Platelets hemostasis

Platelets are the formed elements of the blood which participate in hemostasis. Platelets are enucleated, discoid fragments which arise from mature megakaryocytes in the bone marrow. Under normal circumstances, platelets do not adhere to endothelial surfaces of blood vessels. However, platelets can adhere to damaged areas of blood vessels and become activated in such a way that they can also bind fibrinogen. 

Thatte, H. S., Zagarins, S., Khuri, S. F. Fisher, T. H. (2004). Mechanisms of pGiy-A-acetyi glucosamine polymer-mediated hemostasis platelet interactions. Journal of Trauma, 57(1), SI3-21. 

Whole blood is seldom used ia modem blood transfusion. Blood is separated into its components. Transfusion therapy optimizes the use of the blood components, using each for a specific need. Red cell concentrates are used for patients needing oxygen transport, platelets are used for hemostasis, and plasma is used as a volume expander or a source of proteins needed for clotting of the blood. 

Each component of blood has a function ia the body. Red cells transport oxygen and carbon dioxide between the lungs and cells ia the tissues. White cells function as defense of the body. Platelets are important for hemostasis, ie, the maintenance of vascular iategrity. Plasma, an aqueous solution containing various proteias and fatty acids, transports cells, food, and hormones throughout the body. Some proteias ia plasma play a role ia clotting, others are messengers between cells. 

Platelets play an important role in hemostasis and are involved in the pathogenesis of many diseases, such as artherosclerosis and asthma. Moreover, an inadequately controlled aggregation may lead to vascular occlusion. 

Platelets play a central role in primary hemostasis. They are also important in pathological processes leading to thrombosis. Antiplatelet drugs are primarily directed against platelets and inhibit platelet activation by a number of different mechanisms. They are used for the prevention and treatment of thrombotic processes, especially in the arterial vascular system. 

Primary hemostasis is the first phase of hemostasis consisting of platelet plug formation at the site of injury. It occurs within seconds and stops blood loss from capillaries, arterioles, and venules. Secondary hemostasis, in contrast, requires several minutes to be complete and involves the formation of fibrin through the coagulation cascade. 

Hemostasis is the cessation of bleeding from a cut or severed vessel, whereas thrombosis occurs when the endothelium lining blood vessels is damaged or removed (eg, upon rupmre of an atherosclerotic plaque). These processes encompass blood clotting (coagulation) and involve blood vessels, platelet aggregation, and plasma proteins that cause formation or dissolution of platelet aggregates. 

We shall first describe the coagulation pathway leading to the formation of fibrin. Then we shall briefly describe some aspects of the involvement of platelets and blood vessel walls in the overall process. This separation of clotting factors and platelets is artificial, since both play intimate and often mutually interdependent roles in hemostasis and thrombosis, but it facifitates description of the overall processes involved. 

A number of laboratory tests are available to measure the phases of hemostasis described above. The tests include platelet count, bleeding time, activated partial thromboplastin time (aPTT or PTT), prothrombin time (PT), thrombin time (TT), concentration of fibrinogen, fibrin clot stabifity, and measurement of fibrin degradation products. The platelet count quantitates the number of platelets, and the bleeding time is an overall test of platelet function. aPTT is a measure of the intrinsic pathway and PT of the extrinsic pathway. PT is used to measure the effectiveness of oral anticoagulants such as warfarin, and aPTT is used to monitor heparin therapy. The reader is referred to a textbook of hematology for a discussion of these tests. 

Hemostasis and thrombosis are complex processes involving coagulation factors, platelets, and blood vessels. 

Describe the processes of hemostasis and thrombosis, including the role of the vascular endothelium, platelets, coagulation cascade, and thrombolytic proteins. 

Von Willebrand factor (VWF) is a large multimeric glycoprotein with two main functions in hemostasis to aid the platelet adhesion to injured blood vessel walls and to carry and stabilize factor VIII in plasma. Table 64—4 represents three main vWD phenotypes, their frequency, and genetic transmission.17... 

Discuss the role of platelets in various aspects of hemostasis... 

Platelets are essential for many aspects of hemostasis, or the cessation of blood loss. Several substances are found within the cytoplasm of platelets that contribute to the arrest of bleeding as well as vessel repair ... 

Platelets play a role in each of the mechanisms of normal hemostasis vasoconstriction, formation of the platelet plug, and blood coagulation. However, they are also involved in pathological processes that lead to atherosclerosis and thrombosis (formation of a blood clot within the vascular system). Antiplatelet drugs interfere with platelet function and are used to prevent the development of atherosclerosis and formation of arterial thrombi. 

Blood coagulation. The third major step in hemostasis is coagulation, or the formation of a blood clot. This complex process involves a series of reactions that result in formation of a protein fiber meshwork that stabilizes the platelet plug. Three essential steps lead to clotting (see Figure 16.1) ... 

Blood platelets release ATP, ADP, serotonin, and other compounds and this plays an important role in thrombosis and hemostasy. To study the mechanism of the release, in vitro release of ATP was followed using the firefly luciferase luminescence method [124],... 

Most clinicians agree that crystalloids should be the initial therapy of circulatory insufficiency. Crystalloids are preferred over colloids as initial therapy for burn patients because they are less likely to cause interstitial fluid accumulation. If volume resuscitation is suboptimal following several liters of crystalloid, colloids should be considered. Some patients may require blood products to assure maintenance of 02-carrying capacity, as well as clotting factors and platelets for blood hemostasis. 

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