Now, on the graph, the y axis describes the fractional saturation of the protein. It tells us what fraction of the total number of proteins in our mixture is fully saturated, is bound to that oxygen, and it ranges from a value of zero to a value of one, where zero basically means none of the proteins contain oxygen, while one means 100% of the proteins or the proteins are bound to oxygen. Now, the x axis describes the concentration of the oxygen. And because oxygen is against, we commonly describe the concentration of oxygen by using the partial pressure of oxygen. So PO two, and this is given in either millimeters of mercury or tor, these two units have the same exact quantities. So one Tor is equal to 1 mercury.