There is an interesting article called “Red blood cells have a unique shape and inner components that allow them to efficiently transport oxygen and direct the elimination of carbon dioxide.”

This article mentions that red blood cells have a unique structure and properties that allow them to fulfill these crucial missions. Red blood cells (RBCs) are the primary cells in human blood. They are biconcave discs, having a depressed center on both sides. These depressed centers allow the cells to have more cell membrane surface which can be exposed to diffusing oxygen while transiting the lungs. This structure also allows them to be more flexible when negotiating tight passages.

If you want to see pictures and learn more, please click on the next link:

Mammal red blood cells lose their nuclei, but should have most of the other things that cells have. Bird red blood cells have everything, including the nuclei.