And this is given in millimeters of mercury. So the range begins at zero to about 100 mercury. So recall that at 100 mercury we are in our lungs and at 40 we are in our tissue. So notice what these two curves actually tell us. At about our 40 mercury partial pressure, which is the partial pressure of oxygen inside our tissues, the value for how much of hemoglobin is actually saturated differs for these two PH. At a PH of 7.4 it's around 70% saturation and at a PH of 7.2 it's around 60% saturation of hemoglobin.