 25 percent of all older Americans live under or near the federal poverty level. These people have little or no money for recreational activities. This poverty and isolation often lead to clinical depression and other problems, such as alcoholism.
IV. CAUSES OF AGING
Although the exact causes of aging remain unknown, scientists are learning a great deal about the aging process and the mechanisms that drive it. Some of the most promising research on the aging process focuses on the microscopic changes that occur in all living cells as organisms age. In 1965 American microbiologist Leonard Hayflick observed that under laboratory conditions, human cells can duplicate up to 50 times before they stop. Hayflick also noted that when cells stop normal cell division (see Mitosis), they start to age, or senesce. Since Hayflick’s groundbreaking observations, scientists have been searching for the underlying cause, known as the senescent factor (SF), of why cells stop dividing and thus age.
Different theories have been proposed to explain how SF works. One theory is based on the assumption that aging, and diseases that occur more frequently with advancing age, are caused by structural damage to cells. This damage accumulates in tiny amounts each time the cell divides, eventually preventing the cell from carrying out normal functions.
One cause of this damage may be free radicals, which are chemical compounds found in the environment and also generated by normal chemical reactions in the body. Free radicals contain unpaired electrons and so carry an electric charge that makes them highly reactive. In an effort to neutralize their electric charge, free radicals constantly bombard cells in order to steal electrons in a process called oxidation. Free radicals are thought to greatly increase the severity of—or perhaps even cause—such life-shortening diseases as diabetes mellitus, strokes, and heart attacks. Researchers have observed that free radicals exist in smaller amounts in those species with relatively long life spans. Increasing the human life span may depend on our ability to prevent free radical damage, and scientists are currently examining the role of chemical compounds, called antioxidants, that prevent or reverse oxidative damage in the aging process.
Another theory suggests that SF is genetically regulated—that is, cells are genetically programmed to carry out about 50 cell divisions and then die. Researchers have identified at least three genes that are involved with human cellular senescence. They have also discovered a protein on the surface membranes of senescent cells that inhibit the production of deoxyribonucleic acid (DNA), the essential molecule that carries all genetic information.
Another theory proposes that extra, useless bits of DNA accumulate over time within a cell's nucleus. Eventually this so-called junk DNA builds up to levels that clog normal cell action. If this idea is correct, scientists may be able to find ways to prevent accumulation of junk DNA, thereby slowing down the process of senescence in cells.
Other studies focus on cell division limits. Each time a cell divides, it duplicates its DNA, and in each division, the sections at the ends of DNA, called the telomeres, are gradually depleted or shortened. Eventually, the telomeres become so depleted that normal cell division halts, typically within 50 cell divisions. Scientists have found that an enzyme produced by the human body, called telomerase, can prolong the life of the telomeres, thus extending the number of cell divisions. In laboratory studies, cells injected with telomerase continue to divide well beyond the normal limit of 50 cell divisions. These promising results have triggered worldwide attention on telomerase and its relationship to aging.
A number of other studies are underway to investigate the effects of aging. Scientists have found, for example, a possible explanation for why women have longer average life spans than men. The difference seems to be biologically determined, and male and female sex hormones are probably responsible. The blood levels of female sex hormones drop sharply during menopause. At that time, the incidence of heart disease and high blood pressure in women increases to match the incidence in men, suggesting that the presence of female sex hormones offers some protection against heart disease.