Biomedical Engineering Reference
In-Depth Information
Thus, in terms of partial pressures, and with the ambient humidity equal to a partial
pressure of 10 mm Hg (as an example), Table 14.1 can be rewritten as follows: The partial
pressure of water vapor is 10 mm Hg, and the partial volume would be equal to
(10/760)
6.5 ml. The dry gases (nitrogen and oxygen) represent
the remainder of the tidal volume, or 500 - 6.5
500 ml tidal volume
¼
¼
493.5. Thus, the nitrogen partial
volume would be 493.5
79%
¼
389.9 ml, and the partial volume of oxygen would be
493.5
103.6 ml.
The nitrogen is not exchanged in the lungs, and thus its partial volume does not change.
However, the oxygen is taken up by the bloodstream, and the carbon dioxide is released by
the bloodstream to be exhaled from the lungs. The partial volumes of the exhaled gases are
based on the metabolic load of these gases in the body. At rest, these are 284 ml/min of oxy-
gen (at body temperature) and 226 ml/min for carbon dioxide. These are values at body
temperature and pressure (BTP). The STP values are 250 ml/min for oxygen and 200 ml/min
for carbon dioxide.
The BTP values are calculated by multiplying the STP values by the ration of the tem-
peratures in absolute scale in
K. The STP temperature is 273
K, while the BTP temperature
is 273
21%
¼
310
K. Thus, the BTP value for oxygen is 250
þ
37
¼
(310/273)
¼
284, and the BTP
value for carbon dioxide is 200
226.
The BTP values equal 23.67 ml/breath for oxygen (284/12 breaths per minute) and
18.83 ml/breath for carbon dioxide (226/12). The water vapor during exhalation equals
the partial pressure of water vapor in the body (47 mm Hg) compared to the dry gas pres-
sure (760 - 47
(310/273)
¼
713 mm Hg) times the dry gas expired volume. Thus, the water vapor
attaches to the dry gas during expiration.
Note that in the calculations shown in Table 14.2, the expired dry gas volume is the sum
of nitrogen, oxygen, and carbon dioxide. The water vapor attaches to the dry gas. Also note
that the dry gas pressure is only 760 - 47
¼
713 mm Hg, since the water vapor accounts for
the remaining 47 mm Hg. Thus, the ratio of the partial pressures of water vapor to dry gas
is 47/713.
The preceding analysis does not consider expansion and contraction due to temperature
differences, which do, in fact, exist. This is beyond the scope of this discussion but is avail-
able from several other sources.
In summary, gas exchange between the lungs and the pulmonary circulation occurs at
the smallest elements of each, which are also in extremely close proximity to each other.
With so many alveoli, the surface area for gas exchange is very large, which promotes faster
¼
TABLE 14.2
Single Breath Analysis Based on Partial Volumes
Air Component
Inspired Gases Partial Volume
Expired Gases Partial Volume
Water Vapor
6.5
6.5
þ
(47/713)
488.63
¼
38.7 ml
Dry Gas
493.5
389.9
þ
79.9
þ
18.83
¼
488.63
Nitrogen
389.9
389.9
Oxygen
103.6
103.6 - 23.67
¼
79.9
Carbon Dioxide
0.0
0.0
þ
18.83
¼
18.83
