348 Chapter 17time’s toll: everybody ages
Cumulative damage to Dna
may also play a role in aging
A “cumulative assaults” hypothesis proposes that aging
results from mounting damage to DNA combined with
a decline in DNA’s mechanisms of self-repair. Chapter 2
described how free radicals can damage DNA and other
biological molecules. If changes in DNA aren’t fixed, they
may prevent cells from making needed enzymes and other
proteins required for normal cell operations.
Ultimately, it’s quite possible that aging involves pro-
cesses in which genes, free radical damage, a decline in
DNA repair mechanisms, and even other factors all come
into play.Visible changes occur in skin, muscles,
and the skeleton
Aging is a gradual loss of vitality as cells, tissues, and
organs function less and less efficiently. Starting at about
age 40, all of us begin to see and feel the effects of
such changes.
Changes in structural proteins may contribute to many
of the more obvious aging-related characteristics. Remem-
ber from Chapter 4 that many connective tissues contain
large amounts of the protein collagen, and some also
contain the flexible protein elastin. As we age, chemical
changes make collagen molecules more rigid and reduce
the amount of elastin in many tissues. For example, as
the elastin fibers that give skin its flexibility are slowly
replaced with more rigid collagen, the skin thins, sags, and
wrinkles. It also becomes drier as sweat and oil glands
begin to break down and are not replaced.
As hair folli cles die or become less active, there is a gen-
eral loss of body hair. And as pigment- producing cells die
and are not replaced, the remaining body hair begins to
appear gray or white.
In general, aging muscles lose mass and strength. The
lost muscle tends to be replaced by fat and, with time, by
collagen. Bone cells become less efficient at taking up cal-
cium and generating new bone tissue, so the risk of osteo-
porosis rises. Osteoarthritis also is more common in older
people. With the passing years, intervertebral disks gradu-
ally deteriorate, reducing the distance between vertebrae.
This is why people tend to get shorter by about a centimeter
(half an inch) every 10 years from middle age onward. Stay-
ing physically active can help slow most of these changes.Most other organ systems also decline
The heart, lungs, and kidneys also function less well with
increasing age (Table 17.3). In the lungs, alveoli break
down, so there is less respiratory surface available for gas
exchange. In a person who does not have cardiovascularn Time takes a toll on body tissues and organs. To some
extent, our genes determine how long each of us will live.Genes may determine the maximum life span
Each species has a maximum life span. For example,
we know the maximum is about 20 years for dogs and
12 weeks for butterflies. So far as we can document, no
human has lived beyond 122 years. The consistency of life
span within species is a sign that genes help govern aging.
One idea is that each type of cell, tissue, and organ is
like a clock that ticks at its own genetically set pace. When
researchers investigated this possibility, they grew normal
human embryonic cells, all of which divided about fifty
times, then died. In the body, most cells divide eighty or
ninety times, at most. As discussed in Chapter 18, a cell
copies its chromosomes before it divides. The ends of chro-
mosomes are capped by numerous segments of DNA called
telomeres. A bit of each telomere is lost each time a cell
divides. The cell dies when only a nub remains.
Cancer cells, and cells in gonads that give rise to sperm
and oocytes, make an enzyme that causes telomeres to
lengthen. Apparently, that is why such cells can divide over
and over, without dying.1 7. 1 3
Jose Carillo/Photo EditBenchmark Lung Muscle Kidney
Age Heart Rate Capacity Strength Efficiency
25 100% 100% 100% 100%
45 94% 82% 90% 88%
65 87% 62% 75% 78%
85 81% 50% 55% 69%Table 17.3 Some Physiological Changes in AgingNote: Age 25 is the benchmark for maximal efficiency of physiological
functions.Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).