Development anD aging 349hoW does aging affect body systems?- Virtually every body system undergoes age-related declines
in structure and functioning. - Aging may result from several factors, including an internal
biological clock that ticks out the life spans of cells, and the
accumulation of DNA damage.
taKe-Home messagethe brain waste away (Figure 17.24). AD symptoms include
progressive loss of normal mental functions, including
short-term memory.
Treatments for AD are limited, although drugs can
temporarily help alleviate some symptoms or slow the
progression of the disease. Even so, researchers are making
major strides in understanding the causes of AD. There is
hope that this growing knowledge will lead to strategies
for preventing the disease.
After about age 60 even otherwise healthy people begin
to have “senior moments,” or occasional difficulty with
short-term memory. Older neurons do not conduct action
potentials as efficiently, and neurotransmitters such as
acetylcholine may be released more slowly. Such changes
are why older people tend to move more slowly, have
slower reflexes, and have more problems with muscle
coordination.
Our sensory organs also become less efficient at detect-
ing or responding to stimuli. The taste buds become less
sensitive over time, and as Chapter 14 noted, people also
tend to become farsighted as they grow older because the
eye lens loses elasticity and is altered in other ways that
prevent it from properly focusing incoming light. Most
people over 60 also have some hearing loss due to “worn
out” sensory nerve cells in the ear.disease (which may cause an enlarged heart), the heart
muscle shrinks slightly and so its strength and blood-
pumping ability decline. The waning blood supply may
be a factor in aging-related changes throughout the body.
Blood transport is also affected by changes in aging blood
vessels. Elastin fibers in blood vessel walls are replaced
with connective tissue containing collagen or become hard-
ened with calcium deposits, and so vessels stiffen. Choles-
terol plaques often cause further narrowing of arteries and
veins (Section 7.8). This is why resting blood pressure may
rise as people get older. However, as with the muscular
and skeletal systems, lifestyle choices such as not smoking,
eating a healthy diet, and getting regular exercise can help
each of us maintain a vigorous respiratory and cardiovas-
cular system well past middle age.
In the immune system, the total number of T cells falls
and B cells become less active. Older people also are more
likely to develop autoimmune diseases. It is possible that
faltering DNA repair mechanisms
no longer fix genetic changes that
alter self-markers. This could pro-
voke immune responses against
the body’s own cells.
In the aging GI tract, mucous
glands in the stomach and intes-
tines gradually break down, and
the pancreas secretes fewer diges-
tive enzymes. Although it is vital
for older people to eat a healthy
diet, we require fewer calories as we age. By age 50, your
basal metabolic rate will be only 80 to 85 percent of what it
was in childhood and will keep declining about 3 percent
every decade. This is why middle-aged people tend to gain
weight unless they compensate by eating less, increasing
their physical activity, or both.
Levels of most hormones stay steady throughout life.
Sex hormones are exceptions, however. As Chapter 16
noted, falling levels of estrogens and progesterone trigger
menopause in women. In older men, falling levels of tes-
tosterone reduce fertility. That said, men have fathered
children into their eighties. Men and women both retain
their capacity for sexual response well into old age.
Aging also alters the brain and senses
Brain neurons die throughout life, so the brain shrinks
slightly over time. It loses about 10 percent of its mass after
80 years. Brain neurons also are damaged by the cumula-
tive effects of free radicals. In addition, in most people
who live to old age, tangled clumps of a protein called tau
develop in the cell bodies of many brain neurons. These
neurofibrillary tangles can disrupt normal cell operations.
Clotlike plaques containing misfolded proteins called beta
amyloid also develop between neurons.
In people who develop Alzheimer’s disease (AD), the
brain becomes riddled with masses of neuro fibrillary tan-
gles and beta amyloid plaques. Over time, large portions of
Figure 17.24 The brain of an Alzheimer’s patient shrinks as
the disease progresses. Dark areas in this MRI scan are regions
where the patient’s brain tissue has wasted away.Zephyr/Science Source© Monkey Business Images/Shutterstock.comCopyright 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).