Final answer:
The presence of nitrogen does not explain why the partial pressure of oxygen is lower in the lung than in external air; it is caused by the other mechanisms like mixing with carbon dioxide, diffusion across membranes, and humidification, which are part of gas exchange processes.
Step-by-step explanation:
The question is asking why the partial pressure of oxygen is lower in the lungs compared to external air. Of the options given, the one that does not explain this difference is the presence of nitrogen. Nitrogen is a major component of both inhaled and alveolar air but does not participate directly in gas exchange, hence it does not affect the partial levels of oxygen in the lungs. The other factors listed contribute to lowering oxygen's partial pressure within the lungs:
- Mixing with carbon dioxide: Carbon dioxide from blood diffuses into the alveoli, reducing oxygen's partial pressure due to dilution.
- Diffusion across membranes: Oxygen passes from the alveoli into the blood, effectively lowering its partial pressure in the lung.
- Moistening of the air: Humidification of inhaled air in the respiratory system increases the water vapor content in the alveoli, which, by Dalton's Law, decreases oxygen's partial pressure.
It's evident that nitrogen's presence, being consistent in both inhaled and alveolar air, does not cause the decrease in oxygen partial pressure. Instead, the decrease is due to the other mechanisms that facilitate gas exchange and maintain physiological functions.