So when we break down sugar, specifically glucose, into Pyruvate molecules to form ATP, in the process of cellular respiration, we essentially form the two three BPG as an intermediate. Now, when our cells are exercising, when they have a relatively high rate of metabolism, they can produce an excess amount of two, three BPG. And some of these two three BPG molecules can ultimately leave these exercising cells and enter the capillaries that are found next to these exercising cells. And once the two three BPG molecules are inside the capillaries, they enter the red blood cells where we have the hemoglobin molecule. So to see what we mean, let's take a look at the following diagram. So, these are the exercising cells that have a relatively high rate of metabolism found within our tissue.