The composition of blood

Blood transports materials and distributes heat around the body. It also helps to protect against disease. Blood contains plasma, which is a liquid that the other components of blood are suspended in.

Plasma is a straw-coloured liquid that makes up just over half the volume of blood.

Pie chart showing the composition of blood

This table explains the functions of various blood components.

PlasmaTransporting dissolved carbon dioxide, digested food molecules, urea and hormones; distributing heat
Red blood cellsTransporting oxygen
White blood cellsIngesting pathogens and producing antibodies
PlateletsInvolved in blood clotting

Red blood cells

Red blood cells (also called erythrocytes) transport the oxygen required for aerobic respiration in body cells.

They must be able to absorb oxygen in the lungs, pass through narrow blood capillaries, and release this oxygen to respiring cells.

Red blood cells have several adaptations that enable them to carry out this function:

  • They contain the proteinhaemoglobin, which gives them their red colour
    • \[\text{haemoglobin} + \text{oxygen} \rightleftharpoons \text{oxyhaemoglobin}\]
    • Haemoglobin can combine reversibly with oxygen. This is important - it means that it can combine with oxygen as blood passes through the lungs, and release the oxygen when it reaches the cells.
  • They have no nucleus so they can contain more haemoglobin.
  • They are small and flexible so that they can fit through narrow blood capillaries.
  • They have a biconcave shape - they are the shape of a disc that is curved inwards on both sides - to maximise their surface area for oxygen absorption.
  • They are thin, so there is only a short distance for the oxygen to diffuse to reach the centre of the cell.
A picture of red blood cells. Each cell is circular with a dent in the centre
Scanning electron micrograph of red blood cells

White blood cells

There are several types of white blood cell.


About 70 per cent of white blood cells are phagocytes. Phagocytes engulf and destroy unwanted microorganisms that enter the blood, by the process of phagocytosis.


Lymphocytes make up about 25 per cent of white blood cells. Lymphocytes produce soluble proteins called antibodies when a foreign body such as a microorganism enters the body.

Antibodies neutralise pathogens in a number of ways:

  • they bind to pathogens and damage or destroy them
  • they coat pathogens, clumping them together so that they are easily ingested by phagocytes
  • they bind to the pathogens and release chemical signals to attract more phagocytes

Lymphocytes may also release antitoxins that stick to the toxins that the microorganism makes, which stops it damaging the body.

Both phagocytes and lymphocytes are part of the body's immune system.

A scanning electron micrograph of a lymphocyte
A scanning electron micrograph of a lymphocyte


Platelets are cell fragments produced by giant cells in the bone marrow.

Platelets stop bleeding in two main ways:

  • they have proteins on their surface that enable them to stick to breaks in a blood vessel and clump together
  • they secrete proteins that result in a series of chemical reactions that make blood clot, which plugs a wound
Image showing how platelets stop bleeding

Platelets help blood to clot