Starting at what is commonly called middle age, operations of the human body become more vulnerable to daily wear and tear. There is a general decline in physical, and possibly mental, functioning. In the Western countries, the length of life often extends into the 70s. However, the upward limit of the life span can be as high as 120 years. During the latter half of life, an individual is more prone to problems with the various functions of the body, and to a number of chronic or fatal diseases. The cardiovascular, digestive, excretory, nervous, reproductive, and urinary systems are particularly affected. The most common diseases of aging include Alzheimer's, arthritis, cancer, diabetes, depression, and heart disease.
Description
Human beings reach a peak of growth and development during their mid 20s. Aging is the normal transition time after that flurry of activity. Although there are quite a few age-related changes that tax the body, disability is not necessarily a part of aging. Health and lifestyle factors, together with the genetic makeup of the individual, determine the response to these changes. Body functions that are most often affected by age include:
Hearing, which declines especially in relation to the highest pitched tones.
The proportion of fat to muscle, which may increase by as much as 30%. Typically, the total padding of body fat directly under the skin thins out and accumulates around the stomach. The ability to excrete fats is impaired, and therefore the storage of fats increases, including cholesterol and fat-soluble nutrients.
The amount of water in the body, which decreases, reducing the body's ability to absorb water-soluble nutrients. Also, there is less saliva and other lubricating fluids.
Liver and kidney activities, which become less efficient, thus affecting the elimination of wastes.
The ease of digestion, which is decreased, resulting in a reduction in stomach acid production.
Muscle strength and coordination, which lessens, with an accompanying loss of mobility, agility, and flexibility.
Sexual hormones and sexual function, which both decline.
Sensations of taste and smell, which decrease.
Cardiovascular and respiratory systems, with changes leading to decreased oxygen and nutrients throughout the body.
Nervous system, which experiences changes that result in less efficient nerve impulse transmission, reflexes that are not as sharp, and diminished memory and learning.
Bone strength and density, which decrease.
Hormone levels, which gradually decline. The thyroid and sexual hormones are particularly affected.
Visual abilities, which decline. Age-related changes may lead to diseases such as macular degeneration.
A compromised ability to produce vitamin D from sunlight.
Protein formation, which is reduced, leading to shrinkage in muscle mass and decreased bone formation, possibly contributing to osteoporosis.
Causes & symptoms
There are several theories on why the aging body loses functioning. It may be that several factors work together or that one particular factor is the culprit in a given individual. These theories include:
Programmed senescence, or aging clock, theory. The aging of the cells for each individual is programmed into the genes, and there is a preset number of possible rejuvenations in the life of a given cell. When cells die at a rate faster than they are replaced, organs do not function properly, and they become unable to maintain the functions necessary for life.
Genetic theory. Human cells maintain their own seed of destruction at the chromosome level.
Connective tissue, or cross-linking theory. Changes in the makeup of the connective tissue alter the stability of body structures, causing a loss of elasticity and functioning, and leading to symptoms of aging.
Free-radical theory. The most commonly held theory of aging, is based on the fact that ongoing chemical reactions of the cells produce free radicals. In the presence of oxygen, these free radicals cause the cells of the body to break down. As time goes on, more cells die or lose the ability to function, and the body ceases to function as a whole.
Immunological theory. There are changes in the immune system as it begins to wear out, and the body is more prone to infections and tissue damage, which may ultimately cause death. Also, as the system breaks down, the body is more apt to have autoimmune reactions, in which the body's own cells are mistaken for foreign material and are destroyed or damaged by the immune system.
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