Haemostasis Flashcards
LIst 2 functions of Haemostasis?
Functions of Haemostasis
Prevention of blood loss from intact vessels
Arrest of bleeding from injured vessels
Haemostatic Plug Formation - Overview
Vessel Constriction - when there is an injury to the blood vessel, the smaller blood vessels will constrict in a contractile response to injury which, in itself, can stop blood loss
The main mechanism of haemostasis involves:
………….
……………….. ……………..
The formation of an unstable platelet plug involves …………….. …………….. and ………………. ……………….
Stabilisation of the platelet plug involves activation of ……………… ……………… and the activation of ………………
These TWO stages are called PRIMARY and SECONDARY HAEMOSTASIS
In the initial response, the platelets identify ……………… damage and forms a clump
Coagulation takes place and is accelerated by the ………………
Coagulation is localised on the surface of the ………………
Haemostatic Plug Formation - Overview
Vessel Constriction - when there is an injury to the blood vessel, the smaller blood vessels will constrict in a contractile response to injury which, in itself, can stop blood loss
The main mechanism of haemostasis involves:
Platelets
Coagulation Cascade
The formation of an unstable platelet plug involves platelet adhesion and platelet aggregation
Stabilisation of the platelet plug involves activation of blood coagulation and the activation of fibrin
These TWO stages are called PRIMARY and SECONDARY HAEMOSTASIS
In the initial response, the platelets identify endothelial damage and forms a clump
Coagulation takes place and is accelerated by the platelets
Coagulation is localised on the surface of the platelets
Define coagulation?
What cells produce Von Willebrand factor?
DEFINITION: Coagulation = the process by which blood is converted from a liquid to a solid state
NOTE: Von Willebrand factor is a plasma protein secreted by endothelial cells and platelets
Illustrate the Homeostatic Plug Formation
Illustration of Homeostatic Plug Formation
Endothelial cells line the blood vessel and in the images, there is a site of damage to the right on the vessel wall where the subendothelial layer (RICH IN COLLAGEN) is exposed
The collagen is recognised in TWO ways:
……………. ………… ………. binds to the collagen and attracts platelets - the platelets bind to the Glycoprotein 1b Receptor (…….) on the ……. ………………… …….
OR
………………… ………….. ………….. on the ………….. directly binds to the collagen in the subendothelial layer
There are two mechanisms because the circumstances within the vasculature will vary - the stressors in the blood can be different
In a small blood vessel there could be very high ………….. ………….. which favours the ……. ………….. mechanism
When the glycoprotein receptors on the platelets are engaged either to the Von Willebrand Factor or directly to the collagen, the platelets become …………………
When activated, the platelets will release ……. and ……………………….(………………… in particular)
………………… activate other platelets so the platelets aggregate - the glycoproteins ……. and ……. receptors become available which the fibrinogen can bind to
………….. and ………………… are released from the platelet as it becomes activated - the ………………… acts on the surface of the receptor
In the blood coagulation cascade, a protease called ………………… gets generated which can also directly activate the platelets so that they aggregate
There are specific receptors for thrombin on the platelets which are therapeutic targets
Illustration of Homeostatic Plug Formation
Endothelial cells line the blood vessel and in the images, there is a site of damage to the right on the vessel wall where the subendothelial layer (RICH IN COLLAGEN) is exposed
The collagen is recognised in TWO ways:
Von Willebrand Factor binds to the collagen and attracts platelets - the platelets bind to the Glycoprotein 1b Receptor (GlpIb) on the Von Willebrand Factor
OR
Glycoprotein 1a Receptor on the platelets directly binds to the collagen in the subendothelial layer
There are two mechanisms because the circumstances within the vasculature will vary - the stressors in the blood can be different
In a small blood vessel there could be very high shear stress which favours the Von Willebrand mechanism
When the glycoprotein receptors on the platelets are engaged either to the Von Willebrand Factor or directly to the collagen, the platelets become ACTIVATED
When activated, the platelets will release ADP and PROSTAGLANDINS (Thromboxane in particular)
Prostaglandins activate other platelets so the platelets aggregate - the glycoproteins IIa and IIIb receptors become available which the fibrinogen can bind to
ADP and thromboxane are released from the platelet as it becomes activated - the thromboxane acts on the surface of the receptor
In the blood coagulation cascade, a protease called thrombin gets generated which can also directly activate the platelets so that they aggregate
There are specific receptors for thrombin on the platelets which are therapeutic targets
What happens to platelets once they become activated and why is this important?
The platelets normally circulate as dormant cells but when it becomes activated it changes its shape and hence changes its membrane composition
Certain phospholipids which are usually inside the platelet come to the outside
This is important because it is those phospholipids that bind to the coagulation factors
The platelet presents new or activated proteins on their surface (e.g. GlpIIb and IIIa - this becomes an active conformation so it can react with the FIBRINOGEN )
Platelets have a life span of around 8 days
Haemostatic Plug Formation: An Overview
Draw a flow diagram showing an overview of the haemostatic plug formation
include primary and secondary haemostasis
List the steps for platelet adhesion and platlet aggregation?
Platlet Adhesion
1) Damage to the layer of endothelial cells which exposes the underlying subendothelial connective tissue which contains collagen
2) that collagen attracts a protein that circulates in the blood called the Von willebrand Factor which binds to the collagen
3) The Von Willebrand factor binds to the exposed collagen
4) The platlet binds to the Von Willebrand factor using the GIp1b (glycoprotein 1 b)- this mecahnism through von willebrand factor occurs in high sheer occasions- narrow blood vessels
5) OR In larger vessels- low sheer stress- different mechanism- The direct interaction of GIp1a (platlet receptor) with the exposed collagen
6) Once they bind to the collagen, signals are sent through the receptor and inside the platlet causing activation- this causes release of ADP and prostaglandins
Platlet Aggregation
7) Glp2b/3a changes conformation when activation and can now bind to the circulating Fibrinogen (acts like a glue which clumps the platlets togethere)
Sites of synthesis of clotting factors, fibrinolytic factors and inhibitors
Where are most of these factors made?
Where is Von Willebrand factor made in high concentrations in?
Where is Factor 5 synthesised?
Sites of synthesis of clotting factors, fibrinolytic factors and inhibitors
Liver
Most of these factors are made in the LIVER
Endothelial Cells
Von Willebrand factor is made in high concentration in the endothelium
Megakaryocytes (–> Platelets)
Factor V is synthesised in the megakaryocyte
Draw a diagram showing the intrinsic pathway?
Intrinsic Pathway
Intrinsic Pathway - involves the activation of a …………… (a precursor of a protease) - this is …………….. ……….
Factor 12 is converted to Factor ………. (active) which can then convert Factor …….. to Factor ……… and the cascade continues as shown above with one activated protease activating the next zymogen
FACTOR ……. is a COFACTOR - this is NOT a protease
Factor 8 gets converted to Factor …….
………………… take their place on the surface of the platelet
The coagulation factors bind to the activated phospholipid
Intrinsic Pathway
Intrinsic Pathway - involves the activation of a ZYMOGEN (a precursor of a protease) - this is Factor XII
Factor 12 is converted to Factor 12a (active) which can then convert Factor 11 to Factor 11a and the cascade continues as shown above with one activated protease activating the next zymogen
FACTOR 8 is a COFACTOR - this is NOT a protease
Factor 8 gets converted to Factor 8a
Phospholipids take their place on the surface of the platelet
The coagulation factors bind to the activated phospholipid
Draw a diagram shiwing the relationship between the intrinsic and the extrinsic pathway?
Extrinsic Pathway
There is an Extrinsic Pathway by which coagulation can be activated
When a vessel is damaged, the blood comes into contact with …………… …………
…………… ………… is a membrane protein which isn’t normally found in the blood - smooth muscle cells contain…………… …………
…………… ………… is a potent initiator of the clotting cascade
…………… ………… binds to Factor 7 converting it to Factor 7a which then converts …………… ……. to …………… …….
Extrinsic Pathway
There is an Extrinsic Pathway by which coagulation can be activated
When a vessel is damaged, the blood comes into contact with TISSUE FACTOR
Tissue factor is a membrane protein which isn’t normally found in the blood - smooth muscle cells contain tissue factor
Tissue factor is a potent initiator of the clotting cascade
Tissue factor binds to Factor 7 converting it to Factor 7a which then converts Factor 10 to Factor 10a
Draw the full coagulation cascade system?
Common Pathway
…… converts ………………. to thrombin
…………….. can further activate the platelet - it forms a fibrin clot
…………….. …… helps Factor 10a convert prothrombin to thrombin faster
Factor 5a is generated from Factor 5 by trace amounts of ……………..
Thrombin converts …………….. (soluble) to …………….. forming a …………….. ……………..
The fibrin clot is ……………..
The clot can be crosslinked by ……………. …….. - crosslinking is covalently crosslinking the fibrin clot so that it is stabilised and can’t be broken down by the shear forces
Tissue factor can also activate …………….. ….. to …………. …. when tissue factor binds to Factor 7
So there are THREE pathways:
…………….. (involves factor 12)
…………….. (involves tissue factor and factor 7)
…………….. …………….. (where the intrinsic and extrinsic pathways converge - from factor 10a onwards)
Tissue factor is the PHYSIOLOGICAL INITIATOR of coagulation
Coagulation is NOT initiated by Factor 12 to Factor 12a
Tissue factor is the main driver of coagulation and haemostasis
Factor 12 is important in laboratory tests
Common Pathway
10a converts prothrombin to thrombin
Thrombin can further activate the platelet - it forms a fibrin clot
Factor 5a helps Factor 10a convert prothrombin to thrombin faster
Factor 5a is generated from Factor 5 by trace amounts of thrombin
Thrombin converts fibrinogen (soluble) to fibrin forming a FIBRIN CLOT
The fibrin clot is insoluble
The clot can be crosslinked by Factor 13a - crosslinking is covalently crosslinking the fibrin clot so that it is stabilised and can’t be broken down by the shear forces
Tissue factor can also activate Factor 9 to Factor 9a when tissue factor binds to Factor 7
So there are THREE pathways:
Intrinsic (involves factor 12)
Extrinsic (involves tissue factor and factor 7)
Common Pathway (where the intrinsic and extrinsic pathways converge - from factor 10a onwards)
Tissue factor is the PHYSIOLOGICAL INITIATOR of coagulation
Coagulation is NOT initiated by Factor 12 to Factor 12a
Tissue factor is the main driver of coagulation and haemostasis
Factor 12 is important in laboratory tests
What is the physiological initiator of coagulation?
What is the main drive of coagulation and haemostasis?
Tissue factor is the PHYSIOLOGICAL INITIATOR of coagulation
Coagulation is NOT initiated by Factor 12 to Factor 12a
Tissue factor is the main driver of coagulation and haemostasis
SUMMARY
- Simply viewed, it is a cascade or amplification system
- There are zymogens which are converted to proteinases,
cofactors which need to be activated and surfaces
- The surface is made of activated platelets (Pl) which localise
and accelerate the reactions
- The trigger to initiate coagulation in vivo is tissue factor
- Although FXII can be activated to FXIIa, this is mainly
an in vitro reaction, useful for some diagnostic tests
What does tPA stand for and where is it made and what is its role?
Tissue Plasminogen Activator (tPA) is a protein made by the endothelial cells
The role of tPA is to convert plasminogen from an inactive zymogen to an active protease (PLASMIN)
Normally, there is no interaction that takes place between these two proteins
WHat does the fibrin clot do the the tPA and plasminogen?
What is plasmin?
What are the breakdown products of the fibrin clot?
Name a substance that are used in therapuetic thrombolysis for myocardial infarction?
The fibrin clot assembles the tPA and the plasminogen on its surface, bringing them close together and triggers the cleavage reaction where the tPA can convert plasminogen to plasmin
Plasmin is a powerful proteolytic enzyme that can break down the fibrin clot
The breakdown products of the fibrin clot are Fibrin Degradation Products (FDP)
FDP can be measured when we are giving thrombolytic therapy
tPA and Streptokinase (a bacterial activator) are used in therapeutic thrombolysis for myocardial infarction
The amount of circulating tPA is usually quite low, it is made locally in the endothelial cells, right next to where the clot forms
The clotting cascade is an amplification system, a small amount of factor VIIa producing a large amount of thrombin.
Why does blood not clot completely whenever clotting is initiated by vessel injury?
Coagulation inhibitory mechanisms prevent this
Name the two Coagulation inhibatory mechanisms?
DIRECT INHIBITION
INDIRECT INHIBITION
Name the inhibitor for Direct Inhibition?
DIRECT INHIBITION
Direct inhibition of the activated coagulation factors
This takes place by an inhibitor that circulates in the blood in quite high concentrations
This inhibitor is called ANTITHROMBIN
Antithrombin is a broad scale inhibitor of most of the coagulation proteinases (not just thrombin)
Antithrombin is sometimes known as antithrombin III
What is Indirect Inhibition?
What molecule become activated in what pathway?
INDIRECT INHIBITION
Mechanism that slows down the amount of thrombin that is generated
This involves the activation of PROTEIN C in the Protein C Anticoagulant Pathway
Many of the reactions involve the conversion of a zymogen to an active proteinase - the ones in RED are proteinases
List all the active proteinases that can be inhibited by antithrombin?
What happens if you have a reduction in Antithrombin?
What effect does herapin have on thrombin?
All the active proteinases can be inhibited by antithrombin
So when there is an excess of these coagulation factors, antithrombin will inhibit them by forming a complex with these coagulation factors and then the complexes are cleared from the circulation
Reduction in antithrombin —-> Higher risk of thrombosis
IMPORTANT: HEPARIN ACCELERATES THE ACTION OF ANTITHROMBIN
So heparin is used for immediate anticoagulation in venous thrombosis and pulmonary embolism
Factor 8 and Factor 5 are activated by TRACE AMOUNTS OF THROMBIN and become COFACTORS
Factor 8 and Factor 5 bind to the surface of platelets and make the coagulation occur faster
The platelet localisation process makes the reaction occur much faster
The cofactors are an important acceleratory mechanism
The protein C pathways downregulates thrombin generation
Protein C inactivates the cofactors (…………… and ……………)
…………… and …………… accelerate thrombin formation about 10,000 fold
The thrombin generated can bind to a protein on the surface of the endothelium called ………………………… which changes the conformation of thrombin
…………… is normally involved in forming the clot, activating the platelets and activating Factor 8 and Factor 5
When the thrombin binds to thrombomodulin, it changes its ……………
It activates……………which is an inactive zymogen to activated protein C (along with Protein S)
Activated Protein C and Protein S inactivates ……………and ……………
The inactivation of …………… and …………… by activated protein C and protein S is the second anticoagulant mechanism
If you have a DEFICIENCY OF ONE OF THE INHIBITORY PROTEINS then you can’t really control the amount of coagulation via the indirect inhibitory pathway (protein C pathway)
The protein C pathways downregulates thrombin generation
Protein C inactivates the cofactors (Factor 8 and Factor 5)
Factor 8 and Factor 5 accelerate thrombin formation about 10,000 fold
The thrombin generated can bind to a protein on the surface of the endothelium called THROMBOMODULIN which changes the conformation of thrombin
Thrombin is normally involved in forming the clot, activating the platelets and activating Factor 8 and Factor 5
When the thrombin binds to thrombomodulin, it changes its specification
It activates Protein C which is an inactive zymogen to activated protein C (along with Protein S)
Activated Protein C and Protein S inactivates Factor 5a and Factor 8a
The inactivation of Factor 8a and Factor 5a by activated protein C and protein S is the second anticoagulant mechanism
If you have a DEFICIENCY OF ONE OF THE INHIBITORY PROTEINS then you can’t really control the amount of coagulation via the indirect inhibitory pathway (protein C pathway)
What is Factor 5 leiden.
What does this cause?
Factor 5 Leiden is a COMMON POLYMORPHISM in the population
There is an amino acid change in Factor 5 called Factor 5 Leiden which cannot be inactivated as well as wild type Factor 5
If you have this polymorphism then the protein C anticoagulant pathway can’t inactivate Factor 5 Leiden as well and so there is higher risk of thrombosis
More thrombin is generated if you have Factor 5 Leiden
Equilibrium
Platelets:
1) Bind to damaged endothelium through tissue factor
2) Are inhibited by thromboxane
3) Trigger Coagulation at the site of injury
4) Are activated and aggregate in response to ADP
5) Have a life span of 100 days
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Tissue factor triggers coagulation
Coagulation:
1) FVIIIa is aprotease, generated by thrombin
2) Factor XII interacts with tissue factor to initiate coagulation
3) Activated coagulation complexes assemble on the platlet surface
4) Fibrinogen must be converted to fibrin by thrombin in order to support platlet aggregation
5) FIXa is a cofactor that accelerates FX activation
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