PaO2 is the partial pressure of oxygen in the blood
SaO2 is the saturation of oxygen on the haemoglobin of the red blood cells.
Different terms lets tackle them one at a time. Here I am giving you the "rough and dirty" explanation.
PaO2 - partial pressure. Look up Dalton's law to help you understand this. Basically all the gases dissolved in blood make up a pressure that can be measured. The amount of that total pressure that the oxygen is responsible for is called the patial pressure. It is a way of working out how much oxygen is dissolved.
Saturation on the other hand is different because it is usually measured by measuring how "red" blood is. Each haemoglobin molecule in your blood can carry 4 oxygen molecules. The more it carries the redder blood looks. If only 3 out of 4 of the sites on the haemoglobin molecules have O2 then the SaO2 is only 75% (for the sake of argument) SaO2 measures how much oxygen is being carried by the haemoglobin.
Last edit by Fluesy on Nov 28, '04