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.
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.
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.
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.
AGING
POPULATIONS
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.