Simplifying Fick's law and lung gas exchange

In this article, learn the essentials about Fick’s law, and how it describes gas diffusion across the alveolar-capillary membrane.
Last update24th Nov 2020

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.

Figure 1. Fick’s law describes the diffusion of gases across the alveolar-capillary membrane.

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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 (DM). And the membrane’s diffusing capacity, DM, is dependent on a number of factors: solubility (Sol), membrane area (A), membrane thickness (T), and molecular weight of the gas (MW).

Figure 2. Fick’s law describes how the diffusing capacity of a membrane (DM) is dependent on factors such as membrane area (A), membrane thickness (T), the solubility of the gas (Sol), 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 DL is substituted for DM.

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Recommended reading

  • Grippi, MA. 1995. “Gas exchange in the lung”. In: Lippincott's Pathophysiology Series: Pulmonary Pathophysiology. 1st edition. Philadelphia: Lippincott Williams & Wilkins. (Grippi 1995, 137–149)
  • Grippi, MA. 1995. “Clinical presentations: gas exchange and transport”. In: Lippincott's Pathophysiology Series: Pulmonary Pathophysiology. 1st edition. Philadelphia: Lippincott Williams & Wilkins. (Grippi 1995, 171–176)
  • Grippi, MA and Tino, G. 2015. “Pulmonary function testing”. In: Fishman's Pulmonary Diseases and Disorders, edited by MA, Grippi (editor-in-chief), JA, Elias, JA, Fishman, RM, Kotloff, AI, Pack, RM, Senior (editors). 5th edition. New York: McGraw-Hill Education. (Grippi and Tino 2015, 502–536)
  • Tino, G and Grippi, MA. 1995. “Gas transport to and from peripheral tissues”. In: Lippincott's Pathophysiology Series: Pulmonary Pathophysiology. 1st edition. Philadelphia: Lippincott Williams & Wilkins. (Tino and Grippi 1995, 151–170)
  • Wagner, PD. 2015. The physiologic basis of pulmonary gas exchange: implications for clinical interpretation of arterial blood gases. Eur Respir J45: 227–243. PMID: 25323225

About the author

Michael A. Grippi, MD
Vice Chairman, Department of Medicine | Pulmonary, Allergy, and Critical Care Division, Perelman School of Medicine, University of Pennsylvania, USA.
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