Haemostasis Flashcards
What is haemostasis?
The stopping of flow of blood.
Three steps of haemostasis?
Vasoconstriction. Formation of unstable platelet plug. Formation of stable fibrin mesh. Breakdown of clot and vessel repair.
What is primary haemostasis?
Formation of unstable platelet plug.
What is secondary haemostasis?
Formation of stable fibrin mesh.
What are platelets?
Discoid (lens shaped), non-nucleated, granule-containing cells that are derived from myeloid stem cells.
Circulating lifespan of a platelet?
10 days.
What is the first way platelets can bind to collagen?
Directly using their glycoprotein Ia receptor.
What is the second way platelets can bind to collagen?
Indirectly using their glycoprotein Ib receptor to bind to Von Willebrand factor (VWF).
Where is Von Willebrand factor (VWF) found?
Produced and released from endothelial cells. Produced in megakaryocytes and released from alpha granules in platelets.
What happens after platelet adhesion to collagen?
They become activated.
What happens after platelets become activated?
They become more rounded and form spicules to encourage platelet-platelet interaction. Contents of platelet granules are also released.
How are the contents of platelet granules released?
Platelet membrane is invaginated to form a surface-connected cannalicular system through which the contents of platelet granules are released.
What are released from platelet granules?
ADP, fibrinogen and von Willebrand factor.
What else along with content from granules is produced and released from platelets?
Thromboxane A2
What does thromboxane A2 do?
Vasoconstrictor and aids with platelet aggregation.
What 2 main molecules encourage platelet platelet aggregation?
ADP and Thromboxane A2
What are the two main steps of primary haemostasis?
Platelet adhesion and platelet aggregation.
What receptor does ADP bind to promote platelet aggregation?
P2Y12 receptor found on platelets.
What receptor does Thromboxane A2 bind to promote platelet aggregation?
Thromboxane A2 receptor
What does fibrinogen do in primary haemostasis?
Binds to glycoprotein IIb/IIIa which further activates platelets. Also has a key role in linking platelets together to form a platelet plug.
How is platelet activation and aggregation counterbalanced?
Active flow of blood and the release of prostacyclin (PGI2) from endothelial cells; prostacyclin is a powerful vasodilator and suppresses platelet activation, thus preventing inappropriate platelet aggregation.
How does aspirin work as an anti platelet drug?
Aspirin inhibits the production of thromboxane A2 by irreversibly blocking the action of cyclo-oxygenase (COX) in platelets, resulting in a reduction in platelet aggregation.
How does clopidogrel work?
Blocks P2Y12 Receptor and so ADP can’t bind therefore reducing platelet aggregation.
VWF role apart from helping platelets with adhesion to collagen?
Carrier for factor VIII.
Where are most clotting factors made?
In the liver
What clotting factors aren’t produced in the liver?
Factor VIII and VWF.
Why is vitamin K essential?
Carboxylation of glutamic acid residues which is essential for clotting factor function.
Which factors require vitamin K?
Factors II (prothrombin), VII, IX and X
What factor initiates coagulation?
Tissue factor (TF).
What is important about tissue factor (TF)?
TF is mainly located at sites that are not usually exposed to the blood under normal physiological conditions. As a result, blood only encounters TF at sites of vascular injury.
What are the three phases of coagulation?
Initiation, amplification and propagation.
What does TF bind to?
Factor VIIa.
What does binding of TF do?
Activates factors IX,X. This leads to factor II (prothrombin) being converted to small amounts of factor IIa (thrombin).
What do calcium ions do?
Aid in binding of activated clotting factors to the phospholipid surfaces of platelets.
What happens in the amplification step?
Small amount of thrombin mediates the activation of the co-factors V and VIII, the zymogen factor XI and platelets.
What happens in the propagation phase?
Factor XI converts more factor IX to IXa, which in concert with factor VIIIa,amplifies the conversion of factor X to Xa, and there is consequently a rapid burst in thrombin generation which cleaves the circulating fibrinogen (soluble) to form the insoluble fibrin clot.
What are the three main natural anticoagulant molecules?
Protein C, Protein S and antithrombin.
What activates protein C?
Thrombin binding to thrombomodulin on the endothelial cell surface.
What does activated protein C do?
Inactivates factors Va and VIIIa in the presence of a co-factor protein S.
What does antithrombin do?
Inactivates thrombin and factor Xa.
What makes antithrombin work?
Binding of antithrombin to endothelial cell-associated heparins.
Main anticoagulant drugs?
Heparin, Warfarin and direct oral anticoagulants.
How does heparin work?
Works indirectly by potentiating the action of antithrombin leading to the inactivation of factors Xa and IIa (thrombin).
How is heparin administered?
Intravenously or by subcutaneous injection.
How does warfarin work?
Vitamin K antagonist that works by interfering with protein carboxylation. It therefore reduces synthesis of functional factors II, VII, IX and X by the liver.
How is warfarin administered?
Given as an oral tablet and its anticoagulant effect needs to be monitored by regular blood testing
Why does warfarin take longer to take effect compared to heparin?
Reduces synthesis of coagulation factors in the liver rather than inhibiting existing factor molecules.
How do direct oral anticoagulants?
Directly inhibit either thrombin or factor Xa.
What lyses fibrin?
Plasmin.
What is key for fibrin to be lysed?
Both plasminogen and t-PA need to bind to lysine residues on fibrin.
Why do plasmin levels need to be regulated?
Plasmin isn’t specific for fibrin. They can cause break down fibrinogen, factor Va and VIIIa.
What inhibits plasmin?
alpha 2 macroglobulin and anti-plasmin.
When would thrombolytic agents like t-PA be used?
To treat PE or ischaemic stroke.
Risks of thromobolytic agents?
Bleeding.
When would anti-fibrinolytic drugs be used?
Trauma, surgical patients and patients with bleeding disorders.
Examples of anti-fibrinolytic drugs?
Tranexamic acid and aminocaproic acid.
How do anti-fibrinolytic drugs work?
Competitive inhibition. Bind to plasminogen and so plasminogen can’t bind to fibrin and so fibrin isn’t lysed.
When is prothrombin time used?
To evaluate extrinsic pathway and common pathway.
What factors does pro thrombin test for?
VII, X, V, II and fibrinogen.
When performing a pro thrombin time test, sodium citrate is used. Why is sodium citrate used?
To prevent clotting by chelating calcium.
When performing a pro thrombin time test, the sample is centrifuged. Why?
To produce platelet poor plasma.
How is prothrombin time done?
A source of TF and phospholipid is added to the citrated plasma sample, together with calcium to start the reaction; the length of time taken for the mixture to clot is recorded.
What does recombinant thromboplastin contain?
Tissue factor and phospholipids.
How is APTT done?
Contact activator, together with phospholipid, is added to the citrated plasmasample followed by calcium; the time taken for this mixture to clot is measured
What pathway does APTT (Activated partial thromboplastin time) test for?
Intrinsic and common pathway.
What factors does APTT (Activated partial thromboplastin time) test for?
XII, XI, IX, VIII, X, V, II and fibrinogen.
Why is international normalised ratio (ITR) used as a way to express prothrombin time results?
INR is used because different thromboplastin reagents are used by different laboratories. All laboratories would be expected to obtain the same INR result for a given sample irrespective of the source of thromboplastin.
Difference in preparation of PT and APTT?
PT tissue factor is used. APTT a contact activator is used.
Prolonged APTT causes?
Haemophilia A (Factor VIII deficiency). Haemophilia B (Factor IX deficiency). Haemophilia C (Factor XI deficiency)
Increased bleeding causes?
Thrombocytopenia. Use of anti platelet drugs (e.g aspirin). Reduction in coagulation factors. Increased fibrinolysis (Use of t-PA).
What is disseminated intravascular coagulation (DIC)?
Generalised and uncontrolled activation of coagulation followed by marked activation of the fibrinolytic system. This activation results from the expression of TF within the circulation.
Causes of reduction of clotting factors?
Liver disease, Disseminated intravascular coagulation
Increased fibrinolysis causes?
Disseminated intravascular coagulation or use of t-PA.
3 principle causes of thrombosis (Virchow’s triad)
Stasis of blood flow (veins). Endothelial injury (artery). Hyper-coagulability (both).
Causes of thromobosis?
Inherited thrombophilia (reduced levels of anticoagulant proteins such as antithrombin). Reduced fibrinolytic activity (seen in pregnancy). Hyperviscocity (polycthaemia).
Why can’t anti thrombin activated by low molecular weight heparin inhibit thrombin (factor IIa), but can only inhibit clotting factor Xa?
Inactivation of IIa (thrombin)
requires longer chains of heparin
chains, which are able to wrap
around both the antithrombin and
thrombin. LMW heparin mainly
inactivates Xa.
What factor is increased in pregnancy?
Factor VIII
What does the single point mutation causing factor V Leiden result in?
Factor V Leiden makes factor V more resistant to inactivation by protein C. Type of inherited thrombophilia.
