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Blood Clotting - Biology - AS Level

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Red blood cells - note the concave shape.

Red blood cells - note the concave shape.

What is Blood Clotting?

Blood clotting is a vital process in the human body that prevents blood loss from blood vessels (arteries and veins).

However, when blood clots form inside of blood vessels, there is a chance that they will cause a blockade, restricting or denying blood to particular areas of the body.

This is called thrombosis and the chances of acquiring this disease is greatly increased by atherosclerosis - hard plaques that also block up blood vessels.

Platelet Plug

Platelet Plug

'The Platelet Plug'

Much like in atherosclerosis, blood clotting is a result of damage to the wall of a blood vessel. It also occurs when blood flows too slowly.

Platelets are a type of blood cell. They contain no nucleus. When they make contact with the damaged vessel wall, platelets change their shape. They turn from flattened discs to spherical shapes, with long thin projections stemming out from them.

As well as shape change, the surfaces of the platelets change and become sticky. This causes platelets to stick to other platelets as well as the exposed collagen in the damaged wall. This forms a ‘platelet plug’ that prevents further blood leaking out of the vessel. This is what a ‘scab’ is.

As well as this, the platelets release substances that ‘activate’ even more platelets to change shape and stick to the plug.

Platelet Changes

Platelet Changes

Fibrin Mesh

Fibrin Mesh

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'The Fibrin Mesh'

At the same time as the platelet plug is forming, the direct contact of blood with the exposed collagen of the vessel wall results in a cascade of changes (that you don’t need to know about) that leads to the formation of thrombin from prothrombin.

This thrombin is an enzyme that is used to catalyse the conversion of soluble protein: fibrinogen into the insoluble protein: fibrin.

Fibrin is long and insoluble and therefore easily creates a tangled mesh around any nearby blood cells. Because Fibrin is insoluble, it will not simply dissolve back into the blood plasma around it.

The End Result: A Blood Clot

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DK (author) from London on January 09, 2013:

Thank you very much! :)

Muhammad Hajjo from Dubai, UAE on January 08, 2013:

Good job, I'm a Pharmacy graduate and we studied this subject intensively, but to see it put out in the style you chose was a new experience.

Daniel Clark on October 18, 2012:


Thanks a lot, you made me pass my AS biology :)

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