Number one is their shape. They have a biconcave shape. And that not only increases the surface area of the red blood cells and makes them very efficient in exchanging oxygen, but it also allows them to actually squeeze and travel through the really tiny capillaries. So as the red blood cells with the biconcave shape squeeze through the capillaries surrounding our tissues and cells, we basically exchange that oxygen for carbon dioxide. Now, the next important structure point of our red blood cell is the fact that they do not actually have any organelles. So they don't have nuclei, they don't have mitochondria or any other organelle, for that matter.