What is Blood Clotting Process?

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What is blood clotting process? How does blood clotting work? Information and facts about blood clotting.

Blood ClottingBlood Clotting; When a blood vessel is cut, bleeding would continue indefînitely were it not for a physiological process of repair known as hemostasis. This process occurs spontaneously and takes place through the combined action of the blood vessels, blood platelets, and various plasma proteins known as clotting factors.

As soon as a blood vessel is cut, its lumen (channel) automatically constricts to reduce the flow of blood. The platelets promptly adhere to the rough surface of the cut vessel and form clumps to produce a temporary plug. The platelets then release serotonin, a chemical that causes the blood vessels to constrict further. The actual clotting of the blood results from the conversion of two plasma proteins in two reactions: in one of them prothrombin is converted to thrombin; in the other the resulting thrombin converts fibrinogen to fibrin.

The conversion of prothrombin to thrombin is a complex process, requiring the presence of various clotting factors. Recent studies ha ve shown that there are actually two processes needed for prothrombin conversion, one occurring in the blood itself and the other occurring in the body tissues. The first process, called the intrinsic system, begins the moment there is any disturbance of the surface of the blood vessel. When this occurs, one factor, known as factor XII or Hageman factor, activates the next factor, factor XI, which in turn activates the next factor. The entire intrinsic system involves six factors in all—factor XII, factor XI, factor IX, factor VIII, factor X, and factor V—and requires the presence of calcium and phospholipid, both of which act as catalysts. The calcium is normally present in the blood at all times, and the phospholipid is released by the platelets.

The second process, known as the extrinsic system, is activated by actual damage to the blood vessel and surrounding tissue. In this process there are at least four known factors involved—a tissue factor (as yet unidentified), factor VII, factor X, and factor V. As in the intrinsic system, calcium and phospholipid are necessary as catalysts, but in this case the phospholipid is provided by the body tissue.

Both the intrinsic system and extrinsic system are necessary to convert prothrombin to thrombin. However, because the intrinsic system begins the moment there is any contact with the surface of the blood vessel, this part of the clotting process is well under way by the time there is any actual tissue damage. Thus the conversion of prothrombin to thrombin, although it requires many steps, takes only a few seconds, and clotting begins almost immediately af ter there is any tissue damage.

Although little is known about the exact mechanism of how the factors interact, it is known that if one factor is not present the blood will not clot. For example, in patients with hemophilia factor VIII is missing, and in people with Christmas disease, a disorder similar to hemophilia, factor IX is missing. Although there are several theories of how the clotting factors work, many scientists support the theory that each factor becomes converted to an enzyme, which then acts to convert the next factor to another enzyme. For example, it is believed that in the intrinsic system, factor XII is converted into an enzyme designated as XIIa, which in turn activates a larger amount of factor XI to become another enzyme, known as XIa. This “cascade” effect produces an increasing wave of activity in each successive stage, resulting in an almost explosive production of thrombin.

Once thrombin is formed, it activates the fibrinogen, which is circulating in the blood, to form fibrin. Fibrin is an insoluble protein in the form of a fine network, and it is this network that traps the blood cells and forms the clot. The clot serves to plug the injured blood vessel and prevent the loss of blood.

In addition to the complex system of blood clotting, the blood has a system that can dissolve blood clots. In this system, called the fibrinolytic system, the enzyme plasmin breaks down the fibrin threads and also destroys the surrounding substances, including prothrombin, fibrinogen, and the clotting factors. The function of this system is believed to be the removal of any tiny blood clots that might clog the capillaries. It is also important in removing any blood that has leaked into a body tissue and clotted.

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