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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 Galápagos 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 |
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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 |
Parlor of a Senior Center
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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.
Ronald Reagan Copes with Alzheimer's Disease |
Former United States President Ronald Reagan announced in early November 1994 that he had been diagnosed with Alzheimer’s disease. Reagan wished to increase awareness of Alzheimer’s, a degenerative disease of the brain that is characterized by confusion, loss of memory, and other symptoms. Committed to finding a cure for the disease, Reagan and his wife, Nancy, along with the national Alzheimer’s Foundation, started the Ronald and Nancy Reagan Research Institute in 1995. |
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.
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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 leads
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 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 inhibits
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.
V. | AGING POPULATIONS |
Life Expectancy by Age, Race, and Sex in the United
States
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In developed nations, life expectancy has
increased more in the 20th century than it has in all of recorded history. A
person born in the United States in 1995 can expect to live more than 35 years
longer than a person born in 1900. Today more than 34 million Americans are 65
or older, accounting for about 13 percent of the population. By the year 2030,
their numbers will more than double: One in every five Americans will be over
age 65. A person who lives 100 years or more—a centenarian—was once a rarity,
but today about 60,000 Americans are 100 years or older. By the year 2060, there
may be as many as 2.5 million centenarians in the United States. The number of
supercentenarians—people 105 years of age and older—will probably be as
commonplace in the next century as centenarians are fast becoming now.
In some parts of the world, 16 to 18 percent
of the population is already age 65 or older. By the year 2025, Japan is
expected to have twice as many old people as children. Also by that time, there
will be more than one billion older people worldwide. This increase in life
expectancy is the result of better public health measures,
improvements in living conditions, and advances in medical care. A marked
reduction in infant mortality rates has also contributed to increased life
expectancy statistics.
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Aging populations are expected to have
profound effects on the way societies care for their elderly members. With a
larger proportion of the population over age 65, medical care must become better
equipped to deal with the disorders and diseases of the elderly. All health care
professionals should have special training in geriatrics. As the percentage of
older people in the population exceeds the percentage of young working people,
traditional methods for caring for older people may need to be modified. For
example, in the United States, workers pay taxes throughout their careers so
that when they retire, usually around the age of 65, they can receive money from
the federal government to survive. This system, called Social
Security, may be in jeopardy as the percentage of retired people
increases, placing inordinate demands on the smaller number of people working
and supporting them.
In many parts of the world, including the
United States, older people who cannot work and have health problems live in
long-term care facilities such as nursing homes, where they receive care 24
hours a day. But many families are unable to bear the costs of nursing homes and
medical care for the elderly, and health insurance is unable to cover the
expense. Other countries face similar problems, and multinational efforts are
underway to explore new methods to finance the care of the world’s older
persons, soon to number one billion
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