Lung Volumes and Capacities

An example of a spirogram is shown in figure 16.16, and the various lung volumes and capacities are defined in table 16.3. A lung capacity is equal to the sum of two or more lung volumes. During quiet breathing, for example, the amount of air expired in each breath is the tidal volume. The maximum amount of air that can be forcefully exhaled after a maximum inhalation is called the vital capacity, which is equal to the sum of the inspiratory reserve volume, tidal volume, and expiratory reserve

■ Figure 16.15 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.

Showing Volume And Capacity

■ Figure 16.16 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.

Table 16.3 Terms Used to Describe Lung Volumes and Capacities

Term

Definition

Lung Volumes

The four nonoverlapping components of the total lung capacity

Tidal volume

The volume of gas inspired or expired in an unforced respiratory cycle

Inspiratory reserve volume

The maximum volume of gas that can be inspired during forced breathing in addition to tidal volume

Expiratory reserve volume

The maximum volume of gas that can be expired during forced breathing in addition to tidal volume

Residual volume

The volume of gas remaining in the lungs after a maximum expiration

Lung Capacities

Measurements that are the sum of two or more lung volumes

Total lung capacity

The total amount of gas in the lungs after a maximum inspiration

Vital capacity

The maximum amount of gas that can be expired after a maximum inspiration

Inspiratory capacity

The maximum amount of gas that can be inspired after a normal tidal expiration

Functional residual capacity

The amount of gas remaining in the lungs after a normal tidal expiration

Table 16.4 Ventilation Terminology

Term

Definition

Air spaces

Alveolar ducts, alveolar sacs, and alveoli

Airways

Structures that conduct air from the mouth and nose to the respiratory bronchioles

Alveolar ventilation

Removal and replacement of gas in alveoli; equal to the tidal volume minus the volume of dead space times the breathing rate

Anatomical dead space

Volume of the conducting airways to the zone where gas exchange occurs

Apnea

Cessation of breathing

Dyspnea

Unpleasant, subjective feeling of difficult or labored breathing

Eupnea

Normal, comfortable breathing at rest

Hyperventilation

Alveolar ventilation that is excessive in relation to metabolic rate; results in abnormally low alveolar CO2

Hypoventilation

Alveolar ventilation that is low in relation to metabolic rate; results in abnormally high alveolar CO2

Physiological dead space

Combination of anatomical dead space and underventilated or underperfused alveoli that do not contribute normally to blood gas exchange

Pneumothorax

Presence of gas in the intrapleural space (the space between the visceral and parietal pleurae) causing lung collapse

Torr

Unit of pressure nearly equal to the millimeter of mercury (760 mmHg = 760 torr)

volume (fig. 16.16). Multiplying the tidal volume at rest by the number of breaths per minute yields a total minute volume of about 6 L per minute. During exercise, the tidal volume and the number of breaths per minute increase to produce a total minute volume as high as 100 to 200 L per minute.

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 (table 16.4). This dead space comprises the conducting zone of the respiratory system— nose, mouth, larynx, trachea, bronchi, and bronchioles—where 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. Since the air in the dead space enters the alveoli first, the amount of fresh air reaching the alveoli with each breath is less than the tidal volume. But, since the volume of air in the dead space is an anatomical constant, the percentage of fresh air entering the alveoli is increased with increasing tidal volumes. For example, if the anatomical dead space is 150 ml and the tidal volume is 500 ml, the percentage of fresh air reaching the alveoli is 350/500 x 100% = 70%. If the tidal volume is increased to 2,000 ml, and the anatomical dead space is still 150 ml, the percentage of fresh air reaching the alveoli is increased to 1,850/2,000 x 100% = 93%. An increase in tidal volume can thus be a factor in the respiratory adaptations to exercise and high altitude.

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