Final answer:
The decrease in alveolar oxygen partial pressure is mainly dependent on the total amount of oxygen that diffuses into the blood, driven by the pressure gradient between alveoli and capillary blood.
Step-by-step explanation:
The decrease in the partial pressure of oxygen in alveoli during gas exchange is most dependent upon the total amount of oxygen that diffuses into the blood. This process is governed by Dalton's law, which states that each gas in a mixture of gases exerts its own pressure, and by Henry's law, which explains that the amount of a gas that dissolves in a liquid is proportional to its partial pressure. In the alveoli, the partial pressure of oxygen is about 104 mm Hg, while in the blood of the capillaries it is approximately 40 mm Hg.
This gradient promotes the diffusion of oxygen from the alveoli into the blood, where it binds to hemoglobin in red blood cells. Simultaneously, carbon dioxide diffuses from the blood into the alveoli owing to its higher partial pressure in the pulmonary capillaries and its greater solubility in both blood and alveolar fluids.