Answer:
Red blood cells are adapted to their function by:
1) They contain haemoglobin - a red protein that combines with oxygen. they have no nucleus so they can contain more haemoglobin.
2) They are small and flexible so that they can fit through narrow blood vessels.
3) They have a biconcave shape (flattened disc shape) to maximise their surface area for oxygen absorption.
Hope this helps! :D