Final answer:
The two key players in gas exchange within the alveolus are oxygen and carbon dioxide, where oxygen diffuses into the blood and carbon dioxide is expelled from it.
Step-by-step explanation:
The two key players in gas exchange in the alveolus are oxygen and carbon dioxide. During respiration, oxygen diffuses across the thin alveolar membranes and enters the blood through the surrounding network of capillaries because there is a higher concentration of oxygen in the alveoli compared to the blood. Simultaneously, carbon dioxide diffuses from the blood, where its concentration is higher, into the alveoli to be expelled from the body during exhalation. This gas exchange is facilitated by the alveoli's highly permeable surface provided by type I alveolar cells and the decrease in surface tension due to the surfactant secreted by type II alveolar cells.