Cliff Notes on Hemostasis (apologies to any hemotologists)
Hemostasis is the medical term for the body's method of stopping bleeding. Hemostasis is devided into primary hemostasis and secondary hemostasis. Although "clotting" is best known to lay people actually in most cases clotting is not what stops bleeding.
Primary hemostasis consists of vasoconstriction and platelet aggregation. These are fast processes occuring within seconds of the blood vessel being cut. Platelets are small fragments of larger cells called megakeriocytes. Platelets are normally part of the blood circulating along with white cells and rbcs. When the internal lining of the blood vessel (the endothelium)is transected it triggers a local reaction which causes the platelets to become "sticky" they aggregate to form a platelet plug which clogs the hole. The arteries and arterioles contain smooth muscle cells in their lining which further aid by constricting blood flow into the area. This is how bleeding is stopped.
Secondary hemostasis is coagulation ie. clotting. is a process requiring several minutes to complete and is too slow to stop the bleeding. Coagulation is very important because though primary hemostasis is fast, it is only temporary. The platelet plugs are friable and within a couple of hours the constricted vessels relax. Blood clots are a more permanant seal and are ultimately organised in scar. Coagulation as readers on this site know is set up as a cascade. What is meant by that is one chemical reaction creates a catalyst (enzyme) which then fascilitates another reaction which produces another enzyme and so forth. The important thing about the cascade is that each reaction requires time and a critical concentration of enzyme to continue to the next step. This is the built in safety factor which allows the clot to occur in the area of injury where blood flow is slow, but prevents it from extending into the rest of the circulation, because as the factors are swept into the open circulation they are rapidly diluted to levels too low to continue the cascade.
The intrinsic cascade is the classical factorXII down to fibrin. It is effected in the sex chromosome hereditary coagulopathies such as hemophylia or Christmas Disease. The intrinsic system originates inside the circulation.
The extinsic system operates outside the blood vessels and by-passes most of the early steps by utilizing tissue thromboplastin which acts to produce thrombin. Of the two, the intrinsic system is much more important. For effective hemostasis there must be clotting inside the vessels. The extrinsic system merely adds a margin of safety, a bolster from the outside. For example in Hemophylia the extrinsic system does not require factor VIII and is fully functional, Hemophyliacs actually form clots, but they are not effective.
