Biomedical Engineering Reference
In-Depth Information
9.4 OXYGEN/CARBON DIOXIDE DIFFUSION
Diffusion is the random thermal motion of molecules, which incurs a net movement of
molecules from a high concentration to a lower concentration. Diffusion is the only way
that oxygen enters the blood from alveolar air and that carbon dioxide enters the alveolar
air from the blood (meaning that there is no active transport mechanism for these mole-
cules). Atmospheric air is composed of a number of molecules and the pressure that each
exerts is directly related to the concentration of that particular molecule. The normal atmo-
spheric air contains 21% oxygen; therefore, 21% of atmospheric pressure is contributed by
oxygen (i.e., 0.21 * 760 mmHg
5
160 mmHg). This partial pressure is denoted by P O 2 , for
oxygen. Gases that are dissolved in liquids also exert a pressure. The partial pressure of a
gas dissolved within a fluid is determined by its concentration and its solubility coeffi-
cient. Henry's Law describes the relationship between a gas partial pressure and the fac-
tors that affect the pressure. Henry's Law takes the form of
Gas Concentration
Solubility Coefficient
P 5
ð
9
:
5
Þ
Looking at this relationship, it shows that for a given concentration, if the gas solubility
is high, the gas will exert a lower pressure as compared with a gas that has a lower solu-
bility. Gases that are highly soluble exert a lower pressure because there will be an attrac-
tion between the liquid molecules and the gas molecules, which prevents the gas from
moving randomly and quickly through the fluid. Also, it is easy for gas molecules to enter
the liquid, and the molecules would prefer to be associated with the liquid molecules.
However, when the solubility is low, there is a very small attraction between the liquid
and gas molecules which allows for the gas to move more freely. Also, it is harder for gas
molecules to enter the liquid and the molecules would prefer to not be in the liquid. At
body temperature, the solubility coefficients for oxygen and carbon dioxide are 0.024 and
0.57, respectively. Therefore, the net movement of gas molecules across the respiration
boundary depends on the relationship between the partial pressure of the gas in atmo-
spheric air and the partial pressure of the gas within blood. Under normal conditions, the
partial pressure for oxygen is higher in the alveoli, so that there is a net movement of oxy-
gen into the blood. Also, the partial pressure for carbon dioxide is typically higher in the
blood, so that there is a net movement of carbon dioxide into the alveoli.
Like diffusion of any other molecule, the rate of diffusion is not solely based on the
pressure gradient (for solids/liquids we typically talk about a concentration gradient,
regardless of the phase of the molecule there is some driving force for diffusion to occur).
The other factors that affect gas diffusion are the temperature of the exchange medium,
the distance that the gas must diffuse over, gas solubility, the cross-sectional area, and the
molecular weight of the gas. In equation form, the diffusion rate is proportional to
D ~ Δ PAs
Δ x M
p
ð
:
Þ
9
6
Δ P is the pressure difference, A is the cross-sectional area,
where D is the diffusion rate,
Δ x is the distance, and M is the molecular weight of the gas.
s is the solubility coefficient,
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