And these hemoglobin carry the oxygen. Now, as the red blood cells travel, let's say, from this side to this side, inside the blood plasma, we have a relatively high concentration of oxygen. And so the partial pressure of oxygen inside our capillary will be relatively high compared to the partial pressure of oxygen inside the cells of our tissue. 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.