Why are RBCs efficient in their functions though they lack nucleus and mitochondria?

RBCs are efficient in their functions though they lack nucleus and mitochondria because:

1. Loss of nucleus makes the red cells biconcave, thus increasing their surface area to absorb more oxygen.
2. Loss of mitochondria means that they cannot use the absorbed oxygen themselves. Thus, all the oxygen absorbed from the lungs is transported and delivered to the tissues unconsumed.