Cooperativity and we can plot something called the oxygen hemoglobin dissociation curve on the following x Y axis, where the X axis is the partial pressure of the oxygen within our tissue, and our Y axis is the percent saturation of hemoglobin. So we have from zero to 100 and from zero to 100. And the units of our x axis are millimeters per mercury. So as the red blood cells travel along our capillary, the hemoglobin unloads that oxygen. The oxygen moves through our cell membrane into the blood plasma and then travels into the tissues and to the cells in the tissue, where it is picked up by other molecules, other proteins within those tissue cells. So the tissue and organs of our body have a low partial pressure of oxygen, about 40 mercury of mercury.