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Why can healthy parts of the lung compensate for carbon dioxide during shunting more effectively than for oxygen?

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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.

Healthy parts of the lung can compensate for carbon dioxide during shunting more effectively than for oxygen due to the fundamental differences in their respective blood-dissociation curves.1

The carbon dioxide-dissociation curve is steep and relatively linear, meaning that as the partial pressure of carbon dioxide increases, the content of the gas in the blood continues to rise without reaching a plateau. When shunting occurs, the healthy portion of the lung can increase its ventilation to lower the partial pressure of carbon dioxide in its alveoli. This allows the blood leaving the healthy area to have a lower-than-normal carbon dioxide content, which effectively counterbalances the higher carbon dioxide content coming from the shunted, unventilated areas.1

In contrast, the oxygen-dissociation curve is S-shaped and reaches a plateau near the normal alveolar oxygen partial pressure. Because of this plateau, increasing ventilation in healthy lung areas results in only a negligible increase in oxygen content. Consequently, the healthy lung cannot increase its oxygen output enough to compensate for the blood that bypassed gas exchange, leading to a net reduction in arterial oxygen levels.1