Density (concentration) of gases in diffusion
- There is always a net diffusion from areas
of high concentration (or pressure) towards the area of low concentration
(or pressure)
- Factors that determine the concentration of
a gas in solution: [Henry’s Law]: Concentration
of dissolved gas = pressure x solubility coefficient
|
|
Solubility
Coeff |
|
Oxygen |
0.024 |
|
Carbon
dioxide |
0.57 |
§ If combine the effects depicted by Graham (lighter gases diffuse faster than heavier gases; Oxygen diffusion coeff: 5.6; Carbon dioxide diffusion coeff: 6.6) with that of Henry’s law (the more soluble the gas in the solution the faster will it diffuse; Carbon dioxide is approximately 24 X more soluble in plasma than oxygen) Carbon dioxide will diffuse about 20 times faster than oxygen
- This has physiological relevance as
although the diffusion gradient for carbon dioxide via the alveolus is
much less than for oxygen, the greater ability of carbon dioxide to
diffuse more than adequately compensates
Net
Rate of Diffusion
- Rate of gas diffusion in a liquid [D]is
dependent on:
1)
Pressure difference [DP]
2)
Solubility of gas in fluid [S]
3)
Cross-sectional area of fluid [A]
4)
Distance through which gas must diffuse [d]
5)
Molecular weight of gas [MW]
6)
Temperature of fluid [assume constant in body]
- The diffusion coefficient (rate of
diffusion through a give area for a given distance & pressure
difference) for any gas is proportional to
where S is the
solubility of the gas - MW is the molecular weight of the gas
