Biomedical Engineering Reference
In-Depth Information
FIGURE 14.49
Commercial greenhouses.
and some water is condensed. However, the inspired air still contains significantly more
heat and water than the inspired air.
Respiration results in a latent heat loss and a sensible heat loss. The latent heat loss is
based on the latent heat of vaporization of water by the equation
Q
el
¼ð
dm
a
=
dt
Þð
Y
o
Y
i
Þl
where
dm
a
/dt
¼
the kilograms of air breathed in and out per hour
Y
o
-Y
i
¼
the difference in the expired and inspired air water content
the latent heat of vaporization of water at the expired air temperature
The pulmonary ventilation rate (dm
a
/dt) is primarily a function of the body metabolic
rate via the following relationship:
l ¼
ð
dm
a
=
dt
Þ¼
0
:
006 M
where M is the metabolic rate in Kcal per hour.
It is somewhat rare to breath in bone-dry air, so the inspired air temperature andwater con-
tent can affect the expired air from the lungs. Fanger and colleagues (1968) found that there is
a relationship between the inspired water content of air and the expired water content
20 Y
i
for normal conditions. Thus, if the entering air is very dry (Y
i
¼
Y
o
¼
0
:
029
þ
0
:
0), the expired air would be
less humid than if the inspired air were humid. McCutchan and Taylor (1951) determined
that the temperature of the expired air is dependent on the inspired air in the following
fashion:
32 Y
i
Although the relationship between T
o
and T
i
is expected, the inspired air humidity also
plays a role. The air, upon being expired, is somewhat cooler than it would have been if
the inspired air had been humid.
T
o
¼
32
:
6
þ
0
:
006 T
i
þ
