human aging
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human aging, physiological changes that take place in the human body leading to senescence, the decline of biological functions and of the ability to adapt to metabolic stress. In humans the physiological developments are normally accompanied by psychological and behavioral changes, and other changes, involving social and economic factors, also occur.
Aging begins as soon as adulthood is reached and is as much a part of human life as are infancy, childhood, and adolescence. Gerontology (the study of aging) is concerned primarily with the changes that occur between the attainment of maturity and the death of the individual. The goal of research in gerontology is to identify the factors that influence these changes. Application of this knowledge can reduce the severity of some disabilities commonly associated with aging.
The biological-physiological aspects of aging include both the basic biological factors that underlie aging and the general health status. Since the probability of death increases rapidly with advancing age, it is clear that changes must occur in the individual which make him or her more and more vulnerable to disease. For example, a young adult may rapidly recover from pneumonia, whereas an elderly person may die.
Physiologists have found that the performance of many organs such as the heart, kidneys, brain, or lungs shows a gradual decline over the life span. Part of this decline is due to a loss of cells from these organs, with resultant reduction in the reserve capacities of the individual. Furthermore, the cells remaining in the elderly individual may not perform as well as those in the young. Certain cellular enzymes may be less active, and thus more time may be required to carry out chemical reactions. Ultimately the cell may die.
Effect of aging of the body systems
Cardiovascular system
Diseases of the heart are the single largest cause of death after age 65. Thus, with increasing age, the heart becomes more vulnerable to cardiovascular disease. Even in the absence of detectable disease, the heart undergoes deleterious changes with advancing age. Structural changes include a gradual loss of muscle fibres with an infiltration of adipose tissue (fat) and connective tissue. There is a gradual accumulation of insoluble granular material (lipofuscin, or “age pigment”) in cardiac muscle fibres. These granules, composed of proteins and lipids, make their first appearance by age 20 and increase gradually, so that by age 80 they may occupy as much as 5–10 percent of the volume of a muscle fibre.
The heart also shows a gradual reduction in performance with advancing age. The amount of blood pumped by the heart diminishes by about 50 percent between ages 20 and 90 years. There are marked individual differences in the effects of age. For example, some 80-year-old individuals may have cardiac function that is as good as that of the average 40-year-old individual.
Under resting conditions, the heart rate does not change significantly with age. During each beat, however, the muscle fibres of the heart do not contract as rapidly in the old as in the young. This reduction in power, or rate of work, is due to the age-associated reduction in the activities of certain cellular enzymes that produce the energy required for muscular contraction.
In spite of these changes, the heart, in the absence of disease, is able to meet the demands placed upon it. In response to physical exercise, it can increase its rate to double or triple the amount of blood pumped each minute, although the maximum possible output falls, and the reserve capacity of the heart diminishes with age.
Arteriosclerosis, or hardening of the arteries, increases markedly in incidence with age and is often regarded as part of aging. This is not necessarily true. Arteriosclerosis may appear even in adolescents. It is a progressive disorder and is present to some extent in practically all individuals by middle life. It is, therefore, impossible to make a clear distinction between the effects of aging and the effects of disease in blood vessels in humans. In some animal species—for example, the rat—that do not develop arteriosclerosis, age changes in the heart and blood vessels can be identified.
In general, blood vessels become less elastic with advancing age. There is a progressive thickening of the walls of larger blood vessels with an increase in connective tissue. The connective tissue itself becomes stiffer with increasing age. This occurs because of the formation of cross-links both within the molecules of collagen, a primary constituent of connective tissue, and between adjacent collagen fibres. These changes in blood vessels occur even in the absence of the deposits on the arterial wall characteristic of atherosclerosis, which interfere with blood flow through the arteries. The gradual loss of elasticity increases with resistance to the flow of blood so that blood pressure may increase. This in turn increases the work that the heart must do in order to maintain the flow of blood.
While both systolic and diastolic blood pressures (blood pressures at contraction and dilation of the heart, respectively) increase with age, the rate of systolic increase exceeds that of diastolic so that the pulse pressure widens. The increase in pressure stops in the eighth decade of life, and there may even be a slight decline in pressure in extreme old age.
On the average, obese people have higher blood pressures than those with normal body weights. Since the incidence of obesity increases with age at least up to age 55–60, this factor may contribute in part to the increase in blood pressure with age.
Digestive system
Loss of teeth, which is often seen in elderly people, is more apt to be the result of long-term neglect than a result of aging itself. The loss of teeth and incidence of oral disease increase with age, but, as programs of water fluoridation are expanded and the incidence of tooth decay in children is reduced, subsequent generations of the elderly will undoubtedly have better teeth than the present generation.
While it is true that the secretion by the stomach of hydrochloric acid, as well as other digestive enzymes, decreases with age, the overall process of digestion is not significantly impaired in the elderly. Sugar, proteins, vitamins, and minerals are absorbed from the stomach and intestine as well in the elderly as in the young. Some investigations indicate a slight impairment in fat absorption, but the reduction is probably of little practical significance.
These findings have important implications for nutrition of the elderly. There is no evidence that the intake of any nutrient, such as vitamins and minerals, need be increased in the elderly because of impaired absorption. Nutritional deficiencies can be avoided as long as the diet is varied to assure adequate intake of all nutritional elements. Deficiencies are most likely to develop from poor eating habits, such as excessive intake of carbohydrate with a reduction in protein. In the elderly these deficiencies are most apt to be in the intake of protein, calcium, iron, vitamin A, and thiamine (also called vitamin B1).
Nervous system
Changes in the structures of the brain due to normal aging are not striking. It is true that with advancing age there is a slight loss of neurons (nerve cells) in the brain. The total number of neurons is extremely large, however, so that any losses probably have only a minor effect on behaviour. Since the physiological basis of memory is still unknown, it cannot be assumed that the loss of memory observed in elderly people is caused by the loss of neurons in the brain.
Neurons are extremely sensitive to oxygen deficiency. Consequently, it is probable that neuron loss, as well as other abnormalities observed in aging brains, results not from aging itself, but from disease, such as arteriosclerosis, that reduces the oxygen available to areas of the brain by reducing the blood supply. Genetic and environmental factors, such as exposure to certain chemicals, smoking, or lack of exercise, may also contribute to memory impairment and reduced cognitive ability in the elderly. For example, increased waist circumference and obesity later in life are linked to thinning of the cerebral cortex and cognitive decline; the cerebral cortex is composed primarily of neuronal cell bodies, the deterioration of which is associated with memory and cognitive impairment.
There are probably functional changes in the brain that account for the slowing of responses and for the memory defects that are often seen in the elderly, and even small changes in the connections between cells of the brain could serve as the basis for marked behavioral changes, but, until more is known about how the brain works, behavioral changes cannot be related to physiological or structural changes. It is known that, because of the slow course of aging, the nervous system can compensate and maintain adequate function even in centenarians.
Human behaviour is highly dependent on the reception and integration of information derived from sensory organs, such as the eye and ear, as well as from nerve endings in skin, muscle, joints, and internal organs. There is, however, no direct relation between the sensitivity of receptors and the adequacy of behaviour, because the usual level of stimulation is considerably greater than the minimum required for stimulation of the sense organs. In addition, an individual adapts to gradual impairments in one sensory organ by using information available from other sense organs. Modern technology has also provided eyeglasses and hearing aids to compensate for reduced acuity in the sense organs.
The incidence of gross sensory impairments, of which many are the result of disease processes, increases with age. One survey conducted in the United States classified 25.9 per 1,000 persons aged 65–74 as blind, in contrast to 1.3 per 1,000 aged 20–44 years. In the age group 65–74, 54.7 per 1,000 persons were classified as functionally deaf, compared with 5.0 per 1,000 in the age range 25–34 years.
Vision
Visual acuity (ability to discriminate fine detail) is relatively poor in young children and improves up to young adulthood. From about the middle 20s to the 50s there is a slight decline in visual acuity, and there is a somewhat accelerated decline thereafter. This decline is readily compensated for by the use of eyeglasses. There is also reduction in the size of the pupil with age. Consequently, vision in older people can be significantly improved by an increase in the level of illumination.
Aging also brings about a reduction in the ability to change the focus of the eye for viewing near and far objects (presbyopia), so that distant objects can ordinarily be seen more clearly than those close at hand. This change in vision is related to a gradual increase in rigidity of the lens of the eye that takes place primarily between ages 10 and 55 years. After age 55 there is little further change. Many people in their 50s adopt bifocal glasses to compensate for this physiological change.
The sensitivity of the eye under conditions of low illumination is less in the old than in the young; that is, “night vision” is reduced. Sensitivity to glare is also greater in the old than in the young.
The incidence of diseases of the eye, such as glaucoma and cataracts (characterized, respectively, by increased intra-ocular pressure and opaque lenses), increases with age, but advances in surgery and contact lenses have made it possible to remove cataracts and restore vision to many individuals.
Hearing
Hearing does not change much with age for tones of frequencies usually encountered in daily life. Above age 50, however, there is a gradual reduction in the ability to perceive tones at higher frequencies. Few persons over age 65 can hear tones with a frequency of 10,000 cycles per second. This loss of perception of high frequencies interferes with identifying individuals by their voices and with understanding conversation in a group but does not ordinarily represent a serious limitation to the individual in daily life. Listening habits and intellectual level play an important role in determining the ability to understand speech, so that there is often a disparity between measurements of pure tone thresholds and ability to perceive speech.
Other sensory impairment
After age 70 other sense organs may show a reduction in sensitivity. Reduced taste sensitivity is associated with atrophy and loss of taste buds from the tongue in the elderly. The effect of aging on the sense of smell has not been precisely determined because this sense is extremely difficult to assess quantitatively; in addition, smoking and exposure to occupational odours and noxious substances in the air influence sensitivity to smells.
Sensitivity to pain is difficult to evaluate quantitatively under controlled laboratory conditions. There is some evidence that it diminishes slightly after age 70.
There is a general slowing of responses in the elderly. Reflexes become slightly more sluggish, and the speed of conduction of impulses in nerves is slightly slowed. Old people require more time to respond to the appearance of a light than do young. The slowing with age is greater in situations where a decision must be made. For example, more time is required to initiate a response in experiments in which the instructions are “Press the button with your right hand when the green light comes on, but with your left hand when the red light comes on” than if the instructions are, “Push the button if either light comes on.” From these and other experiments it is concluded that the primary site of slowing of responses is within the brain rather than in the end organ (eye) itself.
