The stages of life and the activities associated with those stages are issues of great interest in today’s society. We view life stages very differently today than we did in the past. For example, in 1960, 20% of males and 12% of females graduating from high school attended college. Today, over half of all people 25 and older have attended college for some period of time. In addition, there are many more nontraditional college students than there were just a few years ago.
The stages of life from fertilization to death are divided into three prenatal stages and five postnatal stages as follows:
1. Germinal (jer′mi-n̆al) period, fertilization to 14 days
2. Embryo, 14–56 days after fertilization
3. Fetus, 56 days after fertilization to birth
4. Neonate, birth to 1 month after birth
5. Infant (in′f̆ant), 1 month to 1 or 2 years after birth (the endof infancy is sometimes
6. set at the time that the child begins to walk)
7. Child, 1 or 2 years old to puberty (about 11–14 years)
8. Adolescent (ad-̄o-les′ent), teenage years, from puberty to20 years old
9. Adult, 20 years old to death
During childhood, the individual grows in size and develops considerably. Many of the emotional characteristics a person pos-sesses throughout life are formed during early childhood.
Major physical and physiological changes occur during ado-lescence, and many of these changes also affect the emotions andbehavior of the individual. Other emotional changes occur as the adolescent attempts to fit into an adult world. Puberty (pū′ ber-tē) is the time when reproductive cells begin to mature and when gonadal hormones are first secreted in substantial amounts. These hormones stimulate the development and maturation of secondary sexual characteristics, such as enlargement of the female breasts and growth of body hair in both sexes. Puberty usually begins in females at about 11–13 years of age and in males at about 12–14 years of age. The onset of puberty is usually accompanied by a growth spurt, followed by a period of slower growth. Full adult stature is usually achieved before age 17 or 18 in females and before age 19 or 20 in males.
The Aging Process
Development of a new human being begins at fertilization, and so does the process of aging. Cell division occurs at an extremely rapid rate during early development and then begins to slow as various cells become committed to specific functions within the body.
Many cells of the body, such as skin cells, continue to divide throughout life, replacing dead or damaged tissue. But other cells, such as mature neurons in the brain, cease to divide. Dead neurons tend not to be replaced. After the number of neurons reaches a peak (at approximately the time of birth), the number begins to decline. Neuronal loss is most rapid early in life and then decreas-es to a slower, steady rate.
Young embryonic tissue has relatively small amounts of col-lagen, and the collagen that is present is not highly cross-linked, making it very flexible and elastic. However, many of the collagen fibers produced during development are permanent components of the individual. As the individual ages, more and more cross-links form between the collagen molecules, rendering the tissues more rigid and less flexible.
The tissues with the highest collagen content and the greatest dependency on collagen for their function are the most severely affected by the collagen cross-linking and tissue rigidity associ-ated with aging. One of the first structures to exhibit age-related changes as a result of this increased rigidity is the lens of the eye. Seeing close objects becomes more difficult with advancing age until most middle-aged people require reading glasses . Loss of elasticity also affects other tissues, including the joints, kidneys, lungs, and heart, and greatly reduces their functional ability.
As with nervous tissue, the number of skeletal muscle fibers declines with age. The strength of skeletal muscle reaches a peak between 20 and 35 years of life and usually declines steadily thereafter. Skeletal muscle strength depends primarily on the size of the muscle fibers, but the total number of fibers is probably also important to muscle strength. As most people age, both the number of fibers and the size of each tend to decline. The decline in muscle fiber size may be more related to a general decrease in activity than to any specific age-related changes. Like the colla-gen of connective tissue, however, the macromolecules of skeletal muscle cells undergo biochemical changes during aging, rendering the muscle tissue less functional. A good exercise program can slow or even partially reverse the process of muscular aging.
Cardiac muscle cells also do not normally divide after birth. Age-related changes in cardiac muscle cell function probably contribute to a decline in cardiac function with advancing age.
The heart loses elastic recoil ability and muscular contractility. As a result, total cardiac output declines, and less oxygen and fewer nutrients reach the body cells supplied by the cardiovascular system. This decline in blood flow can be particularly harmful to cells that require high oxygen levels, such as neurons of the brain, and cells that are already compromised, such as cartilage cells of the joints, contributing to the general decline in these tissues.
Reduced cardiac function also can result in decreased blood flow to the kidneys, contributing to decreases in the kidneys’ filtration ability. Degeneration of the connective tissues as a result of collagen cross-linking and other factors also decreases the fil-tration efficiency of the glomerular basement membrane.
Arteriosclerosis (ar-tēr′ ē-ō-skler-ō′ sis) is general hardening ofthe arteries affecting mainly arterioles. Atherosclerosis (ath′ er-ō-skler-ō′ sis) is the gradual formation of lipid-containing plaques in the arterial wall of large and medium-sized arteries . These plaques then may become fibrotic and calcified, resulting in arteriosclerosis. Atherosclerosis interferes with normal blood flow and may result in a thrombosis (throm-bō′ sis), the formation of a clot or plaque inside a vessel. An embolus (em′ bō-lŭs; a patch) is a piece of a clot that has broken loose and floats through the circulation. An embolus can lodge in smaller arteries and cause heart attacks or strokes. Although atherosclerosis affects all middle-aged and elderly people to some extent and can even occur in young people, some people appear more at risk because of high blood cholesterol levels. This condition seems to have a hereditary component, and blood tests are available to screen people for high blood cholesterol levels.
Many other organs, including the liver, pancreas, stomach, and colon, undergo degenerative changes with age. The ingestion of harmful agents can accelerate such changes. Examples of these types of accelerated changes are the degenerative changes induced in the lungs (aside from lung cancer) by cigarette smoke and sclerotic changes in the liver resulting from excessive alcohol consumption.
In addition to the previously described changes, cellular wear and tear, or cellular aging, contributes to aging. Progressive damage from many sources, such as radiation and toxic substances, can result in irreversible cellular insults and may be one of the major factors leading to aging. Although the data are mixed and their interpretation is controversial, some studies suggest that ingesting moderate amounts of vitamins E and C in combination may help slow aging by stimulating cell repair. Vitamin C also stimulates collagen production and may slow the loss of tissue elasticity associated with aging collagen.
According to the free radical theory of aging, free radicals, which are atoms or molecules with an unpaired electron, can react with and alter the structure of molecules that are critical for normal cell function. Alteration of these molecules can result in cell dysfunction, cancer, or other types of cellular damage. Free radicals are produced as a normal part of metabolism and are introduced into the body from the environment through the air we breathe and the food we eat. The damage caused by free radicals may accumulate with age. Antioxidants, such as beta carotene (provitamin A), vitamin C, and vitamin E, can donate electrons to free radicals without themselves becoming harmful. Thus, antioxi-dants may prevent the damage caused by the free radicals and may ward off age-related disorders ranging from wrinkles to cancer. Again, the data are mixed and their interpretation is controversial.
One characteristic of aging is an overall decrease in ATP production. This decline is associated with a decrease in oxidative phosphorylation, which has been shown in many cases to be asso-ciated with mitochondrial DNA mutations.
Immune system changes may also be a major contributing factor to aging. The aging immune system loses its ability to respond to outside antigens and begins to be more sensitive to the body’s own antigens. Immune responses to one’s own tissues can result in the degeneration of the tissues and may be responsible for such conditions as arthritic joint disorders, chronic glomerulonephritis, and hyperthyroidism. In addition, T lymphocytes tend to lose their functional capacity with aging and cannot destroy abnormal cells as efficiently. This change may be one reason that certain types of cancer occur more frequently in older people.
One of the greatest disadvantages of aging is the increasing lack of ability to adjust to stress. Homeostasis is far more precari-ous in elderly people, and eventually the body encounters some stressor so great that the body’s ability to recover is surpassed and death results.
Death is usually not attributed to old age. Some other problem, such as heart failure, renal failure, or stroke, is usually listed as the cause of death.
Death was once defined as the loss of heartbeat and respira-tion. In recent years, however, more precise definitions of death have been developed because both the heart and the lungs can be kept working artificially, as occurs during cardiopulmonary resuscitation, and the heart can even be temporarily replaced by an artificial device. Modern definitions of death are based on the per-manent cessation of life functions and the cessation of integrated tissue and organ function. The most widely accepted indication of death in humans is whole brain death, which is manifested clinically by the absence of (1) response to stimulation, (2) natural respira-tion and heart function, and (3) brainstem reflexes, in addition to an electroencephalogram that remains isoelectric (“flat”) for at least 30 minutes.
When determining death, certain conditions also need to be ruled out. For example, some central nervous system poisons can cause a flat electroencephalogram, but the patient can be revived if the effects of the poison are eliminated. Hypothermia slows all chemical reactions, including those involved in degenerative changes that begin at the time of death. As a result, a person suf-fering from hypothermia can exhibit no response to stimulation, exhibit no respiration or heartbeat, and have a flat electroencepha-logram for more than 30 minutes and still be revived.
Neocortical (nē-ō-kōr′ti-kăl) death is a condition in whichmajor portions of the cerebrum are no longer functioning. The patient is comatose and incapable of responding to stimuli. However, heartbeat and respiration still continue because of some relatively unimpaired brainstem functions. Also, because some brainstem function remains, the electroencephalogram is not flat but exhibits some level of activity. Under these conditions, some state laws require that the patient be kept alive by intravenous feeding and by other support equipment. The patient may have previously stated in a living will that, if neocortical death occurs and he or she cannot be returned to a reasonably normal level of function, no artificial sup-port should be applied in an attempt to keep the body alive.
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