More than 30 years ago, I invented the diagram below to show the changes in arterial blood gas, and since then, my students and residents refer to it as "The Fish". Just one look at it will help you to see why.
The diagram shows point A and B have the same values for PaCO2 and pH. However, the PaO2 at point B is much lower than at point A, which shows that the severity of the disease at point B exceeds point A.
This diagram presupposes that the patient has not received oxygen (O2) in the 4 hours prior to the drawing of the blood gases.