HUMAN BIOLOGY

182 ChapteR 10

10.4 Breathing: air in, air out

Figure 10.9 Animated! The volume of the chest cavity

increases, then decreases during a respiratory cycle. The

X-ray image in a shows how taking a deep breath changes

the volume of the chest (thoracic) cavity. Part B shows how

the volume shrinks after exhalation.

Pr. M. Brauner/Science Source

© iStockphoto.com/Purdue9394

n You will take about 500 million breaths by age 75—and

probably more because young children breathe faster than

adults do.

n Links to the Axial skeleton 5.3, Skeletal muscles 6.2

a Inhalation. The thoracic cavity and lungs expand as the

diaphragm contracts and moves downward, while the

external intercostal muscles contract, lifting the

rib cage.

B Exhalation. The thoracic cavity and lungs shrink as

the diaphragm relaxes and moves upward and the

external intercoastal muscles relax.

Outward

flow of air

© Cengage Learning

Pr. M. Brauner/Science Source

Inward

flow of air

© Cengage Learning

When you breathe, air pressure

gradients reverse in a cycle

Breathing ventilates the lungs in a continuous in/out

pattern called a respiratory cycle. Ventilation has two

phases. First, inspiration—or inhalation—draws a breath

of air into the airways. Then, in the phase of expiration, or

exhalation, a breath moves out.

In each respiratory cycle, the volume of the chest cav-

ity increases, then decreases (Figure 10.9). At the same

time, pressure gradients between the lungs and the air

outside the body are reversed. To understand how this shift

affects breathing, it helps to remember that air in your air-

ways (oxygen, carbon dioxide, and the other atmospheric

gases) is at the same pressure as the outside atmosphere.

Before you inhale, the pressure inside all your alveoli

(called intrapulmonary pressure) is also the same as that of

outside air.

the basic respiratory cycle As you start to inhale, the

diaphragm contracts and flattens, and external intercostal

muscle movements lift the rib cage up and out (Figure

10.9A). As the chest cavity expands, the lungs expand too.

At that time, the air pressure in alveoli is lower than the

atmospheric pressure. Fresh air follows this gradient and

flows down the airways, then into the alveoli. If you take a

deep breath, the volume of the chest cavity increases even

more because contracting neck muscles raise the sternum

and the first two ribs.

During normal, quiet breathing, expiration is passive.

The muscles that contracted to bring about inspiration sim-

ply relax and the lungs recoil, like a stretched rubber band.

As the lung volume shrinks, the air in the alveoli is com-

pressed. Because pressure in the sacs now is greater than

the outside atmo spheric pressure, air follows the gradient

and moves out of the lungs (Figure 10.9B).

If your lungs must rapidly expel more air—for instance,

when you huff and puff while working out—expiration

becomes active. Muscles in the wall of the abdomen con-

tract, pushing your diaphragm upward, and other muscle

movements reduce the volume of the chest cavity even

more. Add to these changes the natural recoil of the lungs,

and a great deal of air in the lungs is pushed outward.

another pressure gradient aids the process

A negative pressure gradient outside the lungs contributes

to the respiratory cycle. Atmospheric pressure is a little

bit higher than the pressure in the pleural sac that wraps

around the lungs. The pressure difference is enough to

make the lungs stretch and fill the expanded chest cavity.

It keeps the lungs snug against the chest wall even when

air is being exhaled, when the lung volume is much smaller

than the space inside the chest cavity. As a result, when the

chest cavity expands with the next breath, so do the lungs.

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