Platelets and Blood Clotting – Grade 9 Understanding for iGCSE Biology 2.64B

There is a specification bullet point in bold (paper 2 only) about blood clotting and the role of platelets, and students are sometimes not sure to what level of detail is needed for a full GCSE understanding of this topic.  Well the good news is that the only questions I can recall are very straightforward indeed.  But in this post I will give you a little more detail than the minimum needed for A* answers so that you can be confident you are completely clear on this part of the specification.

Why does blood need to clot?

Capillaries have a very thin wall (one cell thick in fact) so can easily tear and get damaged.  This means that damage causes blood to leak into tissues forming a bruise and if the skin is broken, blood can be lost from the body entirely.  Blood clotting is the response in the blood that ensures that blood loss is minimised and also that the time micro-organisms have to get into the blood stream is kept as short as possible.  The surface of the skin is covered with millions of pathogenic organisms (mostly bacteria) all waiting for the chance to get into the blood stream through a cut or tear.

What are platelets?

Platelets are small fragments of cells found in bone marrow that then get into the blood and are carried round in the plasma. They are not entire cells as they lack a nucleus but they do play an essential role in blood clotting.

How does blood clotting work?

When the lining of a capillary is broken, platelets initially stick to the site of damage.  They then trigger a series of reactions in the blood plasma that causes a clot to form.  The details of how this works are too complicated to go into here but the basic idea is that in the blood plasma are a whole family of proteins called clotting factors. There is a cascade of reactions such that one clotting factor is activated and in turn, activates the next in the sequence.   The final reaction in the clotting cascade is that a soluble protein called fibrinogen is converted into an insoluble fibrous protein called fibrin.  Fibrin forms a mesh around the platelet cap covering the site of damage and this mesh traps red blood cells forming the final clot.


The roman numerals on the diagram above refer to clotting factors and each in turn is activated.  You can see the final stage of the cascade is that soluble fibrin is converted into the fibrin clot.

Many of you will know of the disease where blood doesn’t clot called haemophilia.  The commonest type of haemophilia is a genetic disease where patients cannot produce clotting factor VIII.  This means one step in the clotting cascade does not work and so the blood cannot clot normally.

(Extension idea:  find out the link between Haemophilia, the British Royal family and the 20th century history of Russia)

One comment

  1. Pingback: Blood (part 2) White Blood Cells – A* understanding for GCSE Biology | PMG Biology

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