PSIO 202, lecture 4 - PSIO 202 PSIO Human Anatomy and...

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Unformatted text preview: PSIO 202 PSIO Human Anatomy and Physiology Lecture 4 Cardiovascular System: The Blood; Platelets and Clotting Objectives and Reading Assignment • Reading: Tortora, pages 703-707 • Objectives – Describe the structure, function and production of platelets – Teach a friend about the series of chain reactions that control blood loss after an injury – Describe the series of events leading to platelet plug formation – Diagram and compare and contrast the key elements of the extrinsic and intrinsic clotting pathways Platelets (thrombocytes) • Cell fragments that circulate for 5-9 days, then die. • 2/3 of mature platelets circulate, 1/3 reside in the spleen. • “Thrombosis” refers to clot formation; A clot is called a thrombus. • An embolus is a circulating clot. • “Hemorrhage” is severe, uncontrolled bleeding. “Thrombocytopoiesis” • Myeloid stem cells produce megakaryocytes • They have a diameter of 160 um • “thrombopoietin”, or TPO, causes fragments to slough off the megakaryocyte • 2,000-3,000 fragments, or platelets, enter the circulation • Platelet diameter is roughly 2-4 um, and they are about 1 um thick. Hemostasis • A series of reactions designed for stoppage of bleeding • During hemostasis, three phases occur in rapid sequence – Vascular spasms – immediate vasoconstriction in response to injury – Platelet plug formation – Coagulation (blood clotting) Vascular Spasm • Spasm is the spontaneous contraction of the smooth muscle that lines the walls of blood vessels • This presses the endothelial surfaces together • This process is effective only in very small vessels (arterioles; Capillaries) Platelet Plug Formation • Platelets NORMALLY do not stick to each other or to the endothelial lining of blood vessels • Upon damage to a blood vessel, platelets: – Stick to exposed collagen fibers and are activated, allowing them to stick to one another – Liberate thromboxane A2,serotonin and ADP, which attract & activate still more platelets – ADP makes platelets sticky, while thromoboxane A2 and serotonin cause vasoconstriction – This results in formation of the platelet plug Platelet Adherence to Damaged Collagen Release of ADP and Thromboxane Platelet Plug Formation Coagulation • A set of reactions in which blood is transformed from a liquid to a gel • Coagulation follows intrinsic and extrinsic pathways Coagulation • The final thee steps of this series of reactions are: – Prothrombin activator is formed – Prothrombin is converted into thrombin – Thrombin catalyzes the joining of fibrinogen into a fibrin mesh The Common Pathway • The common pathway is responsible for the conversion of thrombin into fibrin • Thrombin has two main functions: – Converts fibrinogen (soluble) to fibrin (insoluble, stable threads) – Activates factor XIII, which stabilizes the fibrin network Formation of Clotting Factors • Prothrombin, fibrinogen, and factors V, VII, IX, and X are synthesized in the liver • Vitamin K is needed for the synthesis of factors II, VII, IX, and X. As a result, vitamin K deficiency can lead to failure of blood clotting • Individuals lacking factor VIII are hemophiliacs Fibrinolysis • Clots must be dissolved so that they do not enter the circulation as an embolus • The dissolution of a clot is known as fibrinolysis. • Tissue plasminogen activator, thrombin and plasminogen all react to form plasmin, which then digests the fibrin strands and breaks the clot. ...
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