Now, the main function of red blood cells is to carry oxygen. So red blood cells contain many, many hemoglobin proteins inside the cytoplasm. And these hemoglobin proteins inside the red blood cells are responsible for picking up that oxygen inside the lungs and dropping, delivering that oxygen to the tissues of our body. Now, as with any other cell in our body, these red blood cells age with time. And what that means is, with time, as they collide with other cells of our blood, with the walls of the capillaries, and as they squeeze through the tiny capillaries of our body, they are damaged. And eventually they must be destroyed.