If an area of the lung is not ventilated, how is the pulmonary capillary blood's PO2 affected?

When an area of the lung is not ventilated, also known as a "shunted" region, the blood flowing through the pulmonary capillaries does not receive fresh oxygen. This is a critical aspect of gas exchange; normal ventilation allows for oxygen to diffuse into the blood from the alveoli. In a non-ventilated region, however, the alveolar PO2 remains low due to the lack of oxygen intake, and as a result, the blood that circulates through that area will equilibrate with whatever low oxygen level exists in that part of the lung.

Consequently, the pulmonary capillary blood's PO2 will approach the level of mixed venous blood, which is the blood returning to the heart after delivering oxygen to the body's tissues and carrying carbon dioxide away, typically a lower PO2 compared to arterial blood. This leads to the conclusion that the PO2 of capillary blood in a non-ventilated region will equal the mixed venous PO2 rather than the arterial PO2, which is usually much higher due to the contribution of well-ventilated areas of the lung.

Understanding this concept is crucial, especially in conditions like pneumonia or pulmonary embolism, where parts of the lung are not effectively ventilated