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How does carbon dioxide become bicarbonate in red blood cells?

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Medical illustration of the human chest showing the lungs, diaphragm, ribs, trachea, bronchus, and bronchioles, with one lung cut away to reveal internal structures. — human respiratory system The lungs serve as the gas-exchanging organ for the process of respiration.

In the human respiratory system, carbon dioxide ( $C O_{2}$ ) is converted into bicarbonate ( $H C O_{3}^{-}$ ) within red blood cells through a rapid chemical process.

When $C O_{2}$ enters the red blood cell from the tissues, it combines with water ( $H_{2} O$ ) to form carbonic acid ( $H_{2} C O_{3}$ ). While this reaction is naturally slow, it is catalyzed by carbonic anhydrase, an enzyme located inside the red blood cell, allowing the conversion to occur in a fraction of a second. The carbonic acid then dissociates into hydrogen ions ( $H^{+}$ ) and bicarbonate ions ( $H C O_{3}^{-}$ ).12

To maintain efficiency, the red blood cell utilizes an ion transport system to move the bicarbonate out of the cell and into the plasma in exchange for a chloride ion. This process, known as the chloride shift, allows the plasma to serve as a storage site for bicarbonate without altering the electrical charge of the blood components. The hydrogen ions released during this process are buffered by hemoglobin, which prevents significant changes in blood acidity.12