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The blood clotting mechanism
Flowing blood is essential to circulate food and oxygen
to all parts of the body. Within the blood there is a system known as the
clotting mechanism. The clotting mechanism performs two vital but opposite
functions – its first function is to keep the blood flowing and the second,
to form a 'plug' or clot to stop us bleeding.
Under normal circumstances the blood clotting function remains dormant. However when we injure ourselves by cutting a blood vessel, this function is activated and the blood forms a solid mesh, or clot, to protect us from loss of blood. The clotting mechanism, therefore, is highly effective in keeping the balance between the blood flowing, clot formation and clot breakdown.
But sometimes this mechanism can go wrong and a clot can develop within a blood vessel that has not been cut.
What is thrombosis?
A clot occurs when the blood stops flowing freely,
the clotting mechanism is activated and blood becomes a solid mass. Doctors
use the term thrombus to describe this.
When a clot is formed inside a blood vessel, blocking the whole of a vein or artery, this is known as thrombosis. This can happen in any part of the body as there are blood vessels throughout the body, circulating the food and oxygen needed to keep us alive.
Thrombosis develops when:
- The blood is more sticky than normal
- There is damage to the blood vessel wall
- The blood flow is reduced for some reason
- There is a combination of these factors
Examples of thrombosis are:
Blood clots in veins
These are known as :
- A blood clot in a deep vein, usually in the leg, is known as a deep vein thrombosis
- A blood clot in the retinal vein of the eye is known as a retinal vein thrombosis
Blood clots in arteries
These are known as :
- A blood clot in an artery of the heart, such as the coronary artery is known as a heart attack, or myocardial infarction
- A blood clot in the brain is known as a stroke,
or cerebrovascular accident
Pulmonary embolism
Someone who has a thrombosis is at risk from an embolism.
When this happens, part of the clot can break off, travel around the body
and eventually block an artery. This process is known as embolisation
and the piece of clot is called an embolus, or emboli.
An example of this is when part of the clot from a deep vein thrombosis breaks off, goes up the leg, through the right side of the heart and lodges in a lung artery (pulmonary artery). This is known as a .
Download our leaflet about thrombosis (pdf format)
Coloured scanning electron micrograph (SEM) of clotted human blood
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