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Respiratory Membrane, Factors Affecting Gas Exchange, and Principle of the CO Method

(Guyton and Hall, Textbook of Medical Physiology, South Asia Edition)

1. The Respiratory Membrane

Gas exchange between alveolar air and pulmonary capillary blood occurs across the respiratory membrane (pulmonary membrane), formed by all the terminal gas-exchanging structures of the lung - the respiratory bronchioles, alveolar ducts, atria, and alveoli (together called the respiratory unit/lobule). There are about 300 million alveoli, each roughly 0.2 mm in diameter, surrounded by an almost continuous mesh of pulmonary capillaries described as a "sheet of flowing blood."
Layers of the respiratory membrane (alveolus to capillary lumen):
  1. A layer of fluid containing surfactant lining the alveolus (reduces surface tension)
  2. The alveolar epithelium (thin epithelial cells)
  3. An epithelial basement membrane
  4. A thin interstitial space between alveolar epithelium and capillary wall
  5. A capillary basement membrane (often fused with the epithelial basement membrane)
  6. The capillary endothelial membrane
Despite these six layers, overall thickness averages only about 0.6 micrometer (as little as 0.2 micrometer in places). Total surface area is about 70 square meters, and pulmonary capillary blood volume at any instant is only 60-140 mL - this huge area combined with minimal blood volume and membrane thickness explains the speed of gas exchange. Pulmonary capillaries average only ~5 micrometers in diameter, so red cells squeeze through with their membrane touching the capillary wall, further speeding diffusion by minimizing the plasma layer gases must cross.

2. Factors Affecting Rate of Gas Diffusion Through the Respiratory Membrane

The same principles governing diffusion of gases in water apply here. Four factors determine diffusion rate:
  1. Thickness of the membrane - diffusion is inversely proportional to thickness. It increases in pulmonary edema (fluid in interstitial space/alveoli) and pulmonary fibrosis. A 2-3 fold increase in thickness significantly impairs gas exchange.
  2. Surface area of the membrane - decreased by pneumonectomy (halves area) or emphysema (alveolar wall destruction and coalescence can reduce area up to 5-fold). Once surface area falls to one-third to one-fourth of normal, gas exchange is impeded even at rest, and even mild reduction becomes critical during exercise.
  3. Diffusion coefficient of the gas - depends on the gas's solubility in the membrane and is inversely proportional to the square root of its molecular weight. Because of its higher solubility, CO2 diffuses about 20 times faster than O2 for the same pressure gradient, and O2 diffuses about twice as fast as nitrogen.
  4. Partial pressure difference across the membrane - the difference between alveolar partial pressure and pulmonary capillary blood partial pressure determines the net driving force. O2 moves alveoli -> blood (alveolar PO2 > blood PO2); CO2 moves blood -> alveoli (blood PCO2 > alveolar PCO2).

3. Diffusing Capacity of the Respiratory Membrane

Diffusing capacity is defined as the volume of a gas that diffuses through the respiratory membrane per minute for a partial pressure difference of 1 mm Hg.
  • O2 diffusing capacity at rest averages 21 mL/min/mm Hg. With a mean O2 pressure gradient of ~11 mm Hg, this gives ~230 mL O2/min diffusing - matching resting O2 consumption.
  • During strenuous exercise, O2 diffusing capacity rises to about 3 times resting values, due to (i) opening/dilation of previously dormant pulmonary capillaries (increasing surface area) and (ii) improved ventilation-perfusion matching.
  • CO2 diffusing capacity has never been directly measured (the alveolar-capillary PCO2 gradient is under 1 mm Hg, too small to measure), but since CO2's diffusion coefficient is ~20 times that of O2, it is estimated at 400-450 mL/min/mm Hg at rest and 1200-1300 mL/min/mm Hg during exercise.

4. Principle of the Carbon Monoxide (CO) Method

Direct measurement of O2 diffusing capacity requires knowing alveolar PO2, pulmonary capillary blood PO2, and O2 uptake rate - but capillary PO2 is very difficult to measure accurately. Physiologists therefore measure CO diffusing capacity instead and calculate O2 diffusing capacity from it.
Principle:
  • A small, known amount of CO is breathed into the alveoli, and alveolar CO partial pressure is measured from alveolar air samples.
  • Because hemoglobin binds CO so avidly and rapidly, the CO pressure in blood remains essentially zero - it never has time to build up.
  • Therefore, the pressure difference of CO across the respiratory membrane equals the alveolar CO partial pressure itself (no need to measure capillary CO pressure).
  • By measuring the volume of CO absorbed over a short time and dividing by the alveolar CO partial pressure, the CO diffusing capacity is calculated directly and accurately.
  • To obtain O2 diffusing capacity, the CO value is multiplied by 1.23 (since O2's diffusion coefficient is 1.23 times that of CO).
Example: Average CO diffusing capacity in healthy young men at rest ≈ 17 mL/min/mm Hg; multiplying by 1.23 gives O2 diffusing capacity ≈ 21 mL/min/mm Hg, matching the value quoted above.
Reference: Guyton and Hall, Textbook of Medical Physiology, 4th South Asia Edition, Chapter 40 - "Physical Principles of Gas Exchange; Diffusion of Oxygen and Carbon Dioxide Through the Respiratory Membrane," pp. 515-518.
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