So as always, the x axis is the partial pressure of oxygen given to us in millimeters of mercury. And the Y axis is the fractional saturation duration of hemoglobin. And what the red curve describes is how much of that hemoglobin inside our blood is saturated with oxygen at some partial pressure value. Now, we know that at sea level, the total atmospheric pressure is 760 mercury. And because the amount of oxygen in the atmosphere is 21%, to find the partial pressure of oxygen, we simply multiply zero point 21 by the total pressure of 760. And that gives us about 159 mercury is the partial pressure of oxygen at sea level.