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. So on this particular graph, we begin at 0 mercury and end up at 100 mercury. And as we'll discuss in the next lecture, a value of 100 mmhg describes the partial pressure of oxygen inside our lungs, while a value of 40 mercury describes the partial pressure inside our resting tissue. And we'll discuss much more of that in the next lecture.