Now, why would the partial pressure inside the tissue be low? Well, that's because the cells inside the tissue continually use, let's say, glucose in the process of cellular respiration to actually break down glucose and form ATP molecules. And in the process, they use up oxygen in the electron transport chain and they produce carbon dioxide. Now, as they use up oxygen, that decreases the concentration of oxygen in the cells of the tissue, and that therefore decreases the partial pressure of oxygen in our tissue. And that's exactly why we have this difference in pressure now, because oxygen will always move naturally from a higher partial pressure to a lower partial pressure. As our red blood cells move along our capillary, our hemoglobin will unload our oxygen.