Flashcards in Normal Hemostasis Deck (87):
What is hemostasis?
It is the process through which the body stops blood hemorrhage from a damaged vessel
What is involved in primary hemostasis? Secondary?
Hemostasis involves endothelial cells lining the vessel, platelets (primary) and coagulation proteins (secondary) to form a platelet-fibrin thrombus at site of injury
What are four ways endothelium play a role in hemostasis?
1. they secrete tissue factor (TF) and inhibitors of TPA after injury
2. They prevent adhesion and aggregation of platelets under normal circumstances via NO and PGI2
3. They regulate thrombin by providing thrombomudulin to activate protein C pathway
4. Regulate fibrinolysis by secreting TPA and dissolving the formed thrombus
Where does tissue factor come from? What does it do?
It is secreted by endothelial cells after injury and it initiates the coagulation cascade
What is vWF?
It is an adhesive glycoprotein exposed upon endothelial injury that augments platelet adhesion to subendothelial collagen/ECM
To regulate thrombin, what do endothelial cells expose?
Thrombomodulin which activates protein C pathway
What regulates the action of tissue plasminogen activator (TPA)?
Where does this regulator come from?
plasminogen activator inhibitor-1 (PAI-1) which is secreted by the endothelium
What are the three steps in primary hemostasis?
1. platelet adhesion
3. platelet aggregation
When does the process of primary hemostasis begin?
When platelets come in contact with injured endothelium and adhere to the collagen.
What structure on the platelet adheres to vWF of the subendothelial collagen?
What platelet glycoproteins attach to the subendothelial collagen?
GpIb- to vWF
GpIa/IIa to collagen
Once the platelet adheres to the subendothelial collagen, what happens to their shape? Why?
They change from discoid to spherical shape with multiple pseudopodia extensions.
This allows further adhesion and aggregation
Once platelet ligands are bound to collagen receptors, intracellular signaling cascades are initiated. What are the three important processes that occur?
1. activation of phospholipase A2 which releases AA from membrane phospholipids
2. fusion of cytoplasmic granules with plasma membrane to secrete biochemical substance
3. conformational change in GpIIb/IIIa which makes it capable of high-affinity fibrinogen binding leading to platelet aggregation
What does activation of phospholipase A2 in platelet activation do?
Releases AA from membrane phospholipids.
Arachadonic acid is a substate for COX-1, an enzyme that syntehsizes TXA2 which is a crucial proaggregant of platelets
What granules fuse with the plasma membrane during platelet activation?
1. alpha granules- contain platelet adhesive molecules like fibrinogen, vWF, thrombospondin
2. dense granules- calcium and ADP
What is the function of ADP in platelet activation?
Where does it come from?
It amplifies aggregation by interacting with receptors on other platelets leading to their activation.
It is released from dense granules from the original platelet that had adhered and activated
Platelet activation causes conformational change in what structure that leads to high-affinity fibrinogen binding and thus aggregation?
What is the most common congenital bleeding disorder?
Von Willebrand disease which presents with mucocutaneous bleeding due to the inability to form a platelet plug
Adhesion is _______ to ________ attachment.
Aggregation is ______ to ________ attachment.
Adhesion- platelet to ECM
Aggregation- platelet to platelet
The platelet plug formed during primary hemostasis is ________ and will dissipate under sheer stress of blood flow unless ___________ occurs.
the plug is fragile and will dissipate if consolidation does not occur
How does consolidation occur in secondary hemostasis?
with the formation of a fibrin cross-linked meshwork (which forms by a complex cascade of protease activity on coagulation factor proteins)
All coagulation factors circulates as _______ until converted to an _________ by proteolytic cleavage.
Zymogens (inactive precursors) until they are converted to an active form
Ex. FVII --> FVIIa
Where are all coagulation factors synthesized?
What is the exception?
They are all synthesized in liver hepatocytes except FVIII (factor 8) which is synthesized by endothelial cells in the liver, spleen and kidney
What cells synthesize factor 8?
endothelial cells in the liver, kidney, spleen
Which factors are non-carboxylated and require gamma-carboxylation to become fully active?
What gamma-carboxylation? What coagulation factors require this?
What is dependent for this process?
Gamma carboxylation promotes calcium dependent binding of coagulation factors to phospholipid surfaces, primarily on platelets.
Factors 2.7. 9. 10 are dependent on gamma-carboxylation
This carboxylation is catalyzed by a Vit. K dependent enzyme
Which coagulation factors are dependent on Vit. K?
What is coumadin?
It is a drug that blocks the vitamin K dependent reductase that would carboxylate factors 22.214.171.124 so it is an anticoagulant
Which coagulation factor has the shortest half life?
7 has a half life of 3-6 hours
The other coag factors have a 12-72 hour half life
Which coagulation factors are heat labile?
How does one of these factors avoid degradation?
5 and 8 degrade in heat. Factor 8 is carried and stabilized by vWF so it doesnt get degraded
Desribe the tissue factor pathway of coagulation cascade.
1. TF, 7a, PL, Ca activate factor 10.
2. 10a, PL, 5a, Ca cleave prothrombin to thrombin
3. Thrombin cleaves 11 to 11a
4. 11a cleaves 9 to 9a
5. 9a, 8a, Ca, PL activate more 10 to 10a to propogate the cycle
Which pathway is necessary for initiation of the coagulation cascade?
What pathway is necessary for propogation?
Initiation- TF pathway (7,10,11,9,8)
Propogation- Intrinsic pathway (12, 11, 9)
Describe the intrinsic pathway.
1. 12 becomes activated by kallikrein, PK, HMWK
2. 12a activates 11 to 11a
3. 11a activates 9 to 9a
4. 9a, 8a, Ca, PL activate 10 to 10a
Which coagulation factor is activated by kallikrein, PK, HMWK?
In the current cell model, coagulation is initiated by what?
Tissue Factor- an apolipoprotein synthesized by endothelial cells after they experience vascular injury
What does TF complex with as the first step of the coagulation cascade?
It complexes with circulating F7a which then complexes with Ca and phopholipid
What is the TF-7a-Ca-PL complex called?
extrinsic tenase because it activates factor 10
Once FX is activated, what does it do?
10a binds to 5a with Ca on PL
What is the 10a, 5a, Ca, PL complex called? What does it do?
It is the prothrombinase complex that converts prothrombin (2) to thrombin (2a)
What is prothrombinase?
10a, 5a, Ca, PL
What coagulation factor is prothrombin?
What does thrombin do?
Cleaves fibrinogen to fibrin allowing clot formation
What prevents further activation of the tissue factor pathway?
tissue factor pathway inhibitor (TFPI) that acts on the extrinsic tenase so no more factor 10 can be activated.
How does thrombin amplify the coagulation cascade?
1. it converts 11 to 11a which activates 9 to 9a.
9a complexes with 8a, Ca, PL to form the intrinsic tenase which activates more factor 10
2. it activates 5 and 8 which are essential cofactors in prothrombinase and intrinsic tenase
3. activates 13 which covalently crosslinks fibrin polymers
What is the extrinsic tenase?
What is the intrinsic tenase?
Extrinsic- 7a, TF, PL, Ca
Intrinsic- 9a, 8a, PL, Ca
What are the regulatory proteins of the coagulation cascade?
3. protein C and S
4. fibrinolytic system
Hemophilia A is a congenital condition that is deficiency in which clotting factor?
factor 8 ( necessary for intrinsic tenase and propogation of the clotting cycle)
Hemonphilia B is a congenital condition that is a deficiency in which clotting factor?
9 which is crucial for instrinsic tenase
What 2 factors are crucial for the amplification phase of clotting cascade?
5 and 8 for prothrombinase and intrinsic tenase
What is the most important regulator of thrombin?
How does it operate?
Antithrombin (AT) binds to serine proteases:
10a= part of prothrombinase
generated during activation of coagulation cascade and inhibit formation of the fibrin clot
Action of antithrombin are potentated by what 3 things present on the endothelium?
1. heparan sulfate
2. dermatan sulfate
3. chondroitin sulfate
Deficiency in antithrombin results in thrombotic complications. Administration of what can also lower AT levels?
How does heparin work?
It speeds the process of bringing Antithrombin to IIa (thrombin) so it is a procoagulant
Coumadin is what kind of drug?
Heparin is what kind of drug?
Coumadin is an anticoagulant because it blocks activation of 2,7,9,10 (the vitamin K dependent factors).
Heparin is a procoagulant because it binds to thrombin to inactivate it
What is protein C?
A vitamin K-dependent anticoagulant that is activated by thrombin-thrombomodulin complex on endothelial surface.
APC inactivates 5a and 8a to downregulate amplification
If there is no vitamin K, what coagulation factors do not function?
What anticoagulation factors do not function?
Coag factors -2, 7, 9, 10
Ant- protein C, protein S
What is protein S?
a vitaminK dependent cofactor of APC which helps inactivate 5a and 8a
What anticoagulant levels are reduced by warfarin therapy (coumadin)?
Protein S and C because they are Vit K dependent
What factor secreted by the endothelium plays the largest role in fibrinolysis?
tPA (tissue plasminogen activator) because it acts on plasminogen of the fibrin clot to generate plasmin which cleaves cross-linked fibrin clots to dissolve the thrombus
What is released from the clot during fibrinolysis that can be measured in the lab?
What does this help monitor?
D-dimers can be measured to monitor the presence or absence of a thrombus or to assess DIC (disemminated intravascular coagulation) in clinical conditions
What neutralizes any free plasmin? Why?
Anti-plasmin neutralizes free plasmin to prevent primary fibrinogenolysis.
What regulates the action of tPA?
plasminogen activator inhibitor-1 secreted by endothelium
What do most screening coagulation tests diagnose?
What does it NOT predict?
The cause of bleeding tendency in bleeding patients.
It does not predict the bleeding risk in non-bleeding patients
What does primary hemostasis assessment begin with?
The platelet count
Thrombocytopenia is a component of several acquired bleeding disorders. Give two examples.
1. Disseminated Intravascular Coagulation (DIC)
2. Liver disease
In most individuals, there is no significant risk of spontaneous bleeding with a platelet count above ______________.
What is the recommended cutoff for prophylactic platelet transfusion in non-bleeding patients?
What are the two newer automated tests that have proven effective in assessing platelet function?
1. Platelet function analyzer (PFA-100) that measures the time required for blood to occlude an aperture in the membrane of collagen/epi or collagen/ADP-coated test cartridge
2. Diagnostic test- aggregation studied by an aggrometer
What test is "closure time" associated with?
How is it assessed?
It is associated with the platelet function analyzer and it tests to see how long it takes platelets to occlude an aperture on the membrane of a collagen/epinephrine or collagen/ADP coated test cartridge
What does the platelet function analyzer effectively screen patients for?
1. platelet function disorders
2. monitor anti-platelet factors
3. monitor treatment in patients with inherited bleeding disorders
What are the three coagulation assessment tests?
1. prothrombin time
2. partial thromboplastin time )PTT_
3. thrombin time
Prothrombin time (PT) helps assess the function of clotting factors associated with ___________________.
TF 7 and common pathways (2,5, 10 and fibrinogen)
What does PT represent?
The time required to form a clot after Ca and tissue thromboplastin (which has TF) are added to patient plasma.
What disorders would have a prolonged PT?
Because 7, 2, 10 all depend on vitK, a prolonged coagulation time would show:
1. vitK deficiency
2. warfarin (coumadin) therapy
3. liver disease or DIC
4. congenital factor 7 deficiency
What is the international normalized ratio used to do?
How is it calculated?
INR- due to heterogeneity of PT reagents, it standardizes the PT to better manage warfarin anti-coagulation
It is the patients PT compared to the mean normal PT and applying it to an international sensitivity index
INR is used to measure _____________________, while PT is used to measure ________________.
INR measures warfarin therapy monitoring
PT is used as a measure of primary hemostasis
PTT assesses the function of which clotting factors?
Intrinsic, contact factors and common pathways (8,9,11,12)
What does PTT represent?
The time required to form a clot after patient plasma is mixed with PL, Ca. and a contact activator.
What conditions would prolong PTT?
1. Hemophilia A and B depending on the extend of deficiency of 8 and 9 respectively
2. Variably prolonged by heparin therapy
3. Lupus anticoagulant
4. Factor 12 deficiency
What clotting factor does not affect PT or PTT?
An isolated prolonged PTT should be investigated by a history and physical looking for what three things?
1. evidence of bleeding
2. history of current heparin therapy
3. family history
What is used to distinguish factor deficiency from circulating inhibitors?
PTT mixing studies where the patients plasma is mixed with normal pooled plasma in 1:1 ratio.
Complete correction of PTT shows factor deficiency.
Partial correction is circulating anticoagulant/coagulant inhibitor
What does TT assess?
The final step in the coagulation cascade where thrombin converts fibrinogen to fibrin
What does TT represent?
The time it takes to clot after bovine thrombin is added to patient plasma
What does TT allow you to distinguish between?
1. Prolonged PTT due to herparin vs. other causes of prolonged PTT
With what disease is dysfibrinogenemia common?
chronic liver disease