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Why does carbon dioxide in the blood affect blood acidity?

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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.

Carbon dioxide ( $C O_{2}$ ) affects blood acidity because it acts as an acidic substance when it dissolves in the blood. As $C O_{2}$ enters the blood from tissues, it reacts with water to form carbonic acid ( $H_{2} C O_{3}$ ). This reaction is catalyzed by the enzyme carbonic anhydrase within red blood cells.12

Once formed, carbonic acid dissociates into hydrogen ions ( $H^{+}$ ) and bicarbonate ions ( $H C O_{3}^{-}$ ). Because acidity is defined by the concentration of free hydrogen ions, the release of these ions during the dissociation of carbonic acid tends to lower the blood's pH, making it more acidic.231

Under normal physiological conditions, the body maintains blood pH at a slightly alkaline level of approximately 7.4. This stability is preserved because blood proteins, particularly hemoglobin, act as effective buffers that neutralize the released hydrogen ions. Additionally, the respiratory system regulates blood pH by adjusting the rate and depth of breathing to control the elimination of $C O_{2}$ from the lungs.12