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Hemostasis is a bleeding process that forms a platlet plug which leads to the formation of a blood clot. Hemostasis is normally prevented in normal blood vessels because the endotehlium cells are in tact and the CD39 present in the cells break AMP down to release a phosphate. Also Prostocyclin and Nitric Oxide are vasodialators that expand the diameter of the endothelium and prevent platelets from sticking together.
Vessels such as capillaries are easily damaged. Platelet plugs are used to fill the tears. When the endothelium is damaged irregular shaped platelets bond to collagen. This bond between platelets and collagen is not very strong so endothelial cells also produce Von Willebrand's Factor Protein. This protein has a platelet and collagen bonding site to strengthen the bond between platelets and collagen. When the platelets bond to the collagen using the Willebrand's factor they degranulate (release contents). When platelets degranulate the vesicles that are contained within them undergo exocytosis to release ADP, Thromboxane A2, Growth factor, and Clotting factors. ADP and Thromboxane A2 make the platelets sticky which increases the chance of a blood clot. Thromboxane A2 is also a vasoconstictor which decreases the blood flowing in the vessels. Growth factors stimulate the regeneration of the damaged connective tissue and Clotting factor helps to form the blood clot. Ailments like Petechiae and Hemophilia can inhibit platelet formation. Petechiae is a problem where low numbers of platelets are made due to a problem with the bone marrow. Hemophilia declines Willebrand's Factor Protein.
Blood clots are formed from fibers made of fibrin which is a polymer of fibrin proteins. The polymer fibrin is produced from fibrinogen. Fibrinogen is a plasma protein and is soluble made by the kidney. Fibrin is insoluble. To convert Thrombin is used which comes from prothrombin which comes from a cascade of events that can be intrinsic, extrinsic, of from the common pathway. Intrinsic pathway involves only blood and the Extrinsic pathway involves blood and tissue chemicals. In vivo, the intrisic pathway may not exist. In the 4 step intrinsic pathway calcium is very important. The extrinsic involves 2 steps. The main chemical that starts the extrinsic pathway is called tissue factor. It is a protein in the plasma membrane of many cells. Tissue factor has cytoplasmic, extracellular, and ? components. When exposed to blood (broken vessels), factor 7 will be bond and activate the extrinsic which leads to the common pathway. Prothrombin is made into thrombin and the clots shrinks
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