Final answer:
The PO₂ decreases and PCO₂ increases in the alveoli due to inhaled air mixing with dead air space, increased water vapor pressure, and rapid gas diffusion, making the correct answer d) all of the above.
Step-by-step explanation:
The PO₂ decreases and PCO₂ increases in the alveoli due to inhaled air mixing with air in the dead air space, the increased partial pressure of water vapor, and rapid diffusion of gases across the respiratory membranes. Therefore, the correct answer is d) all of the above. The presence of moist, CO₂-rich air in the alveoli from previous respiratory cycles mixes with the inhaled air, which dilutes the oxygen content and increases the carbon dioxide content. Additionally, the respiratory system works to humidify incoming air, causing the alveoli to have a greater amount of water vapor than in atmospheric air, which also plays a role in changing the partial pressures of oxygen and carbon dioxide.
During gas exchange, oxygen diffuses across the respiratory membrane from the alveoli into the blood due to its higher partial pressure in the alveoli, while carbon dioxide diffuses from the blood into the alveoli because of its higher pressure in the pulmonary capillaries. This process is governed by Dalton's law and Henry's law, which describe how gases exert force independently and dissolve in liquids based on their partial pressures, respectively.