What is the relationship between PO2 levels in alveolar air and the amount of O2 picked up by the blood?

The correct answer reflects the principle that the amount of oxygen (O2) picked up by the blood in the lungs is directly proportional to the partial pressure of oxygen (PO2) in alveolar air. This relationship is explained by Henry's law, which states that the amount of a gas that dissolves in a liquid is proportional to the partial pressure of that gas above the liquid. When the PO2 in the alveolar air is higher, more oxygen molecules will diffuse across the alveolar-capillary membrane into the blood because the gradient is steeper, driving increased oxygen uptake.

Conversely, when PO2 levels are lower, there is less oxygen available to diffuse into the blood, resulting in reduced oxygen pick-up. This relationship reinforces the importance of maintaining optimal PO2 levels for effective oxygenation of the blood, and it is crucial for ensuring adequate oxygen delivery to tissues throughout the body. Therefore, under normal physiological conditions, a higher PO2 in the alveolar air directly enhances the efficiency of oxygen uptake into the bloodstream.