How your blood clots

Your skin is very tough but if you get a cut, your blood needs to act fast to make sure that you heal as quickly as possible!  The parts of your blood that do this are your platelets.  These are tiny cells that are very, very sticky.  When you get a cut, your platelets rush to the area and join together to form a clot.  As the clot dries out, it forms a scab that stays until the skin underneath is totally healed.  When your skin is totally healed, and ready to face the world again, the scab falls off - all by itself!

There are lots of platelets in each drop of blood – about quarter of a million!  They are made in your bone marrow from large cells that break up into very small pieces so they don’t have a normal shape.  Platelets only live for a short time, about 10 days, but they are constantly made so there is always enough ready to jump into action when you need them.

Apart from being very sticky, platelets have other things that help them to make a clot.  They have chemicals that call out to other platelets to make sure they make a big enough clot and other chemicals to make sure the clot doesn’t break up.  The mineral calcium, vitamin K and a protein called fibrinogen also help the platelets to make a good clot so your skin can heal.  When the platelets reach the cut on your skin, they mix with the fibrinogen and begin to break up.  Little threads called fibrin are made and these make a web, like a spider’s web that has gaps so tiny, your other blood cells cannot get out and the bleeding stops.  When the web dries it goes hard and your skin heals underneath.  When your skin is totally healed, the scab falls off. 

Sometimes you get a blood clot inside that you can’t see.  These can happen in blood vessel in older people and be very serious because they stop the blood flowing properly.  If you have a bad knock or bang against something hard, your body can get a bruise which you will see as a black (or blue or dark red) mark on your skin.  This is a clot under that you can watch heal - the colour will change to a yellow – green colour and then just disappear!

What happens if your body is missing something that helps it clot?  Some boys suffer from a condition called haemophilia.  They are missing a part of their clotting mechanism and if they get a cut it can take a very long time for them to stop bleeding or they just keep on bleeding.  You cannot just keep bleeding - if it is not stopped, there is a point where so much blood is gone that the body doesn’t have enough to keep going and it just shuts down.  Luckily, there is medicine that fixes the problem.   

Did you know…?

Haemophilia is a genetic disorder and it usually only affects boys although women can pass it on to their sons.  It is very rare for a girl to suffer from haemophilia!  There are lots of different factors that help your blood clot – they are part of the coagulation cascade.  It is a very complex system (look at Diagram 1!) but it all works like a row of dominos – the things at the top have to happen before the things at the bottom can!  In haemophilia, either factor VIII or Factor IX is missing so there is a gap in the dominos.  When the cascade reaches that gap, it just stops and the blood can’t clot.  Luckily, there is medicine that replaces the missing Factor VIII or Factor IX so people with haemophilia are not is so much danger any more.