Human Physiology, 14th edition (2016)

Respiratory Physiology 543

no gas exchange occurs. Air within the anatomical dead space

has a lower oxygen concentration and a higher carbon dioxide

concentration than the external air. The air in the dead space

enters the alveoli first, so the amount of fresh air reaching the

alveoli with each breath is less than the tidal volume. But because

It should be noted that not all of the inspired volume reaches
the alveoli with each breath. As fresh air is inhaled, it is mixed
with air in the anatomical dead space (see table 16.4 ). This dead
space comprises the conducting zone of the respiratory system—
nose, mouth, larynx, trachea, bronchi, and bronchioles—where

Atmospheric pressure

(760 mmHg)

Intrapulmonary

pressure

(760 mmHg)

Intrapleural

pressure

(756 mmHg)

763 mmHg

757 mmHg

(b)

757 mmHg

754 mmHg

(a)

Diaphragm

(c)

1. At rest 2. Inspiration
   3. Expiration

Figure 16.14 The mechanics of pulmonary

ventilation. Pressures (at sea level) are shown ( a ) before

inspiration, ( b ) during inspiration, and ( c ) during expiration.

During inspiration, the intrapulmonary pressure is lower than

the atmospheric pressure, and during expiration it is greater

than the atmospheric pressure. The intrapleural pressure

is normally always lower than the intrapulmonary pressure,

so that the difference between the two (transpulmonary

pressure) keeps the lungs stuck to the thoracic wall.

Figure 16.15 A spirogram showing lung volumes and capacities. A lung capacity is the sum of two or more lung
volumes. The vital capacity, for example, is the sum of the tidal volume, the inspiratory reserve volume, and the expiratory reserve
volume. Note that residual volume cannot be measured with a spirometer because it is air that cannot be exhaled. Therefore, the total
lung capacity (the sum of the vital capacity and the residual volume) also cannot be measured with a spirometer.

Inspiratory

reserve volume

Expiratory

reserve volume

Residual

volume

Tidal

volume

6,000

5,000

4,000

3,000

2,000

1,000

0

Lung volume

in cubic centimeters (cc)

Total lung capacity

Functional

residual capacity

Vital capacity

Inspiratory

capacity