Haemostasis Flashcards
What is haemostasis?
- Complex system of pathways designed to keep blood fluid contained within the blood vessels
- These complex pathways maintain delicate balance between tendency to bleed to much and tendency to clot too much (thrombosis)
- Haemostasis is a balance between coagulants and anticoagulants/fibrinolysis
- Thrombosis caused by upregulation of fibrinolysis or anticoagulants..?
Processes in haemostasis
Primary haemostasis: Occurs after damage to vessel wall, involving vasoconstriction and platelet adhesion, followed by further platelet aggregation to form a platelet plug.
Secondary haemostasis: Involves activation of coagulation system, resulting in formation of fibrin strands. Fibrin strands are cross linked to stabilise the platelet plug.
Fibrinolysis: Involves activation of plasminogen. Plasminogen breaks down clots.
Endothelial Cells - Pro-coagulant functions
-Release tissue factor when damaged, which activates the coagulation system.
-Produce von Willebrand factor (vWF), which tethers platelets to the endothelium.
-Produce P-selectin, which activates platelets.
-Produce plasminogen activator inhibitor 1 (PAI-1)
Endothelial Cells - Anticoagulant function
-Produce heparan sulfate, which inhibits activated clotting factors.
-Produce prostacyclin and nitric oxide, which inhibit platelet aggregation and induce vasodilation.
-Produce tissue plasminogen activator (tPA), which promotes dissolution of fibrin
Platelets and their role in haemostasis.
What receptors do platelets have?
What types of granules do platelets contain?
-Produced from megakaryocytes in bone marrow
- Each megakaryocyte produces 1000-2000 platelets that remain in circulation for around 10 days
- Activated platelets have receptors for:
-vWF, which tethers them to endothelial cells
-Subendothelial collagen.
-Fibrinogen
-Thrombin
-Thromboxane
-Plasma coagulation factors such as prothrombin
and factors V,X and X
-Platelets contain two different types of granules:
- alpha granules: release vWF and fibrinogen
- platelet dense granules: release ADP promoting platelet aggregation
Clotting Factors
- Coagulation factors are numbered in the order of their discovery. There are 13 numerals but only 12 factors. Factor VI was subsequently found to be part of another factor.
Factor I - fibrinogen
Factor II - prothrombin
Factor III - tissue thromboplastin (tissue factor)
Factor IV - ionized calcium ( Ca++ )
Factor V - labile factor or proaccelerin
Factor VI - unassigned
Factor VII - stable factor or proconvertin
Factor VIII - antihemophilic factor
Factor IX - plasma thromboplastin component, Christmas factor
Factor X - Stuart-Prower factor
Factor XI - plasma thromboplastin antecedent
Factor XII - Hageman factor
Factor XIII - fibrin-stabilizing factor
Producing Factors: The Liver and Vitamin K.
What can cause Vitamin K deficiency?
-liver uses Vitamin K to produce Factors II, VII, IX, and X.
-Dietary vitamin K widely available from plant and animal sources, also produced by normal intestinal flora.
-Vit K deficiency is rare but may occur: in newborns because they must first develop normal flora to produce Vitamin K, or when the flora is disturbed by broad-spectrum antibiotics.
3 Overlapping phases of blood coagulation. Cell Based Model
-Initiation phase: Where a small amount of thrombin is generated via the TF-induced pathway to activate platelets and coagulation co-factors FV and FVIII and there activated forms (measured by PT)
-Amplification: Coagulation factors bind to receptors and activated platelets
-Propagation: Thrombin is formed via both the contact and TF pathways in order to generate large amounts of thrombin that will transform fibrinogen to fibrin (measured by APTT)
Summary of the process of clot forming/ the coagulation system
- Bleeding initially stopped by vasocontriction with adhesion and aggregation of platelet at the site of vessel injury
- Coagulation system activated by tissue factor
- Causes conversion of FVII to FVIIa which initiate catalytic cascade of reactions leading to rapid generation of small amounts of thrombin
- Small amounts of thrombin trigger a positive feedback loop by activating factors FXI, FVIII and FV causing generation of much larger amounts of thrombin
- Thrombin acts on fibrogen to form fibrin and activates platelets and fibrinolytic inhibtiors. FXIII crosslinks fibrin to form a stable clot
Regulation of clotting
Clotting cascade regulated by clotting inhibitor:
- TF-FVIIa inhibited by Tissue factor pathway inhibtior
- FXIa inhibited by Protease nexin 2
- FIXa and FXa inhibited by antithrombin
- FVa inhibtied by activated protein C
- FVIIIa inhibited by Protein S
Regulation of fibrinolysis: Inhibitors of fibrinolysis and their functions
Plasminogen activator inhibitor-1 (PAI-1), which inhibits tPA.
Antiplasmin, which inhibits plasmin.
Plasminogen activator inhibitor-2, which is synthesised by the placenta and inhibits tPA particularly at the end of pregnancy.
Thrombin activatable fibrinolytic inhibitor, which prevents plasminogen from binding to fibrin
features of Disorders of Primary Haemostasis
Onset of bleeding: Spontaneous or immediately after trauma
Site of bleeding: Skin (Petechiae, ecchymoses) Mucous membranes (nasal, oral, GI)
Primary haemostasis Typical Clinical Presentation
Gingival Bleeding: Bleeding from the gums (gingival bleeding) is another characteristic feature of primary hemostasis disorders. Patients may experience bleeding during brushing, flossing, or dental procedures.
Heavy Bleeding during periods, easy bruising…
Clinical Conditions causing significant platelet disorders
-von Willebrand Disease
-Acquired: Medication, disease related
-Surgically Induced - CABG
-Congenital Platelet Defects
-Vascular Disease
Diagnosis of Primary Haemostasis: PFA-100
CEPI and CADP
-PFA-100 assesses the ability of platelet to adhere to damaged blood vessels and form a platelet plug which is the initial step in the process of hemostasis.
- Catridge Perparation
- Blood Sample Collection
- Blood Sample analysis: blood sample is immediately pipetted onto the membrane-coated apertures within the cartridge.
- Shear stress application: applies high shear stress to blood sample as it glows thorugh the narrow membrane-coated apertures stimulating the conditions of blood flow through damaged blood vessels where platelet adhesion and aggregation are required to stop bleeding. For Collagen/Epinephrine (CEPI) primary screening cartridge, epinephrine is included to further stimulate platelet activation and aggregation.
- Platelet Plug Formation: as blood flows through apertures platelets adhere to collagen-coated membrane and aggregate to form a platelet plug, occluding the aperture. Time taken for this platelet plug formation measured and recorded by the instrument.
- Closures Time Measure: Time taken for platelet plug to form and occlude aperture is the closure time. Used as an indirect measure of platelet function and primary hemostasis. Prolonged time suggests platelet dysfunction or impairment in primary haemostasis
- Collagen/ADP (CADP) cartridge in the Platelet Function Analyzer-100 (PFA-100) system is designed to evaluate platelet function by assessing platelet adhesion and aggregation in response to a combination of collagen and adenosine diphosphate (ADP). Can help differentiate dysfunction due to aspirin.
symptomsofDisorders of Secondary Haemostasis (Coagulation Factor Problem)
-Onset of bleeding – delayed after trauma
-Site of bleeding – deep tissues
-Skin – haematomas
-Mucous membranes – rare
-Common sites – joint, muscle, retroperitoneal
