Aging and Death
Theories On Aging
The relationship between aging and death is complex. The results from many studies indicate that aging decreases the efficiency of the body to operate, defeat infections, and to repair damage. Comparison of people aged 30–75 has demonstrated that the efficiency of lung function decreases by 50% that bones become more brittle, and that the immune system that safeguards the body from infections generally becomes less efficient as we age.
Why this deterioration in the functioning of the body with age occurs is still not clear. Several theories have been proposed to explain this decline. One theory proposes that after the active years of reproduction have passed, chemical changes in the body cause the gradual malfunctioning of organs and other body components. The accumulation of damage to components that are necessary for the formation of new cells of the body leads to death. For example, it has been discovered that the formation of the genetic material deoxyribonucleic acid (DNA) is more subject to mistakes as time goes on. Other theories relating aging with death include the negative effect of stresses to the body, and a theory that proposes that the buildup of non-functional material in the body over time lessens the ability of the body to function correctly.
The strongest arguments on the aging process favor involvement of one of, or a combination of the following: hormonal control, limited cell division, gene theory, gene mutation theory, protein cross-linkage theory, and free radical action. In support of hormonal control, there is the observation that the thymus gland (under the sternum) begins to shrink at adolescence, and aging is more rapid in people without a thymus. Another hormonal approach focuses on the hypothalamus (at the base of the brain), which controls the production of growth hormones in the pituitary gland. It is thought that the hypothalamus either slows down normal hormonal function or that it becomes more error-prone with time, eventually leading to physiological aging.
More recent theories on aging come from cell biology and molecular biology. Cells in culture in the laboratory keep dividing only up to a point, and then they die. Cells taken from embryos or infants divide more than those taken from adults. Hence, it is thought that this is the underlying mechanism of aging—once cells can no longer divide to replenish themselves, a person will begin to die. However, most scientists now accept that most cells (other than brain and muscle cells) are capable of division for a longer time than the normal human lifespan.
Gene theory and gene mutation theory both offer explanations for aging at the level of DNA. Gene theory suggests that genes are somehow altered over time, such that they naturally cause aging. Gene mutation theory is based on the observation that mutations accumulate over time, and it is mutations that cause aging and disease. This view is supported by the fact that samples of cells from older people do generally have more genetic mutations than cells taken from younger people. In addition, some diseases associated with age result from genetic mutations. Cancer is often the result of multiple mutations and some mutations reveal underlying genetic weaknesses, which cause disease in some people. Gene mutation theory also notes that for mutations to accumulate, normal DNA-repair mechanisms must have weakened. All cells have inherent repair mechanisms that routinely fix DNA errors. For these errors to accumulate, the repair system must have gone awry, and DNA-repair failure is thought to be a factor in cancer.
Protein cross-linkage and free radicals are also thought to contribute to aging. Faulty bonds (cross-linkages) can form in proteins with important structural and functional roles. Collagen makes up 25-30% of the body's protein and provides support to organs and elasticity to blood vessels. Cross-linkage in collagen molecules alters the shape and function of the organs it supports and decreases vessel elasticity. Free radicals are normal chemical byproducts resulting from the body's use of oxygen. However, free radicals bind unsaturated fats into cell membranes, alter the permeability of membranes, bind chromosomes, and generally alter cellular function, causing damage. Antioxidants, such as vitamins C and E, block free radicals and are suggested for prolonging life.