ing at 300 million by age eight years. The number remains
constant throughout life, but the alveoli expand. Alveolar
walls thin and may coalesce, and the depth of alveoli begins
to diminish by age forty, decreasing the surface area avail-
able for gas exchange—about three square feet per year. In
addition, an increase in the proportion of collagen to elastin
and a tendency of the collagen to cross-link impair the ability
of alveoli to expand fully. Oxygen transport from the alveoli
to the blood, as well as oxygen loading onto hemoglobin in
red blood cells, becomes less efF cient. Diffusion of CO
of the blood and through the alveolar walls slows too.
As with other organ systems, the respiratory system
undergoes specific changes, but these may be unnotice-
able at the whole-body level. A person who is sedentary or
engages only in light activity would probably not be aware
of the slowing of air ﬂ
ow in and out of the respiratory sys-
tem. Unaccustomed exercise, however, would quickly reveal
cult breathing has become with age.
How does the environment inf
uence the eF
ects o± aging on the
Which aging-related changes raise the risk o± respiratory in±ection?
How do alveoli change with age?
from the lungs and respiratory passages, which increases
susceptibility to and severity of respiratory infections.
Several changes contribute to an overall increase in
effort required to breathe that accompanies aging. Cartilage
between the sternum and ribs calcifies and stiffens, and
skeletal shifts change the shape of the thoracic cavity into a
“barrel chest” as posture too changes with age. In the bron-
chioles, fibrous connective tissue replaces some smooth
muscle, decreasing contractility. As muscles lose strength,
breathing comes to depend more upon the diaphragm. The
vital capacity, which reaches a maximum by age forty, may
drop by a third by the age of seventy years.
Keeping fresh air in the lungs becomes more difF cult
with age. As the farthest reaches of the bronchiole walls
thin, perhaps in response to years of gravity, they do not stay
as open as they once did, trapping residual air in the lower
portions of the lungs. Widening of the bronchi and alveolar
ducts increases dead space. The lungs can still handle the
same volume of air, but a greater proportion of that air is
“stale,” reﬂ ecting lessened ability to move air in and out.
The maximum minute ventilation drops by 50% from age
twenty to age eighty.
Aging-associated changes occur at the microscopic level
too. The number of alveoli is about 24 million at birth, peak-
d. Mucous membrane F
lters, warms, and moistens
e. Particles trapped in the mucus are carried to the
pharynx by ciliary action and are swallowed.
a. Sinuses are spaces in the bones of the skull that
open into the nasal cavity.
b. They are lined with mucous membrane that is
continuous with the lining of the nasal cavity.
a. The pharynx is posterior to the mouth, between
the nasal cavity and the larynx.
b. It provides a common passage for air and food.
c. It aids in creating vocal sounds.
a. The larynx is an enlargement at the top of the
b. It is a passageway for air and helps prevent foreign
objects from entering the trachea.
c. It is composed of muscles and cartilages; some
of these cartilages are single, whereas others are
d. It contains the vocal cords, which produce sounds
by vibrating as air passes over them.
(1) The pitch of a sound is related to the tension
on the cords.
(2) The intensity of a sound is related to the force
of the air passing over the cords.
e. The glottis and epiglottis help prevent food and
liquid from entering the trachea.
INTRODUCTION (PAGE 736)
The respiratory system includes the passages that
transport air to and from the lungs and the air sacs in
which gas exchanges occur. Respiration is the entire
process by which gases are exchanged between the
atmosphere and the body cells.
WHY WE BREATHE (PAGE 736)
Respiration is necessary because of cellular respiration. Cells
require oxygen to extract maximal energy from nutrient
molecules and release carbon dioxide, a metabolic waste.
ORGANS OF THE RESPIRATORY SYSTEM
The respiratory system includes the nose, nasal cavity,
sinuses, pharynx, larynx, trachea, bronchial tree, and
lungs. The upper respiratory tract includes the nose, nasal
cavity, sinuses, and pharynx; the lower respiratory tract
includes the larynx, trachea, bronchial tree, and lungs.
a. Bone and cartilage support the nose.
b. Nostrils provide entrances for air.
2. Nasal cavity
a. The nasal cavity is a space posterior to the nose.
b. The nasal septum divides it medially.
c. Nasal conchae divide the cavity into passageways
and help increase the surface area of the mucous