For the Alveolar partial pressure of oxygen we see, Increasing the inspired concentration (F1) of an anesthetic agent increases the alveolar concentration (FA). A 75% nitrous oxide (N20) in oxygen mixture increases the FI/FA ratio of N2O.
The amount of nitrous oxide absorbed will be greater than the nitrogen given out in the alveoli, hence the alveoli shrink and thereby increase the concentration of the gases (N20) present in the alveoli. This is how the mechanism of alveolar partial pressure of oxygen can be studied.
Oxygen cascade describes the changes in the oxygen content of the air as it moves from the atmosphere till it ultimately reaches the alveoli. The alveolar partial pressure of oxygen is less compared to the atmospheric oxygen partial pressure because as the air enters the airway passages it gets mixed with the expired air, which contains carbon dioxide and water vapor.
This water vapor and carbon dioxide ultimately decrease the partial pressure of the oxygen thus ultimately leading to the final alveolar partial pressure of oxygen.
Thus the partial pressure of oxygen keeps on decreasing from the atmospheric pressure till it reaches the alveoli.
Under normal conditions the alveolar partial pressure of oxygen is 107 mmHg or 14.2 kPa.
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