20
UNIT ONE
1.7
LIFE-SPAN CHANGES
Aging is a part of life. It is the process of becoming mature
or old.
Aging
is the passage of time and the accompanying
bodily changes. The passage of time is inevitable; so, too, is
aging, despite common claims for the anti-aging properties
of various diets, cosmetics, pills, and skin-care products.
Aging occurs from the microscopic to the whole-body
level. Although programmed cell death begins in the fetus,
we are usually not very aware of aging until the third
decade of life, when a few gray hairs, faint lines etched into
facial skin, and minor joint stiffness in the morning remind
us that time marches on. A woman over the age of thirty-
five attempting to conceive a child might be shocked to
learn that she is of “advanced maternal age,” because the
chances of conceiving an offspring with an abnormal chro-
mosome number increase with the age of the egg. In both
sexes, by the fourth or F
fth decade, as hair color fades and
skin etches become wrinkles, the F
rst signs of adult-onset
disorders may appear, such as elevated blood pressure
that one day may be considered hypertension, and slightly
high blood glucose that could become type 2 diabetes mel-
litus. A person with a strong family history of heart dis-
ease, coupled with unhealthy diet and exercise habits, may
be advised to change his or her lifestyle, and perhaps even
begin taking a drug to lower serum cholesterol levels. The
sixth decade sees grayer or whiter hair, more and deeper
skin wrinkles, and a waning immunity that makes vaccina-
tions against infl
uenza and other infectious diseases impor-
tant. Yet many, if not most, people in their sixties and older
have sharp minds and are capable of all sorts of physical
activities.
Changes at the tissue, cell, and molecular levels explain
the familiar signs of aging. Decreased production of the
connective tissue proteins collagen and elastin account for
the stiffening of skin, and diminished levels of subcutane-
ous fat are responsible for wrinkling. Proportions of fat to
water in the tissues change, with the percentage of fats
increasing steadily in women, and increasing until about
age sixty in men. These alterations explain why the elderly
metabolize certain drugs at different rates than do younger
people. As a person ages, tissues atrophy, and as a result,
organs shrink.
Cells mark time too, many approaching the end of a
limited number of predetermined cell divisions as their
chromosome tips whittle down. Such cells reaching the end
of their division days may enlarge or die. Some cells may
be unable to build the apparatus that pulls apart replicated
chromosomes in a cell on the verge of division. Impaired
cell division slows wound healing, yet at the same time, the
inappropriate cell division that underlies cancer becomes
more likely. Certain subcellular functions lose efF
ciency,
including repair of DNA damage and transport of sub-
stances into and out of cells. Aging cells are less efF
cient at
extracting energy from nutrients and breaking down aged
or damaged cell parts.
As changes at the tissue level cause organ-level signs
of aging, certain biochemical changes fuel cellular aging.
Lipofuscin and ceroid pigments accumulate as the cell can
no longer prevent formation of damaging oxygen free rad-
icals. A protein called beta amyloid may build up in the
brain, contributing, in some individuals, to the develop-
ment of Alzheimer disease. A generalized metabolic slow-
down results from a dampening of thyroid gland function,
impairing glucose use, the rate of protein synthesis, and
production of digestive enzymes. At the whole-body level,
we notice slowed metabolism as diminished tolerance to
cold, weight gain, and fatigue.
Several investigations are identifying key character-
istics, particularly gene variants, which people who live
more than 100 years share. These fortunate individuals,
called centenarians, fall into three broad groups: about 20
percent of them never get the diseases that kill most peo-
ple; 40 percent get these diseases but at much older ages
than average; and the other 40 percent live with and sur-
vive the more common disorders of aging. Environmental
factors are important, too—another trait centenarians share
is never having smoked.
Our organs and organ systems are interrelated, so
aging-related changes in one influence the functioning
of others. Several chapters in this book conclude with a
“Life-Span Changes” section that discusses changes spe-
ciF
c to particular organ systems. These changes refl
ect the
natural breakdown of structure and function that accom-
panies the passage of time, as well as events in our genes
(“nature”) and symptoms or characteristics that might
arise as a consequence of lifestyle choices and circum-
stances (“nurture”).
PRACTICE
24
Def
ne aging.
25
List some aging-related changes at the microscopic and whole-
body levels.
1.8
ANATOMICAL TERMINOLOGY
To communicate effectively with one another, investiga-
tors over the ages have developed a set of terms with precise
meanings. Some of these terms concern the relative positions
of body parts, others refer to imaginary planes along which
cuts may be made, and still others describe body regions.
When such terms are used, it is assumed that the body is in
the
anatomical position
—standing erect; the face is forward;
and the upper limbs are at the sides, with the palms forward.
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