Platelets, Blood Clotting & Plasma Proteins PDF

Platelets, Blood Clotting & Plasma Proteins Sept. 2023 Contents Role of Platelets in Haemostasis Role of Normal Endothelium in the Suppression of Haemostasis Primary & Secondary Haemostasis Assessment of Haemostatic Dysfunction Clotting Factor Deficiencies Inhibitors of Haemostasis & Anticoagulants...

Platelets, Blood Clotting & Plasma Proteins Sept. 2023 Contents Role of Platelets in Haemostasis Role of Normal Endothelium in the Suppression of Haemostasis Primary & Secondary Haemostasis Assessment of Haemostatic Dysfunction Clotting Factor Deficiencies Inhibitors of Haemostasis & Anticoagulants Plasma Proteins & their Functions What is Haemostasis? A series of reactions involving platelets and clotting factors that result in the cessation of excessive or unwanted bleeding and promotes wound healing. Primary Haemostasis Begins with vasoconstriction Results in formation of a temporary platelet plug Primary response is ineffective for any duration of time Secondary Haemostasis Secondary haemostasis is the enzymatic activation of the coagulation proteins to produce fibrin from fibrinogen, thereby stabilizing the fragile clot formed during primary haemostasis The cascade of coagulation reactions is a result of complexes that form between enzymes, substrates, phospholipid surfaces, and cofactors Stages of Haemostasis Prevention of blood loss 1. Vascular constriction 2. Formation of a temporary platelet plug 3. Activation of the coagulation cascade resulting in the formation of a blood clot 4.Formation of a fibrin plug growth of fibrous tissue into the blood clot to permanently close the hole in the vessel Steps in Haemostasis 1. Formation of primary platelet plug. – Masses of platelets accumulate at the site of the vessel wall injury, stick to the collagen in exposed sub-endothelial tissue (adhesion) and then aggregate (cohesion). 2. Conversion of primary plug into stable permanent plugs reinforced by a supporting fibrin clot. 3. Lysis of fibrin. Hematopoietic Growth Factors Thrombopoietin A glycoprotein hormone produced mainly by liver and kidney that regulates the production of platelets in bone marrow It stimulates the production and differentiation of Megakaryocytes PLATELETS Non-nucleated biconvex discs Life span of 8-10 days Membrane have adhesive functions Also have contractile properties Have specific surface antigens called HPA Suppresion of haemostasis by Normal Endothelium Because haemostatic reactions are harmful if they occur intralumenally to form a clot (thrombus), endothelial cells suppress haemostasis in several ways. Suppresion of haemostasis by Normal Endothelium 1. Physically separating blood from underlying vessel wall that can initiate blood coagulation 2. Synthesizing & releasing prostacyclin (PGI2) which diminishes platelet responsiveness to activating stimuli. 3.Provide surface binding sites for reactions that turn off blood coagulation. For example, A. Thrombomodulin B. Antithrombin III Function of Haemostasis Hemodynamic Stability – Balance between the procoagulant and anticoagulant pathway – Compromised equilibrium may lead to thrombotic/bleeding complications. – The hemostatic system also helps in wound healing. Function of Haemostasis Cardiovascular System – PGA1 and PGA2 cause peripheral arteriolar dilation. – Prostacyclin produces vasodilation and inhibits platelet aggregation. – Thromboxane A2 and endoperoxides promote platelet aggregation and cause vasoconstriction. – The balance between the prostacyclin and thromboxane A2 determines the degree of platelet plug formation. Thus, prostaglandins greatly influence temporary hemostasis. Primary Haemostasis Primary haemostasis is defined by platelet adhesion to exposed collagen within the endothelium of the vessel wall. Mediators of platelet adhesion: – Platelet membrane receptor (Glycoprotein Ib (GPIb)) – Von Willebrand’s factor The interaction between von Willebrand factor and GPIb platelet receptors will activate platelets, leading to degranulation of platelet content into the intravascular space. This will recruit more platelets to the site of injury, along with other effects that facilitate hemostasis. Platelet aggregation occurs when activated platelets express GPIIb/IIIa receptors that bind fibrinogen to cross-link platelets together and further solidify the platelet plug. Platelet Functions Platelet adhesion Platelet activation Granule release or secretion Platelet aggregation Coagulation Factors 3 categories – based on haemostatic function Substrate – Fibrinogen (Factor I) Cofactors – Labile factor (factor V) – Factor VIII:C (antihaemophilic factor, coagulant portion) Enzymes – Serine proteases IIa, VIIa, IXa, Xa, XIa, XIIa, prekallikrein, Factor II (thrombin) – Transaminase Factor XIIIa (fibrin-stabilizing factor) Extrinsic pathway activated by substances “outside” blood (tissue factors) Fewer steps than the intrinsic pathway Occurs rapidly (within seconds) Less potent than the intrinsic Categories of Haemostatic Defects 3 broad categories – Vascular and platelet disorders – Coagulation factor deficiencies or specific inhibitors – Fibrinolytic disorders Common Laboratory Screening Tests for Haemostatic Disorders Platelet count Peripheral blood smear examination Template bleeding time (TBT) Prothrombin time (PT) Activated partial thromboplastin time (APTT) Thrombin time (TT) Common Laboratory Screening Tests for Haemostatic Disorders Prothrombin time (PT) – Measures factors of the extrinsic and common pathways (factors VII, X, V, II, I) – Used to detect early viamin K defieciencies Activated partial thromboplastin time (APTT) – Measures factors of the intrinsic and common pathways of blood coagulation – deficiencies or inhibitors result in prolonged APTT Both PT and APTT are prolonged with abnormality of shared factors BLEEDING TIME Provides assessment of platelet count and function NORMAL VALUE 2 - 8 minutes CLOTTING TIME Provides assessment of blood clotting factors NORMAL VALUE 5 -10 minutes PROTHROMBIN TIME Measures effectiveness of the Extrinsic pathway and Common pathway NORMAL VALUE 10 -15 seconds PARTIAL THROMBOPLASTIN TIME Measures effectiveness of the Intrinsic pathway NORMAL VALUE 25 - 40 seconds THROMBIN TIME Time for Thrombin to convert fibrinogen to fibrin A measure of fibrinolytic pathway (Thrombin-activated fibrinolysis inhibitor) NORMAL VALUE 9 -13 seconds Consequences of Factor Deficiency Coagulation cannot proceed at a normal rate Initiation of the next subsequent reaction is delayed The time required for the clot to form is prolonged Bleeding from the injured vessel continues for a longer time (or there may be a physiologic tendency toward thrombosis present if the patient is deficient in factor XII). FACTOR DEFICIENCIES Hemophilia A (Classic Hemophilia) 80-85% of all Hemophiliacs Deficiency of Factor VIII Lab results - Prolonged PTT Hemophilia B (Christmas Disease) 10-15% of all Hemophiliacs Deficiency of Factor IX Lab test - Prolonged PTT FACTOR DEFICIENCIES Von Williebrand’s disease Deficiency of VWF and some amount of Factor VIII Lab results - Prolonged BT, PTT INHIBITORS OF HEMOSTASIS Primary – Prostacyclin and nitric oxide (naturally occurring); aspirin (artificial) Secondary – Anti thrombin III, heparin, tissue factor inhibitor Tertiary – Thrombin-activated fibrinolytic inhibitor (TAFI), plasminogen-activated inhibitor (PAI), α-2 antiplasmin ANTICOAGULANTS Coumarin - Prevents thromboembolic events Vit K antagonist Used in vivo Therapy monitored by PT time Heparin - Activates anti thrombin III Used in vivo and in vitro Therapy monitored by PTT time Chelating agents – e.g. oxalates, citrates Remove calcium from blood Used in vitro Anticoagulants Chelate Ca++ ions – EDTA – Citrate Inhibit clotting enzyme – Heparin Inhibit glycolysis – Fluoride OTHER DISORDERS MALABSORPTION Various intestinal diseases will interfere with bile acid metabolism. Bile acids are required for Vit K absorption so you will see a deficiency in Vit K dependent coagulation factors (II,VII,IX,X). OTHER DISORDERS BROAD-SPECTRUM ANTIBIOTICS Change in intestinal flora which might decrease vitamin K production. Vitamin K is necessary for the liver to produce coagulation factors II,VII,IX,X. PLASMA PROTEINS About 7g/dl Made up of albumin (4.5g/dl) and globulin (2.5g/dl) Synthesized in the liver, B-lymphocytes, macrophages, endothelial and intestinal cells Synthesis is increase by increased amino acids, inflammation and antigens Deficiency results from liver diseases and dietary insufficiency Plasma and Plasma Proteins Plasma – liquid portion of blood – serum – remaining fluid when blood clots and the solids are removed Fibrinogen absent 3 major categories of plasma proteins – albumins – smallest and most abundant contributes to viscosity and osmolarity, influences blood pressure, flow and fluid balance – globulins (antibodies) provide immune system functions alpha, beta and gamma globulins – fibrinogen precursor of fibrin threads that help form blood clots Plasma proteins formed by liver – except globulins (produced by plasma cells) FUNCTIONS OF PLASMA PROTEINS Maintenance of colloid osmotic pressure Transport of substances Protein reserve Maintenance of blood viscosity Act as buffers Function as antibodies Acute phase proteins Levels increase in response to inflammatory and neoplastic conditions e.g. C-reactive proteins, fibrinogen, ceruloplasmin, alpha-1 antitrypsin etc. Negative acute phase proteins – levels decrease in response to certain inflammatory conditions e.g. albumin, transthyretin, transferrin etc. Clinical significance Hypoproteinaemia – liver disease, nephrotic syndrome, dietary insufficiency, extensive skin burns, pregnancy, malignancy etc. Hyperproteinaemia – excessive vomiting, diarrhoea, diabetes insipidus, pyloric stenosis, diuresis etc. Clinical indications for assessment of acute phase proteins Presence of inflammatory disease Monitoring of therapeutic effectiveness Post surgical follow-up in patients at risk of post operative infections