Final answer:
The diffusion of gases in the lungs primarily relies on the partial pressure difference between the alveolar air and the blood in the pulmonary capillaries. Therefore, the correct option is D.
Step-by-step explanation:
Gas diffusion at the lung is mainly dependent on the partial pressure difference between air within the alveoli and blood. This process is explained by Dalton's law and Henry's law, which describe the behavior of gases and how they exchange between different media. The partial pressure of oxygen is higher in the alveoli compared to the blood in the pulmonary capillaries, which facilitates oxygen diffusion into the blood. Conversely, carbon dioxide has a higher partial pressure in the blood than in the alveoli, enabling it to diffuse out into the alveoli and be expelled from the body.