Now, as these cells are exercising, they require a greater number of oxygen molecules to produce the ATP. And so inside the red blood cells, the Bore effect takes place. That causes hemoglobin to actually unload and release the oxygen which then travels into those exercising cells inside the cells in a process called aerobic cellular respiration. We produce the energy ATP molecules. Now, these ATP molecules are used for muscle contraction, for the actin contraction. But the CO2 molecules, a byproduct, are produced and these cannot be used in any useful way.