Biomedical Engineering Reference
In-Depth Information
FIGURE 14.6
The close spatial relationship between alveoli and pulmonary capillaries.
The resulting single breath gas exchange between the alveoli and the pulmonary capil-
laries can be simplified via a mass balance. This is summarized in Table 14.1. The gases
(nitrogen, oxygen, and carbon dioxide) are shown with “wet” percentages, which incorpo-
rate the water vapor in the air. Note that not all of the oxygen in the air is taken up by the
lungs and that the expired gases from the lungs are heated to body temperature and
saturated with water vapor—thus the higher water vapor content of the expired air. There
is a trace amount of carbon dioxide in the ambient air but a higher percentage in the expired
air. Thus, oxygen replaces carbon dioxide in the pulmonary bloodstream.
For the purposes of gas exchange, the components of the inspired air are described by
means of their partial pressures. This is the fractional amount of total gas pressure due to
the substance being measured. For example, at sea level, the total atmospheric pressure
is 760 mm Hg. The amount of this pressure that is due to oxygen is approximately
0.21
160 mm Hg. We would say that the partial pressure of oxygen at sea level
in dry air (no vapor in the air) is 160 mm Hg. The partial pressure of carbon dioxide in
dry air at sea level is 0.03
760
¼
22.8 mm Hg. The maximum amount of water vapor in
the air varies with temperature and relative humidity. At body temperature (37
C) air
can be saturated up to 47 mm Hg of water vapor pressure. Therefore, in the lung, where
air is totally water-saturated, the partial pressure of water vapor would be 47 mm Hg.
760
¼
TABLE 14.1
Single Breath Analysis for the Components of Air
Air Component
Atmospheric Air %
Expired Air %
Nitrogen and Inert Gases
78.62
74.9
Oxygen
20.85
15.3
Carbon Dioxide
0.03
3.6
Water Vapor
0.5
6.2
Total
100
100
