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

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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


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 = DO2 x ( PA-O2 - Ppul cap-O2)


ΔP = partial pressure difference = ( PA-O2 - Ppul 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)

PA = alveolar partial pressure

Ppul cap = pulmonary capillary partial pressure

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

V'CO2 = DCO2 x ( Ppul cap-CO2 - PA-CO2 )

E. Diffusing Capacity Typical Values

DO2 = 20 ml/min/mmHg DPO2 (at rest)

= 30-60 (exercise, probably due to increase area as pulmonary capillaries open)

DCO2 = 250+ ml/min/mmHg DPCO2 (at rest)

DCO = DO2 / 1.23 or DO2 = 1.23 x DCO

Note: DCO is useful for estimating DO2

Note utility of adding oxygen to inspired air for patients with low DO2 -- but also note problem of oxygen toxicity

Note: For oxygen, diffusion is the major limitation on rate alveolar exchange. For carbon dioxide, chemical reaction rate is the major limitation of rate of alveolar exchange

Question:  If a patient develops pulmonary edema, thus reducing the diffusing capacity for all gases, will he first get in trouble with hypoxemia or with hypercapnia?