So in the tissues of our body, the partial pressure of oxygen is about 40 mercury. This means that hemoglobin will unload some of that oxygen and it will move from the blood and into the tissue cells down its partial pressure concentration gradient. Partial pressure gradient. Now, if we begin to exercise, if our tissues are exercising, then what happens is the pressure, the partial pressure inside the tissue, drops even more. So let's say now it drops to 20 mercury. Now, because of this sigmoidal curve, we have a very steep slope.