W1 Haemostasis, Platelets and Coagulation Part 1 and 2 Flashcards
what area is a blood clot limited to?
the site of injury
what is haemostasis?
the process in which coagulation is initiated and terminated in a regular way, with the removal of the clot
haemostasis is a physiological process that stops bleeding at the site of injury whilst maintaining what?
blood flow elsewhere in the circulation
what is haemostasis in response to?
a defence mechanism in response to injury in order to preserve the integrity of the vascular system
what are the four steps of haemostasis?
- vasoconstriction restricting blood flow to the damaged vessel
- platelet aggregation and plug formation (primary haemostasis)
- generation of the enzyme thrombin that proteolyzes soluble fibrinogen to insoluble fibrin (secondary haemostasis)
- removal of the clot in a controlled way known as fibrinolysis restores function
what are the five major components of the haemostatic system?
- blood vessels
- platelets
- coagulation factors
- coagulation inhibitors
- fibrinolytic factors (clot dissolving components)
in a high pressure vascular system why does the response need to be rapid but regulated?
to prevent inappropriate clot formation and ensure it is localised to prevent loss of blood
what impact does clotting have on microbes?
prevents them from gaining entry to the body
what are potential problems of inappropriate blood coagulation?
may block vessels which can restrict blood flow, starve tissues of oxygen and thus cell death
what is haemophillia?
failure to be able to generate a fibrin clot (life threatening)
what is DVT?
deep vein thrombosis
what is PE?
pulmonary embolism
what enables coagulation to be so rapid?
platelets are produced in bone marrow and circulate in large numbers, proteins involved in generating fibrin clots are produced in the liver primarily and all components are pre-synthesised and most circulate in the blood
why is coagulation so localised, give three reasons?
any three of:
- platelets sense proteins only at exposed sites of vascular injury
- activated platelets at injury sites expose a surface the promotes assembly of complexes which lead to thrombin generation
- thrombin generation network is triggered by exposure of blood to cellular receptors
- the unperturbed endothelium presents an anticoagulant response that dampens down any inappropriate activation of coagulation
- the anticoagulant response of endothelium limits spread of the response beyond the site of injury and occlusion of injured vessel
platelets are anucleate cells, what does this mean and where are they produced?
they have no nucleus, produced in the bone marrow via budding off from megakaryocytes
what is thrombocytopenia?
a deficiency of circulating platelets due to decreased platelet formation
what is platelet formation regulated by and where is it produced?
thrombopoietin which is produced in the liver and kidney
what percentage of dogs admitted to vet hospitals have low platelet count?
5% according to some surveys
what percentage of king charles spaniels are effected by macrothrombocytopenia which is an inhertied disorder?
around 50%
what shape and colour are resting platelets?
round to oval in shape, they have visible red-pink granules
what are the key binding partners to platelets?
collagen and von Willebrand factor (VWF)
binding to ligands results in the release of granules containing what, that activate platelets?
ADP and thromboxane
activated platelets change shape and generate what?
generation of filopodia
what is VWF?
von Willebran factor, a large protein 250kDa forming multimers of 80 subunits
where is VWF found?
in plasma, platelet storage granules and in endothelial cells
what is VWF associated with?
collagen and laminins found in the subendothelial matrix
under normal conditions is VWF in globular or non-globular form?
globular form that does not bind with platelets
what happens to VWF under conditions of stress or upon binding to collagen?
the globular VWF unfolds to expose platelets binding sites
briefly describe VW disease:
inherited platelet defect caused by VWF that are defective, autosomal recessive, common in certain breeds such as dobermans, rare in cats
what events occur during primary haemostasis?
vascular damage, platelets bind to collagen/VWF, release of ADP and thromboxane, platelets fully activate, negatively charged phospholipid surface promotes thrombin generations, leads to fibrin generation and further platelets activation
what is PS?
phosphatidylserine
activation of platelets results in shape change, adhesion and externalisation of what?
phosphatidylserine
localised thrombin generation leads to what?
further platelet activation and formation of a polymerised fibrin mesh
how are fibrin clots formed?
a complex series of protein-protein interactions
proteases that function in coagulation are what type? and these are members of what family?
serine proteases part of the chymotrypsin super family
what do serine proteases circulate in the plasma as? what are these activated by?
zymogens, activated by limited proteolysis
for maximal activity of proteases what is required and what do they circulate as?
cofactors, they circulate in plasma as pro-cofactors that are activated by limited proeolysis
protein-protein complexes assemble on either of what?
phospholipids surfaces or fibrin clots
coagulation factors are identified by roman numerals, what is used t identify an active or inactive cofactor?
a at the end for active and ai at the end for inactive
what are the common names for FIII, FII and FIIa?
FIII = tissue factor/thromboplastin
FII = prothrombin
FIIa = thrombin
where are clotting factors pre-synthesised? what do they circulate as? (resting phase)
in the liver, circulate in their inactive form
what is the trigger for blood coagulation, what is it? (resting phase)
tissue factor (TF), a cell surface protein
TF is expressed on cells NOT with the blood, what does the expresion pattern generate? (resting phase)
the haemostatic envelope
endothelial cells present an anticoagulat surface through the expression of what molecules? (resting phase)
thrombomodulin, TF pathway inhibitor and antithrombin associated with heparan sulphate and proteoglycans
during the initiation phase vascular damage results in blood being exposed to what?
cells expressing tissue factor (TF)
what leads to explosive generation of thrombin and what does it cause fibrinogen to convert to?
amplification network leads to explosive generation, which causes fibrinogen to fibrin
which is soluble and which is insoluble of fibrinogen anf fibrin?
fibrinogen = soluble
fibrin = insoluble
fibrin crosslinks what to form a stable clot? (initiation phase)
platelets
why is coagulation so controlled?
a series of feedback mechanisms limit the extent of activation
in what ways is coagulation controlled by feedback mechanisms? (3 ways)
- shutting down the initiation complex once it has been activated
- limit generation of fibrin clots by reducing the cofactor effect
- limiting generation of fibrin clots by reducing activity of the activated proteases
tissue factor pathway inhibitor is associated with the epithelial cell surface, what does it inactivate?
inactivates the initiation complex by forming a quarternary complex of TF-FVIIIa-FXa-TFPI
what stimulates the release of TFPI from EC and platelets?
thrombin
what is TM?
thrombomodulin
what is PC?
protein C
what is EPCR?
endothelial protein C receptor
what is PS?
protein S
activated PC in complex with its cofactor PS rapidly inactivates what?
FVa and FVIIIa by limited proteolysis
what is AT?
antithrombin
what does antithrombin inhibit? what is it accelerated by?
coagulation proteases FIXa, FXa, FXIa and thrombin, accelerated by heparin sulfate proteoglycans on the surface of epithelial cells
what is APTT?
activated partial thromboplastin time
describe the process of lab assessment of coagulation (APTT):
- blood sample collected into citrate
- plasma prepared by centrifugation
- coagulation initiated by addition of koalin, silica, calcium - time for formation of fibrin clot
- important to use an appropriate reference plasma (same species from 8< animals)
- tests for deficiencies in the intrinsic pathway
describe the process of lab assessment of coagulation (PT/prothrombin time):
- blood sample collected into citrate
- plasma prepared by centrifugation
- coagulation initiated by addition of TF (thromboplastin), calcium - time for formation of fibrin clot
- important to use an appropriate reference plasma (same species from 8< animals)
- tests for deficiencies in the extrinsic pathway
what are the normal ranges of APTT in dogs, cats and horses?
dogs: 10-17s
cats: 15-19s
horses: 45-66s
what is the normal ranges of PT in dogs, cats and horses?
dogs: 11-16s
cats: 15-20s
horses: 16-20s
what is the basic role of coagulation factors in normal physiology?
vascular integrity and development
what is the basic role of coagulation factors in pathophysiology?
bleeding disorders, thrombosis, infection, cancer, inflammation, tissue remodelling and viral tropism
what is Haemophilia A/B?
a bleeding disorder with identical clinical symptoms, lab test required to distinguish A from B, 400,000 people worldwide have it, only 25% receive treatment, either a deficiency of FVIII (A) or FIX (B)
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