Humans are the only species known to live decades past their reproductive years. Other creatures succumb to predators or diseases while still in their prime.

During the 20th century, the average life span of people living in developed countries increased 60%, from about 50 years to 80 years. As a result, health problems linked to aging, once almost a curiosity, are now an important part of medicine.

The major organ systems change in certain ways as a natural consequence of aging. Here are some of the changes we can all expect as we age.

Skin: Wrinkles, sores, and loss of elasticity result mainly from sun exposure, rather than from aging itself.

Nervous system: Loss of brain cells and slowing of neurotransmitter production leads to somewhat slower responses and diminished sensory perceptions, such as taste and hearing, but the impact is minor.

Cardiovascular system: Heart size increases slightly as muscle cells are replaced by scar tissue, causing slightly decreased efficiency in heart function (though the heart functions well even in very old people unless there is also heart disease).

Respiratory system: Lungs become less elastic and less able to inflate or deflate completely, and have trouble responding effectively to the cough reflex, making older people more vulnerable to lung infections.

Muscles and bones: Loss of bone, called osteoporosis, progresses. Muscle strength and mass decline. Strength training can help improve both conditions.

Digestive system: The amount of digestive enzymes declines, causing fewer vitamins and nutrients to be absorbed.

Immune system: Key cells of the immune system, especially the T cells, function less effectively.

Reproductive system: During menopause, a woman's ovaries stop producing estrogen and progesterone, ending her ability to reproduce. Some estrogen continues to be made by the adrenal glands and fat cells. Among men, testosterone output diminishes steadily after age 65, though this may not prevent a man from having erections, making sperm, and fathering children.

Urinary system: A reduction in blood flow to the kidneys affects their ability to extract wastes from the blood and to form urine. In most people, this slight change barely affects kidney function. Many people find they need to urinate more frequently as they age, including more than one or two times at night. Also, the urge to urinate may come on more suddenly. Leaking urine, however, is not a normal part of aging.

Hormonal system: The production of many hormones diminishes somewhat with aging. Except for the reproductive hormones, this change has few, if any, effects on the body, though science knows relatively little about the true effect.

Each body system ages according to its own timetable. And these schedules vary dramatically from person to person. Despite the inevitable toll that aging takes on your body, you can slow the process by taking several important preventive steps:

  • embrace exercise
  • eat a healthful diet
  • don't get too much sun
  • don't smoke
  • if you drink alcohol, keep it moderate (no more than one drink a day for women, two a day for men)
  • get screening tests for diseases that can be detected early.