How does the chloride shift help the blood carry carbon dioxide?

The chloride shift is a critical mechanism that enhances the blood's capacity to transport carbon dioxide by utilizing the plasma as a storage site for bicarbonate ions.12

Inside the red blood cell, carbon dioxide is converted into bicarbonate ions ($HC{O}_3^{-}$) and hydrogen ions. Because this production occurs rapidly within the cell, bicarbonate would quickly accumulate to levels that inhibit further conversion if it remained trapped there.12

To prevent this, an ion transport system in the red blood cell membrane facilitates the chloride shift: it moves a bicarbonate ion out of the cell into the plasma while simultaneously bringing a chloride ion into the cell. This exchange allows the blood to store the vast majority of its carbon dioxide as bicarbonate in the plasma—where it accounts for approximately 62 percent of total carbon dioxide content—without altering the electrical charge of either the plasma or the red blood cell.12