The PAO2 that's noted in the equation is the alveolar O2 left in the alveoli once the inhaled O2 that was destined to enter the blood has gone into the blood. Maybe it might be more intuitive if we rearrange the equation so it's like PIO2 = PAO2 + PO2delivered to blood. The total inhaled O2 (PIO2) will either stay in the alveoli (PAO2) or will be delivered into the blood (PO2delivered to blood). The output O2 (PO2delivered to blood) is calculated by seeing how much CO2 is in the alveoli since there's the specific exchange ratio between CO2 and O2. The inhaled CO2 is almost zero so all the CO2 seen in the alveoli essentially all came from the blood.

@@medrounds101 Thank you so much! But I feel the PaCO2 should be drawn from the VBG instead of the ABG since VBG has CO2 rich blood and that CO2 in VBG is what is going to be readily picked up by aveoli? Also I feel the PO2 delivered to blood can also be drawn from ABG directly because it's O2 rich blood and the PaO2 reading from ABG should reflect the PO2 delivered to blood? Sorry about all the questions....and again thank you so much for making these videos!