Why is partial pressure used as a metric for oxygen in blood?
The equation P=CRT is the relationship for the oxygen concentration in the gas phase.  To get the mole fraction of oxygen the the liquid phase (blood), you first use the Henry's law constant.  Here is a link that can help:  https://www.google.com/webhp?sourceid=chrome-instant&amp;ion=1&amp;espv=2&amp;ie=UTF-8#q=henry%27s%20law%20constand%20for%20oxygen%20in%20blood

If, after applying Henry's law, you end up with a concentration in units of x ml per unit volume, this result implies that x is the volume that the mass of dissolved gas (solute) would have it were an actual gas at the same pressure as that in the gas phase.