Erythrocytes (or Red Blood Cells) are anucleate blood cells that are produced and develop in the bone marrow and, once mature, circulate in the blood and have the primary fuction of delivering oxygen to the respiring tissues in an organism. They are around 7µm in diameter and, in humans, mature erythrocytes circulate in the blood for 100-120 days.
Erythrocytes take on a biconcave shape which gives the cell a large surface area for maximal oxygen transport. Erythrocytes are rich in haemoglobin, which contains iron groups that the oxygen binds to. Red Blood Cells are flexible which enables them to squeeze through the blood capillaries and deliver the oxygen to the tissues. As well as lacking a nucleus, they also lack most organelles, maximising the available space for haemaglobin. All of the above points make the erythrocyte very efficient at its job.