As the lungs expand, the air molecules inside occupy a
larger volume, which causes the air pressure inside to de-
crease. Because atmospheroic air pressure is now higher
than the pressure inside the lungs, air moves into the lung.
By contrast, when the lung volume decreases, the pressure
inside the lung increases. Air then flows from an area of
higher pressure in the alveoli to the area of lower pressure
in the atmosphere.
The pressure differences during breathing are not
huge. In fact, they are only about 2 mm of mercury (2 mm
Hg), which is equal to 0.26% of atmospheric pressure (760
mm Hg). Yet these small pressure differences are sufficient
to move about 500 mL of air into and out of the lungs with
each breath (Figure 13.4). The greater the pressures cre-
ated, the more air that will be moved.
The pressure in the pleural space is slightly lower
than in the lungs. This pressure difference, along with the
stickiness of the pleural fluid, keeps the lungs “adhered”
to the chest wall so that the lungs and chest wall can
move together. If air is introduced into the pleural space
(as might occur with a puncture wound to the chest), it is
called a pneumothorax. A condition known as
p le u ra l effu -
sion
occurs if fluid (for example, blood, pus, serous fluid)
collects in the pleural space. In either case, the connection
between the thoracic wall and the lung may be disrupted,
and the lung can collapse. Without this connection to the
wall of the chest, the lungs cannot inflate, so ventilation is
not possible.
Respiratory Health Is Sometimes
Tested Using a Spirometer
Like a balloon, your lungs are capable of expanding and
filling with various volumes of air. While at rest, a healthy
adult breathes about 12 times per minute, with each inha-
lation and exhalation moves about 500 mL of air into and
out of the lungs. As illustrated in Figure 13.4a, the tidal
volume is the volume of air that moves in and out during
a normal quiet breath.
Tidal volume varies considerably from one person to
the next and even in the same person at different times.
About 350 mL of the tidal volume, only 70%, actually
reaches the respiratory bronchioles and alveolar sacs to
participate in gas exchange. The other 150 mL (30%) does
not participate in gas exchange because it remains in the
airways of the nose, pharynx, larynx, trachea, bronchi, and
bronchioles. Collectively, these conducting airways are
known as anatomic dead space.
If you consciously take a very deep breath, your lungs
can take in a good deal more air than 500mL. This addition-
al amount of air is referred to as the inspiratory reserve
volume. Similarly, you can expel more air beyond what
exits your body during a normal quiet exhalation; this ad-
ditional volume of air is called the expiratory reserve vol-
ume. Finally, there is a certain volume of air that remains in
your lungs and airways that cannot be expelled because you
cannot contract your airways to total collapse; this remain-
ing volume of air is called the residual volume.
Lung capacities are combinations of specific lung
volumes (the percentage of air that lungs can hold at any
given period of time). The volume of air that is moved
from a maximum inhalation to a maximum exhalation,
for example, is called the vital capacity.
Total lung capacity is the sum of the vital capacity
and residual volume, generally about 6000 mL in males
and 4200 mL in females. Lung volumes and capacities vary
with age, gender, and body size. For example, lung volume
and capacities may be smaller in older people, females,
and shorter people.
Health providers measure all of these volumes, except
residual volume, using a device called a spirometer. The
record produced by a spirometer is called a spirogram
and is shown in Figure 13.4a. The vital capacity measure-
ment is especially important because it can give the doctor
an idea of lung capacity, lung flexibility, and levels of air
flow to the alveolus. For example, people with obstructive
lung disease, which affects the airways, will have normal
or higher than normal lung capacities but lower than nor-
mal airflow. In contrast, people with restrictive pulmonary
disease, which affects the structure of the lungs and their
ability to expand, will have lower than normal lung vol-
umes and capacities.
(For Figure 13.4) If a patient develops pneumothorax:
a. There will be no effect on the ventilation process.
b. Exhalation would be difficult.
c. Abdominal muscles would need to contract to inhale properly.
d. Lung pressures would not be able to decrease to admit air.
e. Vital capacity would increase.
V
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378 CHAPTER 13
The Respiratory System
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