ALVEOLAR-CAPILLARY EXCHANGE

A. Mechanisms

1. Simple diffusion through tissue and fluids
2. Chemical reaction in blood
3. Total time for transport is the sum of diffusion time and chemical reaction time

B. Characteristics of Alveolar-Capillary Diffusion

1. Driving force: partial pressure difference between alveoli and pulmonary capillary blood for each gas (Note:   partial pressure and not concentration because most O₂ and CO₂ is bound or in another chemical form and so cannot diffuse; however, the bound molecules are in equilibrium with the diffusible molecules)

2. Rate of diffusion: Product of driving force and ease of diffusion

3. Gases diffuse independently, according to each gas’s partial pressure difference and ease of diffusion

4. Ease of diffusion depends on

a. Area: the alveolar area available for diffusion (i.e., in contact with pulmonary capillary blood) is very large (order of 70 m²)

Note effect of degenerative lung disease
Note effect of pulmonary embolism

b. Distance: even though gas must pass through several compartments, all are normally thin (even interstitial space), so total diffusion distance is small

Note effect of pulmonary edema on increasing diffusion distance and reducing diffusion area

Definitions:

Pneumonitis:     Inflammation of the lungs

Pneumonia:       Lung inflammation with consolidation (loss of air space) in alveoli;  several possible causes – bacterial or viral infection, parasites, etc.

Pneumococcus:      Bacteria which are a frequent cause of pneumonia

c. Diffusion Coefficient: depends on molecular weight of the gas (not normally a major factor) and its water solubility (major factor)

Note diffusion coefficient of CO₂ >> O₂ (because CO₂ is much more soluble in water than O₂)

C. Rate of Chemical Uptake (required for exchange with blood)

1. Depends on

a. rate of reactions
b. volume of blood in the pulmonary capillaries

D. Equations

Rate = D x ΔP

e.g.   V'_O2 = D_O2 x ( P_A-O2 - P_{pul cap-O2})

where

ΔP = partial pressure difference = ( P_A-O2 - P_{pul cap-O2})

D = Diffusing Capacity (combined diffusion and rx. rate)   (Note:  D is directly proportional to Area and Diffusion Coefficient for the gas and inversely proportional to diffusion Distance)

V'= rate of gas movement (e.g. ml/min)

P_A = alveolar partial pressure

P_{pul cap} = pulmonary capillary partial pressure

Question:  What would be the equation for rate of alveolar diffusion of carbon dioxide?

V'_CO2 = D_CO2 x ( P_{pul cap-CO2} - P_A-CO2 )