What is primarily exchanged during gas exchange in the alveoli?

The primary process of gas exchange in the alveoli involves the movement of oxygen and carbon dioxide between the air within the alveoli and the blood in the surrounding capillaries.

Oxygen from the inhaled air diffuses across the thin walls of the alveoli into the blood, where it binds to hemoglobin in red blood cells for transport to body tissues. Conversely, carbon dioxide, which is a byproduct of cellular metabolism, moves from the blood into the alveoli to be exhaled. This exchange is driven by differences in partial pressures of the gases: oxygen is higher in the alveolar air compared to the blood, facilitating its diffusion into the bloodstream, while carbon dioxide is higher in the blood, promoting its transfer to the alveoli.

Other options don't represent accurate processes of gas exchange in the alveoli. For instance, nitrogen and argon are inert gases and do not undergo metabolic exchange. Carbon monoxide is not a primary gas exchanged at the alveoli under normal conditions; it's a toxic gas that can interfere with oxygen transport if present. Therefore, the correct answer emphasizes the critical exchange of oxygen and carbon dioxide during respiration.