Red blood cells transport oxygen for aerobic respiration. They must be able to absorb oxygen in the lungs, pass through narrow blood vessels, and release oxygen to respiring cells.
Red blood cells have adaptations that make them suitable for this:
Red blood cells have very thin cell membranes – this lets oxygen diffuse through quickly. The cells themselves are thin, so there is only a short distance for the oxygen to diffuse to reach the centre of the cell. The biconcave shape provides a large surface area compared to the volume of the red blood cell, allowing diffusion to happen efficiently.
Red blood cells do not contain a nucleus so they can contain more haemoglobin. Oxygen combines with haemoglobin to form oxyhaemoglobin: