Which of the following reactions takes place at the alveolar site?

In the alveoli, bicarbonate ions re-enter red blood cells to combine with hydrogen ions, forming carbonic acid, which subsequently breaks down into carbon dioxide. This carbon dioxide is then expelled through exhalation.

In the lungs, as the blood reaches alveoli (respiratory site), bicarbonate ions are transported back into red blood cells in exchange for chloride ions. Hydrogen ions dissociate from hemoglobin and bind to bicarbonate ions, forming the intermediate carbonic acid. This carbonic acid is then converted back into carbon dioxide through the enzymatic action of carbonic anhydrase (CA). The resulting carbon dioxide is expelled through the lungs during exhalation.

The chemical reactions involved can be represented as follows:

CO₂ + H₂O ⇌ H₂CO₃ (carbonic acid) ⇌ HCO₃^- + H⁺ (bicarbonate)