we are going to learn hemostasis the term hemostasis means prevention of blood loss as it is made up of two Greek words he means blood and stasis means stagnation hemostasis can be divided into three major stages first is vasoconstriction then comes platelet plug formation and the final is coagulation let's assume that you had a fall and injured yourself the injury has caused a tearing blood vessel if we look into the blood vessel it is made up of innermost layer of endothelial cells surrounding the endothelial cells is extracellular Matrix containing collagen outside the collagen is a layer of smooth muscle cells as the blood is flowing through the vessel it will start to bleed from injured side to prevent the blood loss the vascular smooth muscles will contract by three mechanisms to cause vasoconstriction first is the ruptured endothelial cells release a substance called endothelene that is a powerful vasoconstrictor second is the nerve reflexes initiated by pain itself from the traumatized vessel and third is local myogenic spas as a direct response to injury after vasoconstriction we have second stage of hemostasis which is plated plug formation other than red blood cells there are substances in blood such as platelets one William brand Factor various clotting factors prothrombin and fibrinogen right now we will be looking at platelets and one William brand Factor the clotting factors will come in next stage platelet plug formation can be divided into four parts first is platelet adhesion when one valibrant factor comes in contact with collagen of damaged vascular surface they adhere to exposed collagen and express a receptor called glycoprotein 2B platelets get attracted to this receptor glycoprotein 2B and readily attached to one millibrant factor via this receptor these attached platelets secrete large quantities of adenosine diphosphate and thromboxin A2 and serotonin these substances activate other platelets and cause them to adhere to the original activated platelets this will lead to platelet aggregation thus forming a platelet plug these substances also further increase the vasospas platelet plug is usually successful in blocking the bleeding but it is not firm enough so the punctured vessel needs to be repaired completely by the formation of a blood clot which is the third step of hemostasis this step is a bit more complex and involves various clotting factors Pro thrombin and fibrinogen there are two mechanisms for initiation of clotting which we will learn in a bit in each instance this leads to the formation of prothrombin activator prothrombin activator catalyzes the conversion of prothrombin into thrombin thrombin acts on fibrinogen causing polymerization of its molecules into fibrin fibers fibrin fibers form a mesh work running in all directions and trapping blood cells and platelets and this makes the final blood clot as we mentioned earlier there are two mechanisms for initiation of clotting that are extrinsic and intrinsic pathway extrinsic pathway begins with trauma to vascular wall what happens is that the traumatized tissue release a substance in the blood that is called tissue factor or factor 3. tissue Factor combines with Factor 7 in the presence of calcium to form factor 7 tissue Factor complex this tissue factor factor 7 complex acts and enzymatically on Factor 10 to form activated Factor 10. the activated Factor 10 combines with tissue phospholipids that are a part of tissue factor to form a complex called prothrombin activator it is called extrinsic pathway because it is activated by a factor the tissue factor that normally does not belong to plasma the second mechanism for initiation of clotting is intrinsic pathway intrinsic pathway begins when traumatized blood vessel activates the factor 12. activated Factor 12 acts enzymatically on Factor 11 and activate this Factor as well the activated Factor 11 acts on Factor 9 and activate it as well the activated Factor 9 along with activated factor 8 in the presence of calcium act on Factor 10 and forms activated Factor 10. it is important to note that this step will be deficient if factor 8 is in short supply deficiency of factor 8 is the main cause of classic hemophilia for this reason factor 8 is also called anti-hemophilic Factor this pathway is referred to as intrinsic pathway because the entire Cascade of clotting factors is intrinsic to blood and hence its name after all the subsequent steps of clotting and final clot formation the entrapped platelets contribute to clot retraction by activating thrombocine actin and myosin molecule all these contractile proteins cause strong contraction and the edges of broken blood vessel are pulled together contributing further to hemostasis