What is the fractional concentration of a gas?
The concentration of a single gas, A, in the mixture, M, can be represented in terms of its fractional concentration: the number of molecules of gas A divided by the number of all the molecules in the gas mixture (the amount of gas A plus the amount of gas B plus the amount of gas C).
The atmosphere is a mixture of gases, including nitrogen, oxygen, carbon dioxide, argon, and water vapor. The amounts of argon and carbon dioxide are minimal. Under normal ambient (dry) conditions, there is also little water vapor.
Calculating the fractional concentrations of the gases in our atmosphere
Just as we did for the generic gas mixture above, the concentrations of gases in the atmosphere can be expressed as fractional concentrations. But for these purposes, we can ignore the concentrations of all gases but oxygen and nitrogen. And, since this is the air we breathe in, we can express them as fractional concentrations of inspired gas, X (FIX).
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- 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 J. 45: 227–243. PMID: 25323225