Biomedical Engineering Reference
In-Depth Information
has decreased, its pressure increases to a value that is about 3 mmHg above atmospheric
pressure. Air now moves out of the lungs and into the atmosphere.
The primary purpose of the respiratory system is gas exchange. In the gas-exchange
process, gas must diffuse through the alveolar space, across tissue, and through plasma into
the red blood cell, where it finally chemically joins to hemoglobin. A similar process occurs
for carbon dioxide elimination.
As long as intermolecular interactions are small, most gases of physiologic significance
can be considered to obey the ideal gas law:
pV ¼ nRT
where
pressure, N/m 3
p ¼
volume of gas, m 3
V ¼
n ¼
number of moles, mol
R ¼
gas constant, (N
m)/(mol
K)
T ¼
absolute temperature, K
The ideal gas law can be applied without error up to atmospheric pressure; it can be
applied to a mixture of gases, such as air, or to its constituents, such as oxygen or nitrogen.
All individual gases in a mixture are considered to fill the total volume and have the same
temperature but reduced pressures. The pressure exerted by each individual gas is called
the
of the gas.
Dalton's law states that the total pressure is the sum of the partial pressures of the
constituents of a mixture:
partial pressure
N
P ¼
1 pi
i ¼
where
th constituent, N/m 3
p i ¼
partial pressure of the
i
N ¼
total number of constituents
Dividing the ideal gas law for a constituent by that for the mixture gives
P i V
PV ¼ n i R i T
nRT
so that
p i
p ¼ n i R i
nR
which states that the partial pressure of a gas may be found if the total pressure, mole frac-
tion, and ratio of gas constants are known. For most respiratory calculations,
will be con-
sidered to be the pressure of 1 atmosphere, 101 kN/m2. Avogadro's principle states that
different gases at the same temperature and pressure contain equal numbers of molecules:
V
p
V 2 ¼ nR
nR 2 ¼ R
1
1
1
R 2
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