Red blood cells

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:

  • they contain haemoglobin - a red protein that combines with oxygen
  • they have no nucleus so they can contain more haemoglobin
  • they are small and flexible so that they can fit through narrow blood vessels
  • they have a biconcave shape (flattened disc shape) to maximise their surface area for oxygen absorption
Close-up view of large number of circular red blood cells.
Close-up view of red blood cells

Adaptations for efficient diffusion of oxygen

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.

Adaptations for efficient carriage of oxygen

Red blood cells do not contain a nucleus so they can contain more haemoglobin. Oxygen combines with haemoglobin to form oxyhaemoglobin:

Equation showing that when oxygen combines with haemoglobin it forms oxyhaemoglobin
Oxygen binds to haemoglobin and oxygen being released from haemoglobin.Oxygen binds reversibly to haemoglobin