The nature of how blood clots around any cuts that happen in the skin is another example of an irreducible system. If any part of this system is missing (i.e. not evolved yet), then the whole system fails. Therefore, all the parts of the system must have had to develop at the same time. Here is a quick look at how blood clotting works.

Around 3 % of blood is a protein called fibrinogen......easily remembered because the proteins resemble fibers. These fibers normally float around in blood just minding their own business until a cut happens. When it does happen, another protein called thombrin cuts the ends off of fibrinogen, exposing the sticky tips. These tips match perfectly with other fiber ends and when they form, they make long ropes intertwining like a net. This net is the most efficient way to keep the red blood cells from flowing out through the cut.

However, thombrin which cuts the fibers, cannot exsist active all the time or else the clotting would not stop until the animal was dead. So it exsist in a passive state as prothombrin.

Now, prothombin has to be activated in order to cut the fibers to form the clot. The protein that does this is called Stuart Factor with the help of yet another protein called accelerin. Accelerin is a protein that speeds up the action of Stuart Factor, without which, the transofrming of prothombrin would be too slow to help clot the blood in time.

Guess what? Accelerin also has to be kept in a passive form called proaccelerin or else all the animal's blood would clot, killing the animal. The funny thing is that the thing that activates proaccelerin is thombrin!!! Therefore blood clotting is auto-regulatory, in that a product further down the line helps to initiate the clotting process. Due to the naturally slow action of Stuart Factor, there is always a small amount of free flowing thombrin in the blood to activate proaccelerin when needed.

But like you can guess, Stuart Factor has to be kept inactive so all of the animal's blood will be kept from clotting and killing the animal. Therefore is kept passively as well. So Stuart Factor has to be activated. This is done by one of two methods, called intrinsic and extrinsic.

The intrinsic route starts with a protein called Hageman Factor sticking around the area of a cut. Hageman Factor is then cut by another protein called HMK to get activated Hageman Factor. Immediately, activated Hageman Factor converts a protein called prokallikrein to its active form, kallikrein......this helps HMK convert Hageman Factor even faster. Then together, HMK and activated Hageman Factor, convert yet another protein called PTA to its active form. Then activated PTA and another protein called convertin activate a protein called Christmas Factor.  Finally.......activated Christmas Factor combines with Antihemophilic Factor to activate Stuart Factor.

The extrinsic route is like the intrinsic. Proconvertin is activated by Hageman Factor and thombrin. Then in the presence of Tissue Factor, the activated convertin activates Stuart Factor. But this only happens when the animal's tissue is brought into contact with blood....such as deep cuts.

Finally, we have activated Stuart Factor able to activate thombrin, which then cuts the fibrinogen that forms the clot.

So now you can see that a failure anywhere along the lines of these complex interacting parts, the whole blood clotting system would fail. One cannot look at this system and say, "Oh, this protein was the building block of the whole thing." because any one of these proteins by itself is useless to a living animal without the help of all the others as well. Therefore, each protein involved would have to had been around at the same time, fully formed, in order to be of any use to the creature.
 

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