Porth's Essentials of Pathophysiology, 4e

526

Respiratory Function

U N I T 6

6000

5000

Inspiratory reserve volume 3100 mL

Inspiratory capacity 3600 mL

FIGURE 21-15. Spirometry recording of respiratory volumes (left) and diagram of lung capacities (right).The tidal volume (TV, yellow) is the volume of air inhaled and exhaled during normal breathing; the inspiratory reserve volume (IRV, pink), the maximal volume of air that can be forcefully inhaled in excess of theTV; the expiratory reserve volume (ERV, blue), the maximal volume of air that can be exhaled in excess of theTV; and the residual volume (RV, green), the air that continues to remain in the lung after maximal respiratory effort.The functional residual capacity (FRC) is the sum of the ERV and the RV.The vital capacity is the IRV,TV, and ERV.

4000

Total lung capacity 6000 mL

3000

Tidal volume 500 mL

Functional residual capacity 2400 mL Vital capacity 4800 mL

Expiratory reserve volume 1200 mL

2000

1000

Residual volume 1200 mL

0

mL

Lung volumes

Lung capacities

Many of these volumes can be measured using an instrument called a spirometer . The person is asked first to breathe normally into the spirometer, during which the tidal volume is measured, and then to inhale and exhale maximally, during which the IRV and ERV are measured. The RV cannot be measured with the spirom- eter because this air cannot be expressed from the lungs. It is measured by indirect methods, such as the helium dilution methods, the nitrogen washout methods, or body plethysmography. Lung capacities include two or more lung volumes. The vital capacity (VC) equals the IRV plus the TV and the ERV and is the amount of air that can be exhaled from the point of maximal inspiration. The inspira- tory capacity (IC) equals the TV plus the IRV. It is the amount of air a person can breathe in beginning at the normal expiratory level. The functional residual capac- ity (FRC) is the sum of the RV and ERV; it is the volume of air that remains in the lungs at the end of normal expiration. The total lung capacity (TLC) is the sum of all the volumes in the lungs. Lung volumes and capaci- ties are summarized in Table 21-1.

The previously described lung volumes and capacities are anatomic and static measurements, determined by spirometry and recorded without relation to time. The spirometer also is used to measure dynamic lung volumes (i.e., ventilation with respect to time); these tests often are used in assessing pulmonary function (Table 21-2). The maximum voluntary ventilation measures the volume of air that a person can move into and out of the lungs dur- ing maximum effort lasting for a specific period of time. This measurement usually is converted to liters per min- ute. Two other useful tests are the forced vital capacity and the forced expiratory volume. The forced vital capac- ity (FVC) involves full inspiration to total lung capacity followed by forceful maximal expiration. Obstruction of airways produces an FVC that is lower than that observed with more slowly performed vital capacity measurements. The forced expiratory volume (FEV) is the expiratory volume achieved in a given time period. The FEV 1.0 is the forced expiratory volume that can be exhaled in 1 second. The FEV 1.0 frequently is expressed as a percentage of the FVC. The FEV 1.0 and FVC are used in the diagnosis of obstructive lung disorders. Amount of air that moves into and out of the lungs with each breath Maximum amount of air that can be inhaled from the point of maximal expiration Maximum volume of air that can be exhaled from the resting end- expiratory level Volume of air remaining in the lungs after maximal expiration.This volume cannot be measured with the spirometer; it is measured indirectly using methods such as the helium dilution method, the nitrogen washout technique, or body plethysmography. Volume of air remaining in the lungs at end-expiration (sum of RV and ERV) Maximal amount of air that can be forcibly exhaled from the point of maximal inspiration Total amount of air that the lungs can hold; it is the sum of all the volume components after maximal inspiration.This value is about 20% to 25% less in females than in males. Sum of IRV andTV Measurement

TABLE 21-1 LungVolumes and Capacities Volume Symbol

Tidal volume (about 500 mL at rest)

TV

Inspiratory reserve volume (about 3000 mL) Expiratory reserve volume (about 1100 mL)

IRV

ERV

Residual volume (about 1200 mL)

RV

Functional residual capacity (about 2300 mL)

FRC

Inspiratory capacity (about 3500 mL)

IC

Vital capacity (about 4600 mL)

VC

Total lung capacity (about 5800 mL)

TLC