Final answer:
The partial pressure of O₂, CO₂, and H₂O changes as air travels through the respiratory system. Gas exchange occurs in the alveoli, where the composition of air differs from atmospheric air. The changing partial pressures of oxygen and carbon dioxide affect their diffusion across the membrane.
Step-by-step explanation:
The partial pressure of O₂, CO₂, and H₂O changes as air courses through the airways. In the respiratory system, the process of gas exchange occurs in the alveoli. The composition of air in the alveoli differs from atmospheric air, with a greater amount of water vapor and carbon dioxide and less oxygen. As a result, the partial pressures of oxygen and carbon dioxide change, affecting the diffusion process that moves these gases across the membrane.