Anatomy and Physiology - Introduction to Biomedical Engineering - page 107

Thus,

p i

p ¼ V i V

where is the volume fraction of a constituent in air and is therefore dimensionless.

Table 3.2 provides individual gas constants, as well as volume fractions of constituent gases
of air.
Lung mechanics refers to the study of the mechanical properties of the lung and chest
wall, whereas lung statics refers to the mechanical properties of a lung in which the volume
is held constant over time. Understanding lung mechanics requires knowledge about the
volumes within the lungs. Lung capacities contain two or more volumes. The tidal volume
(TV) is the amount of air that moves in and out of the lungs during normal breathing
(Figure 3.25). The total lung capacity (TLC) is the amount of gas contained within the lungs
at the end of a maximum inspiration. The vital capacity (VC) is the maximum amount of
air that can be exhaled from the lungs after inspiration to TLC. The residual volume (RV)
is the amount of gas remaining in the lungs after maximum exhalation. The amount of
gas that can be inhaled after inhaling during tidal breathing is called the inspiratory reserve
volume (IRV). The amount of gas that can be expelled by a maximal exhalation after exhal-
ing during tidal breathing is called the expiratory reserve volume (ERV). The inspiratory
capacity (IC) is the maximum amount of gas that can be inspired after a normal exhalation
during tidal breathing, and the functional residual capacity (FRC) is the amount of gas that
remains in the lungs at this time (Table 3.3).
All of the volumes and capacities except those that include the residual volume can
be measured with a spirometer. The classic spirometer is an air-filled container that is
constructed from two drums of different sizes. One drum contains water, and the other
air-filled drum is inverted over an air-filled tube and floats in the water. The tube is
connected to a mouthpiece used by the patient. When the patient inhales, the level of the

V i / V

TABLE 3.2 Molecular Masses, Gas Constants, and Volume Fractions for Air and Constituents

Volume Fraction in Air m 3 /m 3 Constituent Molecular Mass kg/mol Gas Constant N m/(mol K)

Air 29.0 286.7 1.0000

Ammonia 17.0 489.1 0.0000

Argon 39.9 208.4 0.0093

Carbon Dioxide 44.0 189.0 0.0003

Carbon Monoxide 28.0 296.9 0.0000

Helium 4.0 2,078.6 0.0000

Hydrogen 2.0 4,157.2 0.0000

Nitrogen 28.0 296.9 0.7808

Oxygen 32.0 259.8 0.2095

Note: Universal gas constant is 8314.43 N m/kg K.