How does blood clot?

08 May 2011


How does blood clot?


Chris - Very good question. I suppose the simplest way of thinking about this is that it's a bit like glue in a tube, in the sense that you've got various components inside the glue in the tube, and it's only when they get squeezed out of the tube and they meet the foreign environment of the air that they then get sticky, and stick things together.

In blood, there are several elements. They include little bits of cells called platelets, which are programmed to recognise holes in blood vessels and bind on to them. There are also proteins which are dissolved in the blood which are able to work like tiny pairs of scissors and cut other proteins to produce a cascade of changes which culminate in formation of a fibrin network. This looks like a fishing net inside a blood vessel which traps blood cells in it, and acts as a plug to block a hole in a blood vessel, for example.

So if we look at how it works: If you have a blood vessel and injure it, what happens first is that the platelets, which are circulating in the blood, recognise the fact that when the blood vessel is punctured there are now foreign surfaces exposed to the blood. The platelets bind on and then release various factors that trigger the blood vessel to constrict, so it gets narrower and reduces blood loss from the area. It also starts to recruit these other proteins dissolved in the blood, the coagulation factors, activating them sequentially. They cut each other, activating other coagulation factors, and culminating in the production of this cleavage of a protein called fibrinogen, to make this fibrin network.

All of this happens in a very short space of time - literally seconds for platelets blocking up the hole, to minutes for the formation of one of these fibrin networks. Once the gap in the vessel has been plugged, then the cells locally which line the vessel over-grow the area which has been breached, and they establish a new smooth lining to the blood vessel. The clot is then slowly metabolised away by other cells called macrophages. Dave - Does the same thing happen if you injure a blood vessel which is inside your body, and it's not exposed to the air and foreign environment?

Chris - Exactly right. The thing that the platelets and some of these other factors are recognising is collagen, the main building block of connective tissue. The proteins, including one called Von Willebrand factor, in the bloodstream are able to recognise the presence of collagen which is not normally ever seen inside a blood vessel or in a healthy organ, because they're normally kept separate from it. So whenever that interaction occurs, this tells the factors in the bloodstream that a vessel must have been breached, and therefore you activate the clotting system and it plugs up the hole wherever this occurs. Normally blood vessels keep themselves clear because the lining of the blood vessel, the endothelium, produces various factors which are anti-thrombotic, they antagonise or prevent blood from clotting. Obviously if you damage the blood vessel, you remove that anti-coagulative ability, so it shifts the blood into a pro-coagulation state, and then it starts to clot.

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