How does breathing help buffer the pH of blood?

In this Medmastery Clinical Guide article, learn how the carbon dioxide-bicarbonate system buffers the body’s pH.
Last update4th Dec 2020

The carbon dioxide-bicarbonate system is the most important of the body’s buffer systems (others include intracellular proteins, hemoglobin, plasma proteins, and constituents of bone). The bicarbonate buffer system modulates pH in both intra- and extra-cellular body fluids.

Bicarbonate, present in most body fluids, constitutes a large reservoir of buffer called the alkali reserve. Bicarbonate reacts with the hydrogen ion to form carbonic acid, which exists in equilibrium with carbon dioxide.

Figure 1. Carbon dioxide and bicarbonate exist in equilibrium. These compounds are part of the carbon dioxide-bicarbonate buffer system which maintains pH of the body.

With hydrogen ion ingestion or production by metabolizing tissues, bicarbonate converts it to carbonic acid and, ultimately, to carbon dioxide and water. And the carbon dioxide is eliminated by the lungs.

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Hendersen-Hasselbalch equation

The system pH in which these reactions take place is calculated using the Henderson-Hasselbalch equation.

Figure 2. The Henderson-Hasselbalch equation calculates system pH from the ratio of the concentrations of bicarbonate (HCO3-) and carbonic acid (H2CO3).

However, from a clinical perspective, using blood gas analysis, it makes more sense to express the concentration of H2CO3 as the partial pressure of CO2 times the solubility factor for CO2.

Figure 3. The Henderson-Hasselbalch equation from a clinically relevant perspective calculates system pH from the concentrations of bicarbonate (HCO3-), solubility factor for carbon dioxide (CO2), and partial pressure of CO2 (PaCO2).

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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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