Red blood cells contain haemoglobin that make them suitable for adaptation. Haemoglobin is a red protein that combines with oxygen. RBC have no nucleus so they can contain more haemoglobin. They are small and elastic so that they can fit through small blood vessels. It carry or transport O2. They have biconcave disc-like structure that allow them to pass through any vessel. Its biconcave shape allows distortion of the shap[e to squeeze through small tubes.
