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Peak muscle strength

Peak of male sexuality

Sexual maturity

Thymus begins to shrink

Brain cells begin to die


Although many biological changes ensue as we grow older, photographs of actress Katharine Hepburn at various stages of her life indicate that we can age with great grace and beauty.


Time Period

Major Events

Neonatal period

Birth to end of fourth week

Newborn begins to carry on respiration, obtain nutrients, digest nutrients, excrete wastes, regulate body temperature, and make circulatory adjustments


End of fourth week to one year

Growth rate is high; teeth begin to erupt; muscular and nervous systems mature so that coordinated activities are possible; communication begins


One year to puberty

Growth rate is high; deciduous teeth erupt and are replaced by permanent teeth; high degree of muscular control is achieved; bladder and bowel controls are established; intellectual abilities mature


Puberty to adulthood

Person becomes reproductively functional and emotionally more mature; growth spurts occur in skeletal and muscular systems; high levels of motor skills are developed; intellectual abilities increase


Adolescence to old age

Person remains relatively unchanged anatomically and physiologically; degenerative changes begin to occur


Old age to death

Degenerative changes continue; body becomes less and less able to cope with the demands placed upon it; death usually results from mechanical disturbances in the cardiovascular system or from disease processes that affect vital organs

Organ System

Aging-Related Changes

Organ System

Aging-Related Changes

Integumentary system

Skeletal system

Muscular system Nervous system

Endocrine system Cardiovascular system Lymphatic system Digestive system Respiratory system Urinary system

Reproductive systems Male Female

Degenerative loss of collagenous and elastic fibers in dermis; decreased production of pigment in hair follicles; reduced activity of sweat and sebaceous glands Skin thins, wrinkles, and dries out; hair turns gray and then white Degenerative loss of bone matrix

Bones become thinner, less dense, and more likely to fracture; stature may shorten due to compression of intervertebral disks and vertebrae Loss of skeletal muscle fibers; degenerative changes in neuromuscular junctions Loss of muscular strength


Decreased metabolic rate; reduced ability to cope with stress; reduced ability to maintain homeostasis Degenerative changes in cardiac muscle; decrease in lumen diameters of arteries and arterioles Decreased cardiac output; increased resistance to blood flow; increased blood pressure Decrease in efficiency of immune system


Degenerative loss of elastic fibers in lungs; reduced number of alveoli

Reduced vital capacity; increase in dead air space; reduced ability to clear airways by coughing Degenerative changes in kidneys; reduction in number of functional nephrons Reductions in filtration rate, tubular secretion, and reabsorption

Reduced secretion of sex hormones; enlargement of prostate gland; decrease in sexual energy Degenerative changes in ovaries; decrease in secretion of sex hormones Menopause; regression of secondary sex characteristics

U How does the body change during adolescence?

^9 Define adulthood.

^9 What changes occur during adulthood?

Q What changes accompany senescence?

Recent public interest in physician-assisted suicide has focused attention on why severely ill people seek to end their lives. While the courts argue the legality of assisted suicide, the medical community is recognizing shortcomings in the treatment of the dying. Curricula for medical students and medical residents are being revamped to increase emphasis on providing palliative care for the terminally ill. Such care seeks to make a patient comfortable, even if the treatment does not cure the disease or extend life.

From 65 to 80% of all deaths in the United States take place in hospitals, often with painful and sometimes unwanted interventions to prolong life. One study found that about half of all conscious patients suffer severe pain prior to death. In Oregon, which has pioneered education on caring for the dying patient, a greater percentage of patients live out their last days at home, in nursing homes, or in hospices, which are facilities dedicated to providing comfort and support for the dying.


The aging process is difficult to analyze because of the intricate interactions of the body's organ systems. Breakdown of one structure ultimately affects the functioning of others. The medical field of gerontology examines the biological changes of aging at the molecular, cellular, organismal, and population levels. Aging is both passive and active.

Passive Aging

Aging as a passive process entails breakdown of structures and slowing of functions. At the molecular level, passive aging is seen in the degeneration of the elastin and collagen proteins of connective tissues, causing skin to sag and muscle to lose its firmness.

During a long lifetime, biochemical abnormalities accumulate. Mistakes occur throughout life when DNA replicates in dividing cells. Usually, repair enzymes correct this damage immediately. But over many years, exposure to chemicals, viruses, and radiation disrupts DNA repair mechanisms so that the error burden becomes too great to be fixed. The cell may die as a result of faulty genetic instructions.

Another sign of passive aging at the biochemical level is the breakdown of lipids. As aging membranes leak during lipid degeneration, a fatty, brown pigment called lipofuscin accumulates. Mitochondria also begin to break down in older cells, decreasing the supply of chemical energy to power the cell's functions.

The cellular degradation associated with aging may be set into action by highly reactive chemicals called free radicals. A molecule that is a free radical has an unpaired electron in its outermost valence shell. This causes the molecule to grab electrons from other molecules, destabilizing them, and a chain reaction of chemical instability begins that could kill the cell. Free radicals are a by-product of normal metabolism and also form by exposure to radiation or toxic chemicals. Enzymes that usually inactivate free radicals diminish in number and activity in the later years. One such enzyme is superoxide dismutase (SOD).

Active Aging

Aging also entails new activities or the appearance of new substances. Lipofuscin granules, for example, may be considered an active sign of aging, but they result from the passive breakdown of lipids. Another example of active aging is autoimmunity, in which the immune system turns against the body, attacking its cells as if they were invading organisms.

Active aging actually begins before birth, as certain cells die as part of the developmental program encoded in the genes. This process of programmed cell death, called apoptosis (ap"o-to'sis), occurs regularly in the embryo, degrading certain structures to pave the way for new ones. The number of neurons in the fetal brain, for example, is halved as those that make certain synaptic connections are spared from death. In the fetal thymus, T cells that do not recognize "self" cell surfaces die, thereby building the immune system. Throughout life, apoptosis enables organs to maintain their characteristic shapes.

Mitosis and apoptosis are opposite, but complementary, processes. That is, as organs grow, the number of cells in some regions increases, but in others it decreases. Cell death, then, is not a phenomenon that is restricted to the aged. It is a normal part of life. Clinical Application 23.4 discusses genetic disorders that greatly accelerate aging.

The Human Life Span

In the age-old quest for longer life, people have sampled everything from turtle soup to owl meat to human blood. A Russian-French microbiologist, Ilya Mechnikov, believed that a life span of 150 years could be achieved with the help of a steady diet of milk cultured with bacteria. He thought that the bacteria would live in the large intestine and somehow increase the human life span. (He died at 71.) Ironically, many people have died in pursuit of a literal "fountain of youth."

The human life span—the length of time that a human can theoretically live—is 120 years. Of course, m

_ Application

Old Before Their Time


In Hutchinson-Gilford syndrome, which affects the Luciano brothers that appear in figure 23D, a child looks normal at birth. Within just a few years, however, the child acquires a shocking appearance of wrinkles, baldness, and the prominent facial features characteristic of advanced age. Arteries clog with fatty deposits. The child usually dies of a heart attack or a stroke by the age of twelve, although some patients live into their twenties. Only a few dozen cases of this syndrome have ever been reported.

An adult form of progeria called Werner syndrome becomes apparent before the twentieth birthday. The person usually dies before age fifty of disorders usually associated with advanced age, such as type II diabetes mellitus, atherosclerosis, and osteoporosis. Curiously, they usually do not develop hypertension or Alzheimer disease.

The cells of progeria patients show profound aging-related changes. Normal human cells in cul-

ture divide only fifty or so times, but cells from progeria patients divide only ten to thirty times, and die prematurely. Certain structures seen in normal cultured cells as they near the fifty-division limit (glycogen particles, lipofuscin granules, many lysosomes and vacuoles, and a few ribosomes) appear early in the cells of people with progeria. Understanding the mechanisms that cause these diseased cells to race through the aging process may help us to understand the biology of normal aging. ■

Figure 23D


Figure 23D


most people succumb to disease or injury long before that point. Life expectancy is a more realistic projection of how long an individual will live, based on epidemio-logical information. In the United States, life expectancy is 76.5 years. Yet in some countries life expectancy is only 52 years, and in African nations being decimated by the AIDS epidemic, it is dropping precipitously.

Life expectancy approaches life span as technology conquers diseases. Technology also alters the most prevalent killers. Development of antibiotic drugs removed some infectious diseases such as pneumonia and tuberculosis from the top of the "leading causes of death" list, a position that heart disease filled. Cancer is currently approaching heart disease as the most common cause of death in developed nations. Infections remain a major cause of death in less-developed countries. Table 23.6 lists the top causes of death in the U.S., and Table 23.7 indicates how age is a factor in the nature of the most common causes of death.

Medical advances have greatly contributed to improved life expectancy. Antibiotics have tamed some once-lethal infections, drugs enable many people with cancer to survive, and such advances as beta-blocking drugs and coronary bypass surgery have extended the lives of people with heart disease. However, a look at the history of two infectious diseases, pneumonia and tuberculosis, sounds a warning against complacency. The top two killers in 1900, pneumonia and tuberculosis, did not make the top five causes of death in the 1986 list. However, they reappear as numbers 4 and 5 in the 1993 list,

I| Leading Causes of


Percent of Total


Percent of Total

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