Simplifying Fick's law and lung gas exchange

The alveolar surface area is expansive and contains an extensive network of pulmonary capillaries, making it ideal for gas exchange.

Gas movement across the alveolar-capillary membrane occurs by diffusion, as described by Fick’s law.

According to Fick’s law, the rate of gas transfer across a tissue plane or membrane (Vgas) is directly proportional to the difference in partial pressures of the gas on the two sides of the membrane (P1 – P2) and the membrane’s diffusing capacity (D_M). And the membrane’s diffusing capacity, D_M, is dependent on a number of factors: solubility (Sol), membrane area (A), membrane thickness (T), and molecular weight of the gas (MW).

Diffusion increases with increases in membrane surface area, gas solubility, and the gas’s pressure gradient across the membrane, and decreases in tissue thickness and molecular weight of the gas. To determine the diffusing capacity of the lung as a whole, the term D_L is substituted for D_M.

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