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 cells and cell fragments are suspended in:

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

Types of blood cell and cell fragments:

  • red blood cells
  • white blood cells
  • platelets
Pie chart showing the composition of blood

This table explains the functions of various blood components.

PlasmaTransporting 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 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 adaptations that enable them to carry a maximum amount of oxygen:

  • They contain the proteinhaemoglobin, which gives them their red colour, at the lungs
    •  \text{haemoglobin} + {oxygen} \xrightarrow {at~the~lungs} \text{oxyhaemoglobin}
    •  \text{haemoglobin} + {oxygen} \xleftarrow {at~the~cells} \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 - they lose it during their development - so they can pack in 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.
Red blood cells
Red blood cells

White blood cells

There are several main types of white blood cell.


About 70% of white blood cells are phagocytes. Phagocytes engulf and destroy unwanted microorganisms that enter the blood, by the process of phagocytosis. They are part of the body's immune system.



Lymphocytes make up about 25% of white blood cells. They are also part of the body's immune system. Lymphocytes produce soluble proteins called antibodies when a foreign body such as a microorganism enters the body.


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.

Blood products

Blood products are components of blood that are given to a patient by transfusion. They include:

Blood products are produced from blood from blood donors.

Blood products can then be given to patients depending on their needs. Patients rarely receive transfusions of whole blood in modern medicine.

Blood products are screened for:

  • infectious agents such as HIV
  • their blood group
  • the presence of certain antibodies

Blood for transfusion must be compatible with that of the patient's blood, for instance, their blood group. Before a transfusion, white blood cells are often removed to reduce the risk of infections or immune reactions.