Animal transport and exchange systems

In mammals, blood consists of plasma, red blood cells and white blood cells.

Nutrients (eg glucose and amino acids), oxygen and carbon dioxide are transported around the body in the blood.

Red blood cells

Red blood cells transport oxygen around the body. They are specialised to carry oxygen because they:

  • contain large quantities of a protein called haemoglobin, which can bind oxygen.
  • don't have a nucleus, so there is more room for haemoglobin.
  • have a biconcave disc shape, which maximises the surface area of the cell membrane for oxygen to diffuse across.
  • are tiny and flexible so can squeeze through the narrowest of blood capillaries to deliver oxygen.

Microscope view of red blood cells
Microscope view of red blood cells


Haemoglobin binds with oxygen in body locations where the oxygen concentration is high (in the lungs) and forms oxyhaemoglobin.

Blood with a high concentration of oxygen is described as oxygenated.

Haemoglobin + oxygen \to oxyhaemoglobin

This makes the blood a bright red colour.

In locations where the oxygen concentration is low (body tissues) haemoglobin releases oxygen.

Oxyhaemoglobin \to haemoglobin + oxygen

The oxygen then diffuses into the cells. Blood that has a low oxygen concentration is a dark red colour and is described as deoxygenated.

Oxygen binds to haemoglobin and oxygen being released from haemoglobin.

White blood cells

White blood cells are part of the immune system and are involved in destroying pathogens (disease-causing micro-organisms bacteria, viruses and fungi).

There are 2 main types of cells involved:

  • Phagocytes which carry out phagocytosis by engulfing pathogens and digesting them (breaking them down).
  • Lymphocytes which produce antibodies which destroy pathogens.

Antibodies are specific to particular pathogens.