Physiology of Platelets and Hemostasis PDF

Physiology of Platelets and Hemostasis 2nd year medical student 2024- 2025 By Dr. Nidhal Gorgees Hanna Unit of Physiology/ College of Medicine/ HMU  Objectives Describe the platelets and their structure. List the functi...

Physiology of Platelets and Hemostasis 2nd year medical student 2024- 2025 By Dr. Nidhal Gorgees Hanna Unit of Physiology/ College of Medicine/ HMU  Objectives Describe the platelets and their structure. List the functions of platelets describe the hemostasis and its different stages Summarize the mechanism of blood coagulation  INTRODUCTION Platelets or thrombocytes are the formed elements of blood. Small colorless, non-nucleated, considered to be the fragments of cytoplasm. Size of Platelets Diameter : 2.5 µ Volume : 7.5 cu µ Shape of Platelets Normally, platelets are of several shapes, Inactivated platelets are without processes or filopodia and the activated platelets develop filopodia  STRUCTURE AND COMPOSITION 1. Cell membrane 2. Microtubules 3. Cytoplasm. Cell membrane Contains different lipids, carbohydrates and proteins. Of these substances, glycoproteins and phospholipids are functionally important. 1. Glycoproteins Glycoproteins prevent the adherence of platelets to normal endothelium, but accelerate the adherence of platelets to collagen and damaged endothelium in ruptured blood vessels. 2. Phospholipids Phospholipids accelerate the clotting reactions. Form the precursors of thromboxane A2  Cytoplasm of platelets Cytoplasm of platelets contains the cellular organelles, also contains some chemical substances such as proteins, enzymes, hormonal substances, etc. Proteins 1. Contractile proteins i. Actin and myosin: Contractile proteins, which are responsible for contraction of platelets. ii. Thrombosthenin: Third contractile protein, which is responsible for clot retraction. 2. von Willebrand factor: Responsible for adherence of platelets and regulation of plasma level of factor VIII. 3. Fibrin-stabilizing factor: A clotting factor. 4. Platelet-derived growth factor (PDGF): Responsible for repair of damaged blood vessels and wound healing. 5. Platelet-activating factor (PAF): Causes aggregation of platelets during the injury of blood vessels. Hormonal Substances 1. Adrenaline 2. serotonin 3. Histamine. Other Chemical Substances 1. Glycogen 2. Substances like blood group antigens 3. Inorganic substances such as calcium, copper, magnesium and iron.  NORMAL COUNT AND VARIATIONS Normal platelet count is 250,000/cu mm of blood. It ranges (200,000 and 400,000/cu mm).„ PHYSIOLOGICAL VARIATIONS 1. Age: Platelets are less in infants (150,000 to 200,000/cu mm) and reaches normal level at 3rd month after birth. 2. Sex: There is no difference in the platelet count between males and females. In females, it is reduced during menstruation. 3. High altitude: Platelet count increases. 4. After meals: the platelet count increases. Platelet under electron microscope  PROPERTIES OF PLATELETS Platelets have three important properties (three ‘A’s): 1. Adhesiveness is the property of sticking to a rough surface. 2. Aggregation is the grouping of platelets. 3. Agglutination is the clumping together of platelets.  FUNCTIONS OF PLATELETS 1. ROLE IN BLOOD CLOTTING 2. ROLE IN CLOT RETRACTION 3. ROLE IN PREVENTION OF BLOOD LOSS (HEMOSTASIS) „ 4. ROLE IN REPAIR OF RUPTURED BLOOD VESSEL 5. ROLE IN DEFENSE MECHANISM „  LIFESPAN AND FATE OF PLATELETS Average lifespan of platelets is 10 days. It varies between 8 and 11 days. Platelets are destroyed by tissue macrophage system in spleen. So, splenomegaly (enlargement of spleen) decreases platelet count and Splenectomy (removal of spleen) increases platelet count.  HEMOSTASIS DEFINITION Hemostasis is defined as arrest or stoppage of bleeding. „ STAGES OF HEMOSTASIS When a blood vessel is injured, the injury initiates a series of reactions, resulting in hemostasis. It occurs in three stages 1. Vasoconstriction 2. Platelet plug formation 3. Coagulation of blood.  1. VASOCONSTRICTION Immediately after injury, the blood vessel constricts The endothelium is damaged and the collagen is exposed. Platelets adhere to this collagen and get activated. Adherence of platelets to the collagen is accelerated by von Willebrand factor. 2. PLATELET PLUG FORMATION 1. Initially, platelets contact and stick to parts of a damaged blood vessel (platelet adhesion). 2. Due to adhesion, the platelets become activated, and they begin to liberate their contents. This phase is called the platelet release reaction. Serotonin and thromboxane A2 function as vasoconstrictors, causing and sustaining contraction of vascular smooth muscle, which decreases blood flow through the injured vessel. 3. The release of ADP makes other platelets in the area sticky,(platelet aggregation). Eventually, the accumulation and attachment of large numbers of platelets form a mass called a platelet plug. PLATELET PLUG FORMATION Collagen fibers and damaged endothelium Liberated ADP, serotonin, and thromboxane A2 Platelet plug  3. COAGULATION OF BLOOD During this process, the fibrinogen is converted into fibrin. Fibrin threads get attached to the loose platelet plug, which blocks the ruptured part of blood vessels and prevents further blood loss completely. STAGES OF HEMOSTASIS  Coagulation of Blood DEFINITION Coagulation or clotting is defined as the process in which blood loses its fluidity and becomes a jelly-like mass few minutes after it is shed out or collected in a container.  Stages of Blood Clotting Three stages: 1. Formation of prothrombin activator 2. Conversion of prothrombin into thrombin 3. Conversion of fibrinogen into fibrin. Thirteen clotting factors are identified: Factor I Fibrinogen Factor II Prothrombin Factor III Thromboplastin (Tissue factor) Factor IV Calcium Factor V Labile factor (Proaccelerin or accelerator globulin) Factor VI Presence has not been proved Factor VII Stable factor Factor VIII Antihemophilic factor (Antihemophilic globulin) Factor IX Christmas factor Factor X Stuart-Prower factor Factor XI Plasma thromboplastin antecedent Factor XII Hageman factor (Contact factor) Factor XIII Fibrin-stabilizing factor (Fibrinase)  BLOOD CLOT „DEFINITION AND COMPOSITION OF CLOT Blood clot is defined as the mass of coagulated blood which contains RBCs, WBCs and platelets entrapped in fibrin meshwork. RBCs and WBCs are not necessary for clotting process. However, when clot is formed, these cells are trapped in it along with platelets. The trapped RBCs are responsible for the red color of the clot. CLOT RETRACTION After the formation, the blood clot starts contracting. And after about 30 to 45 minutes, the straw-colored serum oozes out of the clot. This process is called clot retraction. Contractile proteins, namely actin, myosin and thrombosthenin are responsible for clot retraction. FIBRINOLYSIS Lysis of blood clot inside the blood vessel is called fibrinolysis. It helps to remove the clot from lumen of the blood vessel. This process requires a substance called plasmin or fibrinolysin. THANK YOU