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White blood cells

Defend the body against disease.

Red blood cells

Carry oxygen in the blood. They are also known as erythrocytes.


The amount of a substance (solute) in a solution


The protein that carries oxygen within red blood cells.


Proteins produced by the plasma cells (B cells, a type of white blood cell) of the immune system in response to a specific antigen..

Blood clot

A mass of fibrin, platelets and red blood cells which form a solid mass and stop blood loss at a wound.


Fragments of cells that circulate in the blood and play a role in the formation of blood clots


Single-celled organism. Has a cell wall, cell membrane, cytoplasm. Its DNA is loosely-coiled in the cytoplasm and there is no distinct nucleus


The part of a cell that controls the cell function and contains the chromosomes.


A polymer made up of amino acids joined by peptide bonds. The amino acids present and the order in which they occur vary from one protein to another.


A protein produced during the clotting of the blood which traps platelets and red blood cells to form the clot


The yellow liquid which supports all the cells of the blood and transports dissolved substances around the body


The liquid which leaves your body through the urethra. It contains water, salts urea and other chemicals.

The blood and blood clotting

If you cut yourself you bleed. Your blood looks red but it is really a yellow liquid with lots of red cells floating in it! Your blood is the transport medium for the circulatory system.

The structure of the blood

The components of the blood

The components of the blood

Plasma is a yellow liquid which transports all of the blood cells around your body. It carries waste carbon dioxide from the cells of the body to the lungs to be removed. It takes urea from the liver where it is made to the kidneys to be excreted in the urine. It carries the small dissolved food molecules from the gut to the cells where they are needed.

Red blood cells are small biconcave discs. They contain the red pigment haemoglobin which picks up oxygen in your lungs and carries it to the body cells where it is needed. Mature red blood cells don't have a nucleus so they can pack in more haemoglobin and carry more oxygen.

Haemoglobin and oxygen

Oxygen is taken up in the lungs where it is at a relatively high concentration, and released into cells where it is at a relatively low concentration.

White blood cells are bigger than red blood cells but there are fewer of them.

They are an important part of the way your body defends itself against disease.

There are lots of different types of white blood cells.

Some of them make antibodies against disease-causing organisms.

Some of them engulf bacteria and digest them.

They all have a nucleus.

The diagram opposite shows different types of white blood cells.

Different types of white blood cells

A blood clot forming

A blood clot forming

Platelets are small fragments of cells that have no nucleus. They are very important in the successful clotting of your blood.

If you cut yourself it is very important that the blood forms a solid clot as fast as possible to prevent you from bleeding to death.

The clotting of your blood involves a series of enzyme-controlled reactions which produce a net of fibres made of a protein called fibrin. These fibres then trap lots of red blood cells and platelets and form a jelly like clot which dries to form a scab. The skin heals under the scab.

Healing of a wound

Healing of a wound