Week 31 Haemostasis Flashcards
What is Haemostasis?
Haemostasis is the process which causes bleeding to stop with the aim of keeping it within a damaged blood vessel.
Opposite of haemostasis?
Haemorrhage (escape of blood from circulation)
What kind of response is haemostasis?
Positive feedback loop - Platelet activation and coagulation pathways positively re-enforce each other until the breach is sealed.
What is the 3 stages of haemostasis?
- Vascular spasm –> constriction of damage blood vessel –> blood slowed
- Formation of platelet plug –> Platelets bind to collagen at injury site –> stick together and chemicals released to encourage more platelet binding
- Blood coagulation –> clotting cascade –> intrinsic and extrinsic pathways occur
What are platelets?
basophilic, non-nucleated fragments of megakaryocytes.
How long do platelets last in circulation?
7-10 days
What is Thrombopoiesis?
Platelet production
What drives the body to produce more platelets?
Thrombopoietin (TPO) (produced when platelet numbers are low
Where does Thrombopoietin act?
Areas of Megakaryocyte differentiation
Where is Thrombopoietin produced?
Liver mainly
What do platelet granules store?
Platelet granules store potent substances involved in inflammation & wound healing processes including: 1. Lysosomal enzymes 2. Coagulation factors 3. Immunologic and adhesion molecules 4. Chemokines and growth factors
What is platelet plugging initiated by?
Von Willebrand factor (vWF) (found in plasma) Binding to collagen and platelets.
What is the three main steps in platelet action?
- Adhesion —> Vascular injury exposes collagen for platelet adherence
- Aggregation —> more platelets join, release platelet activation mediators (eg ADP and thromboxane A2)
- Activation —> when activated platelets produce thrombin which catalysts the initiation of the coagulation cascade, resulting in a mesh-like fibrin deposition to seal the breach.
What are some hormones released by platelets and what are their functions?
- Platelet derived PDGF —> proliferation of vascular endothelial cells, vascular smooth muscle fibres and fibroblasts (to help repair damaged vessel wall)
- ADP —> makes nearby platelets “sticky” (increases platelet deposition into plug)
- Thromboxane A2 —> A prostaglandin that strengthens the fibrin clot, also induces vasoconstriction and sustain smooth muscle contraction to decrease blood flow
- Serotonin —> induces vasoconstriction and sustain smooth muscle contraction to decrease blood flow
What are the three pathways for blood coagulation?
- Extrinsic
- Intrinsic
- Common
What happens in the Extrinsic pathway of blood coagulation?
- Tissue factor is released (within seconds if trauma is severe)
- Numerous reactions with factor VII and X.
- Activated factor X (Xa) combines with factor V in the presence of phospholipids and Ca2+ to form prothrombinase which converts prothrombin to thrombin (a serine protease).
- Thrombin converts fibrinogen to fibrin.
What happens in the Intrinsic pathway of blood coagulation?
- Endothelial cell surfaces damaged expose blood to collagen and can activate XII (XIIa).
- XIIa activates XI (XIa) which in turn activates factor IX (IXa).
- IXa activates X to Xa.
- Platelet phospholipids, factors VIII & V Ca2+ and Xa form prothrombinase.
- Prothrombinase converts prothrombin to thrombin which converts fibrinogen into fibrin
What happens in the common pathway of blood coagulation?
- Activation of factor X
- Factor X activates thrombin
- Thrombin converts fibrinogen to fibrin to stabilise.
- Thrombin activates factor XIII which cross links fibrin for clot stabilisation
Where are clotting factors mainly produced?
Liver
What is the role of Vitamin K in clotting?
Involved in the synthesis of many blood clotting proteins:
1. Prothrombin
2. Factors VII IX and X
Also in production of anticoagulation proteins:
1. Protein C
2. Protein S
What is the Activated Partial Thromboplastin Time (aPTT)?
The time required to generate thrombin and fibrin via the intrinsic and common pathways (Reference range: less than 35 seconds).
What is the Prothrombin Time (PT)?
The time for thrombin and fibrin formation via the extrinsic and common pathways (Reference range: 10-13 seconds)
What is the Thrombin time (TT)?
The time it takes for a clot to form in the plasma of a blood sample containing anticoagulant, after an excess of thrombin has been added.
What is International normalised ratio (INR)?
calculates the standardized PT. INR is based on the ratio of the patient’s PT and the normal mean PT.
What are the main two system branches for physiological anticoagulants?
- Anticoagulants —> reduces thrombin production/action —> prevention
- Fibrinolytic —> degrades fibrin in clots —> removal
What is the main inhibitor of the extrinsic pathway?
Tissue factor pathway inhibitor
How is a fibrinogen changed to form a clot in the blood coagulation cascade?
- Fibrinogen (soluble)
- Thrombin –> cleaves fibrinogen
- Fibrin (Insoluble)
- Polymerisation –> fibrin is linked
- Polymerised Fibrin
- Factor XIIIa (Transglutaminase)
- Cross linked fibrin (CLOT)
What is the term for clotting in an unbroken vessel?
Thrombosis
What makes blood clots stay localized to one position?
Thrombin is adsorbed by fibrin
Blood flow dilutes active factors too much in circulation
Blow flow also prevents pooling (stasis) of blood –> does not allow accumulation of coagulation factors
What are the two main physiological mechanisms of anticoagulants?
Anticoagulants –> for prevention of coagulation –> reduces thrombin production or limits its activity
Fibrinolytic –> for removal of clots –> dissolves formed clots by degrading fibrin
Why is it important that the balance of procoagulants and natural anticoagulants are in balance during the formation of a clot?
- Aim is for enough clotting to prevent bleeding
2. Not too much clotting that there is a large blood clot that obstructs vein flow of blood
Where is Tissue factor pathway inhibitor (TFPI) located and what does it do?
Located on the endothelial cell surface and in plasma –> it is the main inhibitor of the extrinsic pathway.
What is the mechanism of Tissue factor pathway inhibitor? (TFPI)
Two mechanisms:
1. Directly inhibits Factor Xa,
2. it complexes with Factor Xa and the complex inhibits TF/FVIIa,
With these mechanisms it impairs the triggering mechanism of the extrinsic pathway.
What are the other main anticoagulants apart from TFPI? (4)
Protein S
Protein C
Protein Z
Antithrombin
What is the anticoagulant mechanism of Protein S?
Enhances the interaction of TFPI with Factor Xa in the presence of Ca2+ and phospholipid
What is the anticoagulant mechanism of Protein C?
- Protein C is a vitamin K-dependent plasma glycoprotein.
- The thrombin–thrombomodulin–EPCR) complex activated protein C.
- Activated protein C (APC) proteolytically deactivates factors Va & VIIIa.
What is the anticoagulant mechanism of Protein Z?
Protein Z and Z protease inhibitor induces degradation of the factor Xa
What is the anticoagulant mechanism of Antithrombin?
It is a Serine protease inhibitor –> inhibits the protease activities of factor Xa and also thrombin
What is Fibrinolysis?
Enzymatic degradation of fibrin
What mediates the fibrinolysis in blood clots?
Plasmin –> which is a serine protease activated by the plasminogen activation cascade
What is involved in the fibrinolytic system?
Fibrinolysis
Tissue repair
Angiogenesis
What are the two fibrinolytic systems?
Cellular –> leukocytes, endothelial, epithelial cells and platelets system
Plasma –> fibrin clot based system
What does the small a next to the number for a factor mean?
Activated form
How many clotting factors are there in the coagulation cascade?
Factors 1-5 and 7-13 (there are 12 because the numbering skips 6)
What is factor I and where does it come from?
Fibrinogen –> comes from the liver
What is factor II and where does it come from?
Prothrombin –> comes from the liver
What is factor III and where does it come from?
Tissue factor (Thromboplastin) –> comes from the endothelium/platelets
What is factor IV and where does it come from?
Calcium –> comes from bone and GIT
What is the other name for factor X (10)?
Stuart factor
What is factor XIII?
Fibrin stabilising factor
What is the origin for most of the coagulation factors?
They are from the liver
What initiates the extrinsic pathway?
- Tissue trauma –> endothelium/vascular damage
2. causes release of TISSUE FACTOR (Thromboplastin)
What does tissue factor do?
Extrinsic pathway component (factor III) –> converts factor VII –> VIIa (active)
What does factor VIIa do in the extrinsic pathway?
Combines with Tissue factor and Ca2+ to cause the conversion of factor X (10) to its active form –> Xa –> in the presence of factor V and Ca2+
What initiates the intrinsic pathway?
Trauma to the blood cells or exposure of blood/platelets to collagen –> Causes factor XII (12) activation –> XIIa
What is the flow of the intrinsic pathway of coagulation?
- XII –> XIIa in presence of trauma or collagen
- XIIa converts factor XI (11) into its active form XIa
- Xia (11) converts factor IX (9) into its active form Ixa
- Factor 9 in combination with factor 8 and Ca2+ activates factor X –> Xa
- Xa in combination with Ca2+ and factor V(5) forms the prothrombin activator
What initiates the common pathway of the coagulation cascade?
Prothrombin activator (or active factor 10 depends how they ask)
What is the steps in the common pathway?
- Prothrombin activator converts prothrombin to thrombin
- Thrombin –> acts on fibrinogen to form fibrin monomers
- Fribrin stabilising factor XIII (13) stabilises the fibrin monomers to form fibrin polymers
- Fibrin polymers –> form a fibrin mesh
- Fibrin mesh –> attracts platelets and phospholipids
- Stable blood clot –> haemostasis
What do both the intrinsic and extrinsic pathway lead to?
prothrombin activator –> which is a complex of coagulation factors
What does activated coagulation factor X do?
In the presence of Ca2+ it converts Prothrombin into Thrombin
What does Thrombin do?
Converts fibrinogen –> Fibrin monomers
What factor is responsible for converting fibrin monomers into polymers?
Factor XIII (13) –> fibrin stabilising factor
What is the Tunica of blood vessels?
3 layers: Tunica Intima (inner) Tunica Media (middle) Tunica Externa (outer)
What is the tunica intima composed of?
Single layer of endothelial cells in contact with the blood supported by an internal elastic lamina