Hi Kaylee.*
Yes, Shom.* is correct, but you could do with an explanation PLUS some other points.
The absence of a nucleus in an erythrocyte (Greek eryth = red; cytos = container here cell SO red blood cell) means that it can carry more haemoglobin to bind oxygen.
The flattened doughnut shape means that there is a large surface area for exchange of gases (in a Q like this show off your knowledge by brief mention of O2 and CO2 exchange in the tissues AND in the lungs).
The diameter of the RBC (about 7 microns [micrometres]) is similar to the diameter of the capillaries, so the RBC just squeezes through -----> slow passage of RBC means more time for gas exchange BUT ALSO there is a very short distance for O2 to diffuse across = capillary wall (unicellular - one cell thick ONLY) and RBC cell membrane (also very thin = 7 nm (nanometres = 10^-9 m) (plus v thin alveolar wall in the lungs).
The fact that the RBCs are minute and that there are a few million of them per square mm of blood (4-5 million in a male, slightly less in a female) also vastly increases the surface area for gas exchange.
The cell membrane of the RBC is very specialized (like no other cell in the body) which makes it survive immense mechanical and osmotic forces as it squeezes through capillaries - it is v flexible and v strong due to the arrangement of specialized proteins helically (sort of in minute spirals) e.g. a protein called spectrin and another called adhesin. During the 90 day life cycle of a RBC, it travels many thousands of miles in the circulation.
The section is italics is beyond A level, but would impress your examiner in the synoptic Q at A2 if you are aiming for an A* - if not, don't worry if you can't remember it.
M.