OVERVIEW: HEMOSTATIC COMPONENTS AND DISORDERS

20 OVERVIEW: HEMOSTATIC COMPONENTS AND DISORDERS



1 What is hemostasis?


Hemostasis is the maintenance of vascular integrity and blood fluidity necessary for the normal function of blood. The term hemostasis implies a balance between the extremes of hemostatic dysfunction: too little hemostasis resulting in hemorrhage and too much hemostasis resulting in thrombosis. When a blood vessel is injured, blood loss must be minimized by the rapid formation of a clot localized at the injury site. Hemostasis is involved in the healing process. When hemostasis is in balance, rapid clotting at the injury and appropriate healing occur. Imbalance resulting in too little hemostasis results in some component of significant blood loss due to hypocoagulation at the injury site. Imbalance resulting in too much hemostasis results in hypercoagulation or thrombosis, obstruction of vascular blood flow, and distal organ hypoxia and injury.



2 When is a hemostatic disorder suspected?


Lengthy or unabated hemorrhage after venipuncture suggests a hemostatic disorder. A hemostatic disorder also is suspected with petechiae, purpura, ecchymoses, body cavity hemorrhage, or the presence of excessive hematoma formation. A family history of bleeding or if the patient has experienced previous bleeding diatheses recently or as a young animal suggests a hemostatic disorder as well. Any evidence of excessive bleeding or bleeding in excess of the anticipated amount of hemorrhage from trauma should increase suspicion for a hemostatic disorder. Sudden onset of dyspnea or acute organopathy suggests a thrombotic process.



3 List the components of hemostasis in sequence.


The process of hemostasis is a diverse interplay among the vascular wall or endothelial cells, circulating platelets, coagulation proteins or factors, and the factors confining hemostasis to the appropriate area—the fibrinolytic system. Once endothelial damage has occurred and there is subendothelial collagen exposure, the sequence of appropriate hemostasis is initiated.


The endothelial response, vascular constriction, occurs first, followed quickly by the attachment of platelets to exposed subendothelial collagen, called platelet adhesion. Platelet adhesion is followed by recruitment of additional platelets to the area, or platelet aggregation. Stabilization of the platelet plug then occurs through the activation of the coagulation proteins or factors, resulting in coagulation or fibrin formation. Fibrin encompasses the platelet plug and provides the superstructure on which healing occurs. To keep the clot localized once fibrin formation has occurred, the backup fibrinolytic system is activated. Fibrinolysis ultimately prevents extension of the fibrin clot and breaks down fibrin to produce fibrin degradation products (FDPs) and fibrin split products (FSPs).



4 List activities of the endothelial cells.


Endothelial cells are among the most metabolically active cells in the mammalian body. Endothelial cells are involved with the material transfer of metabolic substances of varied size between blood and tissues. They also provide a relative barrier to blood cells, plasma, macromolecules, and particulate matter. Endothelial cells synthesize or metabolize numerous mediators, including von Willebrand factor, fibronectin, proteoglycans, and serotonin. These cells maintain thromboresistance, mediate vascular repair, and mediate cell migration and proliferation as well as thrombolysis. Endothelial cells also process antigen in cellular immunity.



5 Which clinical problems are associated with endothelial cells?


Potential problems with endothelial cells include ineffective vascular constriction and the lack of surrounding tissue support that occurs in older individuals as muscle mass decreases. Hyperadrenocorticism due to glucocorticoid increase also diminishes muscle mass, resulting in endothelial damage. Immune and septic activity against endothelial cells will cause vasculitis. Heatstroke is a common cause of severe vasculitis.



6 Why is platelet adhesion clinically important?


Platelet adhesion is required for primary hemostasis. Platelets do not adhere to healthy endothelium. Ligands such as collagen and von Willebrand factor are involved with platelet adhesion and are sequestered in the subendothelium. Intact endothelial cells secrete antithrombotic substances such as prostacyclin (PGI2), a prostaglandin, vasodilator, and platelet inhibitor. Platelets are also repelled by the negatively charged surface of intact endothelium. Most canine cases involving poor platelet adhesion are associated with hereditary von Willebrand factor decrease or dysfunction.



7 What is von Willebrand factor, and why is it a clinical concern?


Von Willebrand factor is a large protein synthesized by megakaryocytes and endothelial cells. It is necessary for platelet adhesion and is the carrier protein for coagulation factor VIII—hemophilia A factor. The von Willebrand molecule is made of a series of subunits. Larger subunits, stored in endothelial cells, are most effective binding platelets. These subunits are released in response to various stimuli. Deficiency of von Willebrand factor results in lack of platelet adhesion, resulting in primary hemostatic dysfunction.



8 Why is platelet aggregation clinically important?


When a blood vessel is damaged, vascular constriction and platelet adhesion begin to reduce the rate of blood flow through that vessel. As platelets adhere, they begin to change physically, promoting platelet-to-platelet aggregation. Platelets also begin to produce and secrete platelet chemotactic substances, effectively recruiting other platelets to the injured area. The most important of these is thromboxane A2 (TXA2), a product of arachidonic acid metabolism. Prostacyclin is a platelet antiaggregant. When primary hemostasis is in balance, PGI2 and TXA2 allow platelet aggregation to occur at the injury site but prevent platelet aggregation distal to the site. The clinical use of most nonsteroidal antiinflammatory drugs (NSAIDs) interferes with the production of these two substances, resulting in inbalance and lack of primary hemostasis.

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Aug 26, 2016 | Posted by in INTERNAL MEDICINE | Comments Off on OVERVIEW: HEMOSTATIC COMPONENTS AND DISORDERS

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