Flow in the Lungs - Biofluidmechanics: An Introduction to Fluid Mechanics Macrocirculation and Microcirculation - page 264

TABLE 9.1 The relative diffusion coefficients for important respiratory gases

Gas Relative Diffusion

Coefficient

Oxygen 1.0

Carbon Dioxide 20.3

Nitrogen 0.53

Carbon Monoxide 0.81

TABLE 9.2 Respiratory gas partial pressures

Gas Atmospheric Air

(mmHg)

Alveolar Air
(mmHg)

Deoxygenated Blood
(mmHg)

Expired Air
(mmHg)

N 2 597 569 563.3 566

O 2 159 104 40 120

CO 2 0.3 40 45 27

H 2 O (as vapor) 3.7 47 Variable 47

Total 760 760 760 760

B

This is intuitive because as the pressure difference, cross-sectional area, and solubility
increase, more gas molecules should be able to diffuse into the medium. As the distance
and molecular weight increase, then the amount of molecules that can diffuse should
decrease. From previous engineering courses, you may be familiar with the concept of a
diffusion coefficient. For gases in solution, the diffusion coefficient is proportional to the
solubility of the gas divided by the square root of the molecular weight of the gas. If we
set the relative diffusion coefficient of oxygen to be 1.0 under normal physiological condi-
tions, then the relative diffusion coefficient for other physiologically relevant gases are
listed in Table 9.1 .
Similar to the diffusion of gases through fluids, the diffusion of gas through the respira-
tory boundary is regulated by the same factors. Namely, the pressure difference, the thick-
ness of the membrane, the cross-sectional area, and the diffusion coefficient (solubility/
molecular weight) are the most important factors that determine the diffusion of gas across
the respiratory boundary. The pressure difference across the respiratory boundary is deter-
mined by the partial pressures of the gas in the alveolar air and in blood (which are listed
in Table 9.2 ). The difference in these two values will provide a means to determine the net
direction of movement for gas molecules across the respiratory boundary. The respiratory
boundary thickness is a measure of the linear distance that the molecules must traverse in
order to be exchanged. As this membrane increases in thickness, which commonly occurs