Aging is a series of biological changes that follow a natural progression from birth through maturity to old age and death. For most people, advancing age is characterized by graying or thinning hair, loss of height, wrinkling of the skin, and decreased muscular strength. Still, an active lifestyle, including both exercise and sound nutrition, can contribute greatly to achieving a long and productive life. Genetics (inherited physical characteristics) also appears to play a role in the process of aging and death. People whose parents or grandparents live to old age seem to have a better chance of living long lives themselves.
Great strides have been made in increasing the average life expectancy in humans in the United States. This is due largely to the elimination of many diseases of early childhood and young adulthood and to the advanced methods of treating diseases that in the past would have resulted in early death. However, the maximum potential life span (how long an organism can exist) of humans appears to remain the same—about 85 years—regardless of efforts to expand it.
Every species has a different normal life span. In most species, death occurs not long after the reproductive phase of life ends. This is obviously not the case for humans. However, women do experience physical changes when they are past their childbearing years. Levels of the hormone estrogen begin to fall, which results in the gradual cessation, or stopping, of menstruation. (Hormones are chemicals produced by the body that regulate various bodily functions. Menstruation is the monthly shedding of the lining of the uterus [womb] in nonpregnant females.) After menopause (when menstruation ceases and childbearing is no longer possible), women produce less facial skin oil (which leads to wrinkling) and are at greater risk of developing osteoporosis (brittle bones). Men continue to produce nearly the same levels of facial oils and are thus less prone to early wrinkling.
As we age, our cells become less efficient and our bodies become less able to carry out their normal functions. Muscles lose strength, hearing and vision become less acute, reflex times slow down, lung capacity decreases, and the heart's ability to pump blood may be affected. In addition, the immune system weakens, making it less able to fight infection and disease.
No single theory on how and why people age is able to account for all aspects of aging, but most take one of two approaches: (1) that humans are genetically programmed to age and die, and (2) that natural wear and tear causes aging and eventual death. Arguments in favor of genetic programming cite hormonal control and/or limited cell division (reproduction) as the mechanisms involved in the aging process. One hormonal approach focuses on the hypothalamus (at the base of the forebrain), which controls the production of growth hormones (which influence growth and development) in the pituitary gland. It is thought that—possibly due to the action of a hormonal clock—the hypothalamus either slows down normal hormonal function or that it becomes more error-prone with time, leading to physiological aging.
Repeated experiments have shown that human cells grown in tissue culture in the laboratory will divide only about 50 times before they die. (Exceptions are cancer cells, which have unlimited growth, and brain and muscle cells, which do not divide after birth). This suggests a genetic clock that limits how many time cells can divide. It is thought that as cell division decreases, as it does in older people, the functioning of the body also begins to slow down, resulting in aging and eventual death.
The wear-and-tear theory of aging and death suggests that genes (the molecules that carry instructions for passing on specific traits from one generation to another) are altered by random mutations (changes in form) that accumulate over time, gradually leading to aging and disease. Environmental agents, such as X rays, ultraviolet radiation, and chemical toxins, can contribute to this mutation process. All cells have the ability to repair damaged DNA (deoxyribonucleic acid, the molecular basis of heredity), but when these repair mechanisms fail, mutations can accumulate. This failure is also thought to be a factor in the development of cancer.
DNA (deoxyribonucleic acid): A compound composed of sugar, base, and phosphate groups and located in the nucleus of cells where hereditary information is stored.
Gene: A section of a DNA molecule that carries instructions for the formation, functioning, and transmission of specific traits from one generation to another.
Gerontology: The scientific study of aging with regard to its physical, social, economic, and psychological aspects.
Hormone: A chemical produced in living cells that is carried by the blood to organs and tissues in distant parts of the body, where it regulates cellular activity.
Life expectancy: The average age of death of an individual within a population.
Life span: The average maximum length of life for members of a species.
Menopause: Period in a woman's life when menstruation stops and childbearing is no longer possible, usually occurring between the ages of 45 and 50.
Proteins: Large molecules that are essential to the structure and functioning of all living cells.
Two other factors are thought to contribute to aging: (1) protein damage from cross linkages and (2) the presence of free radicals in the body. Cross linkages are faulty bonds that can form in proteins, such as collagen. Collagen is a major component of connective tissue, which provides support to organs and elasticity to blood vessels. Cross-linkage in collagen molecules changes the shape and function of an organ's collagen supports and decreases vessel elasticity. Free radicals are normal chemical by products resulting from the body's use of oxygen. However, the buildup of free radicals, often the result of environmental abuses, can alter cellular function, causing damage. Antioxidants, such as vitamins C and E, block the action of free radicals and are therefore thought to retard aging.
Diseases associated with aging include cancer, cardiovascular (or heart) disease, diabetes mellitus, Alzheimer's, and Parkinson's. Diabetes is usually characterized by decreased production of insulin. (Insulin is a hormone essential to the metabolism of carbohydrates, sugars, and starches.) Alzheimer's is a degenerative disease of the brain cells that causes progressive loss of memory and concentration, as well as impaired learning and judgment. It primarily affects people over 65 years of age. As a larger proportion of Americans become elderly in the near future, scientists predict that the number of Alzheimer's cases will continue to rise. By 2050, it is estimated that 14 million Americans will have Alzheimer's disease. Parkinson's is a crippling disease in which the muscles become increasingly rigid, movement—such as walking—is difficult, and involuntary tremors (shakiness) develop.
Death is marked by the end of blood circulation, the end of oxygen transport to organs and tissues, the end of brain function, and overall organ failure. The diagnosis of death can occur legally when breathing and heartbeat have stopped and when the pupils are unresponsive to light. Although Alzheimer's disease and AIDS (acquired immunodeficiency syndrome) receive much attention in the press, the two major causes of death in the United States are heart disease and cancer.
Other causes of death include stroke, accidents, infectious diseases, murder, suicide, and euthanasia (ending a hopelessly sick person's life for reasons of mercy). Loss of life can also result from genetic diseases, bacterial and viral infections, drug use, and alcoholism.