The Composition of Blood

Blood is a tissue that transports useful substances around the body and removes waste substances. Our blood is made up of four components:

  • Red blood cells
  • White blood cells
  • Platelets
  • Plasma

Red blood cells, white blood cells and platelets are all suspended in blood plasma.

Red Blood Cells

Red blood cells are the most well-known of the four and make up almost half of our blood’s volume. The purpose of red blood cells is to carry oxygen from our lungs to the tissues in our body, so our cells can complete aerobic respiration. Red blood cells contain a molecule called haemoglobin.

Haemoglobin binds to oxygen to become oxyhaemoglobin, which is a red pigment. The equation for this is:

Oxygen + Haemoglobin ⇌ Oxyhaemoglobin

  • The double arrow (⇌) indicates that the reaction is reversible

Once the red blood cells have travelled through the blood vessels to the tissues, the oxyhaemoglobin molecule splits, becoming oxygen and haemoglobin again. As the oxygen is now free, it can diffuse into the tissues.

Adaptations of red blood cells

Red blood cells have adaptations that make them more efficient at transporting substances:

  • The cytoplasm of red blood cells contains the protein haemoglobin – which allows oxygen to bind to it.
  • They don’t have a nucleus – There is more space for haemoglobin molecules
  • They are shaped like biconcave disks – Gives them a large surface area to volume ratio, which makes them more efficient at absorbing oxygen.
  • They are small and flexible – Allows them to fit through narrow blood vessels (e.g. capillaries)
  • They have a thin cell membrane – Provides a short diffusion distance

White Blood Cells

White blood cells make up less than 1% of our blood, however, they are a vital part of our immune system. Their purpose is to identify pathogens and protect the body against them. Unlike red blood cells, white blood cells contain a nucleus.

There are many different types of white blood cells, the two main types are:

  • Phagocytes – Can change shape to engulf and digest pathogens, which destroys them
  • Lymphocytes – Identify pathogens as foreign cells, then produce antibodies to attack and destroy them

How white blood cells protect us from infection


Lymphocytes produce soluble proteins called antibodies which attach to antigens on the surface of pathogens. Different antibodies attach to different antigens, which can be very useful. Antibodies neutralise pathogens by:

  • Supporting phagocytes by clumping pathogens together
  • Punching holes in the cell walls of bacterial cells, which can cause them to burst
  • Producing antitoxins, which bind to and neutralise the toxins produced by pathogens


Phagocytosis is the act of engulfing pathogens.

1. The phagocyte binds to the surface of the bacterial cell

2. It draws the bacterial cell inwards, engulfing it and enclosing it in a vacuole

3. Enzymes are secreted into the vacuole, which destroys the bacterial cell


Platelets are not actual cells. They are small circulating fragments of cells, which means they do not have a nucleus. When we get a cut, platelets rush to the wound and patch it up. This process is called clotting.

Without platelets, blood would pour out whenever we got a wound. However, this isn’t the only purpose of platelets. They are also there to prevent microorganisms from getting in through open wounds, as this can lead to an infection.