What happens to free CO2 generated during gas exchange?

The free CO2 generated during gas exchange primarily diffuses into the alveolus to be exhaled. This process occurs in the lungs, where carbon dioxide, a waste product of cellular metabolism, moves from the blood into the alveoli due to the difference in concentration. As blood circulates through the pulmonary capillaries surrounding the alveoli, the partial pressure of CO2 is higher in the blood than in the air within the alveoli. This gradient facilitates the diffusion of CO2 from the blood, where it is dissolved or bound to hemoglobin, into the alveoli, where it can then be exhaled from the body.

The other choices involve mechanisms that do not accurately describe the fate of free CO2 during gas exchange. CO2 is not stored in the liver nor converted back to bicarbonate (HCO3-) in the context of gas exchange in the lungs. The re-conversion to bicarbonate primarily occurs in the systemic circulation, particularly within red blood cells, rather than during the gas exchange process where CO2 is actively being eliminated. Thus, the primary role of free CO2 generated during cellular respiration is to diffuse into the alveoli for exhalation, making this the correct answer.