In a pulmonary embolism, how is the alveolar PO2 in the affected lung typically affected?

In a pulmonary embolism, the affected lung region experiences a blockage in the blood vessels, which interferes with blood flow and can result in areas of the lung that are ventilated but not perfused. This condition leads to an increase in the ventilation/perfusion (V/Q) ratio in the affected area because while there is normal airflow (ventilation), the blood flow (perfusion) is restricted.

Since the alveoli in the affected lung may still be receiving air containing the typical levels of oxygen, the partial pressure of oxygen (PO2) in the alveoli can remain close to that of inspired air, which is around 160 mmHg when taking ambient conditions into account. Therefore, the alveolar PO2 does not significantly decrease, as the oxygen in the alveoli is not effectively removed by blood flow because of the embolism.

This explains why in a pulmonary embolism, the alveolar PO2 in the affected lung is effectively equal to the inspired air PO2, reflecting the altered V/Q dynamics rather than a decrease in oxygen availability in the alveoli themselves.