Diffusion of Gases

Diffusion: Partial P gradient, Total P is equal, Gases move according to individual partial P NOT total content, can occur if total P = 0 but only if a gradient exists

Bulk Flow: Total P gradient, movement is due to total P, molecules of gases move together due to a total P gradient from an external force

Alveolar/Capillary Diffusion
Rate of diffusion = delta x area x (P1-P2) / T

A = surface area for diffusion
P1-P2 = partial pressure gradient between alveolus and pulmonary capillary
T = tissue thickness
delta = solubility / sqrt(MW)

- diffusion of CO2 much greater than O2, but the rate of gas transfer is roughly equal.
- CO2 20x more soluble than O2
- O2 smaller than CO2 and has a greater partial pressure gradient
- PAO2 = 100 to PmvO2 = 40, PACO2 = 46 to PmvCO2 = 40
- with diffusion barriers, O2 more affected due to decreased diffusion coefficient

DL gas = delta x A / thickness
DL gas = Vgas/ (P1-P2), given in pulmonary function test

CO is used since it is highly bound to Hb and consequently the parital P remains nearly zero so that the gradient remains constant and transfer is depenent on the properties of the alveolar capillary membrane

METHOD:
1) subject blows all air out to RV
2) subject takes a max inspiration of air containing 0.2% CO
3) subject holds her breath for 10 sec so some of the inhaled CO diffuses
4) Subject then exhales and the end tidal gas is collected.
5) From the end tidal gas, an estimate of the CO in the alveolus is made, and CO uptake is calculated using the difference between inspired and expired CO

Uptake: Ventilation flow CO = amount inhaled - amount exhaled / 10sec

Now use Fick's law...
DLCO = VCO/PACO - PCCO

P1 = PACO; P2 = PCCO = 0; (A = alveoli, C = pulmonary capillaries)

So: DLCO = VCO/PACO

Once DLCO is calculated we estimate DLO2 (the diffusing capacity of the lung for O2) by multiplying by 1.23 (which corrects for the difference in the MW and solubility of O2

DLO2 = DLCO x 1.23

Thus, if a person has a low diffusing capacity, he will exhale much of the CO that he inhaled, making VCO low. PACO will be high because the gas is not diffusing into the pulmonary capillaries.

1) decreased surface area available for diffusion
- emphysema: decreased surface area for diffusion (increased dead space)
* radiograph = hyperinflated dark lungs
** gross pathology = large dark holes of dead space

2) Thickening of the Alveolar-Blood Gas Barrier
- interstitial/alveolar edema
- interstitial/alveolar fibrosis (sarcoidosis)
* CT= increased thickness decreasing surface area