How is the structure of a red blood adapted to its function?

Red blood cells are shaped like a biconcave disc - increases SA:volume ration -> increase surface for oxygen to diffuse in and out of cell

Red blood cells are very small - 7um, compared with 40um average cell -> allows them to fit through capillaries, haemoglobin molecules close to surface -> quick oxygen exchange

Red blood cells are flexible - able to deform to pass through very tight vessels

Red blood cells have no nucleus/mitochodria/ER - more room for haemoglobin -> maximising oxygen carrying capacity