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Haematology > Haemostasis > Flashcards

Haemostasis Flashcards

1
Q

State the four steps involved in haemostatic plug formation from the time of injury.

A
  1. Vessel constriction
  2. Formation of an unstable platelet plug (platelet adhesion + platelet aggregation)
  3. Stabilisation of plug with fibrin (blood coagulation)
  4. Dissolution of clot and vessel repair (fibrinolysis)
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2
Q

What component found underneath the endothelium is involved in triggering the coagulation cascade?

A

Procoagulant subendothelial structures e.g. COLLAGEN Tissue factor is also expressed on the surface of the cell that underlie blood vessels but it is NOT normally expressed within the circulation itself

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3
Q

State some important factors produced by endothelial cells.

A

Prostacyclin (PGI2)
Thrombomodulin
Von Willebrand Factor
Plasminogen Activator

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4
Q

What process during maturation of the megakaryocytes is important for the formation of platelets?

A

Granulation

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5
Q

How many platelets are produced by one megakaryocyte?

A

4000

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6
Q

What do the dense granules in platelets contain that is important for platelet function?

A

ADP

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7
Q

What do alpha granules in the platelets contain?

A

vWF
Factor V
growth factors
fibrinogen

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8
Q

State the two ways in which platelets can bind to collagen. Name the receptors involved.

A

It can bind via vWF to collagen (via the Glp1b receptor)

It can bind directly to the collagen (via the GpVI and Integrin alpha2beta1 receptor)

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9
Q

What happens following the passive adhesion of platelets and engagement of receptors?

A

The receptors signal inside the cell to release ADP from the storage granules and to synthesise thromboxane
These bind to receptors on the surface of the platelets and activate them
Once activated, GlpIIb/IIIa receptors become available, which can bind to fibrinogen and allows the platelets to aggregate

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10
Q

Which receptors on the platelets become available following activation of the platelets and what do they bind to?

A

GlpIIb/IIIa

These bind to fibrinogen which slows bleeding and provides surface for coagulation

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11
Q

What activates platelets?

A
  1. Thrombin and Collagen

2. ADP and thromboxane

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12
Q

Which enzyme converts phospholipids to arachidonic acid?

A

Phospholipase

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13
Q

What does COX convert arachidonic acid to?

A

PGG2 and PGH2 which are endoperoxides

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14
Q

Describe what happens to endoperoxides in platelets and in the endothelial cells.

A

Platelets – thromboxane synthetase converts endoperoxides to thromboxane (potent inducer of platelet aggregation)
Endothelial Cells – prostacyclin synthetase converts endoperoxides to prostacyclin (inhibitor of platelet function)

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15
Q

What effect does aspirin have on this entire pathway?

A

Aspirin is an irreversible COX1 inhibitor

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16
Q

State some important drug targets in platelet aggregation.

A

COX inhibitor
GlpIIb/IIIa antagonists
ADP Receptor antagonists

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17
Q

State two ADP receptor antagonists.

A

Clopidogrel

Prasugrel

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18
Q

State three GlpIIb/IIIa antagonists.

A

Abciximab
Tirofiban
Eptifibatide

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19
Q

What is the most important test for monitoring platelets and their function?

A

Platelet count

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20
Q

What is a common cause of spontaneous bleeding and what are the symptoms?

A 
Autoimmune thrombocytopenia (autoimmune antibodies clear platelets from the circulation) 
This results in purpura, multiple bruises and ecchymoses
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21
Q

What is the normal range for platelet count?

A

150-400 x 10^9/L

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22
Q

Why do you get thrombocytopenia in leukaemia?

A

Leukaemic cells populate the bone marrow so it crowds out the megakaryocytes so the platelets aren’t produced in sufficient numbers

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23
Q

What is the bleeding time test used to observe?

A

This checks the platelet-vessel wall interaction

This isn’t used any more

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24
Q

Describe the platelet aggregation test.

A

The platelets are stimulated with ADP/thromboxane/collagen to study their function
This is used to diagnose platelet disease e.g. von Willebrand disease

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25
Q

Where is von Willebrand factor produced?

A

Endothelial cells and a little bit by megakaryocytes

26
Q

What factors do megakaryocytes produce?

A

Factor V

Von Willebrand Factor

27
Q

Tissue factor activates the clotting cascade by converting 9 to 9a and by converting 10 to 10a. What difference does this make?

A

9 to 9a – slower but produces more thrombin

10 to 10a – faster

28
Q

State two accelerating factors. What are they activated by?

A

Factor VIII
Factor V
They are activated by trace amounts of thrombin

29
Q

Which factors are activated on the surface of the platelet? Describe how this works.

A

10 to 10a
2 to 2a (prothrombin to thrombin)
For 9a to activate 10 it needs to come in close proximity with 10. They both bind to the surface of the platelet mediated by calcium ions, and factor VIIIa bring the two close together so that 9a can proteolytically cleave 10 to 10a
Factor Va does the same with 10a and 2 (prothrombin)

30
Q

Which factors are affected by warfarin?

A

2, 7, 9, 10

31
Q

What is common to all of these factors (affected by warfarin) and what is the significance of this common feature?

A

They have a cluster of glutamic acid
The glutamic acid is recognised by an enzyme in the liver and undergoes post-translational modification in the presence of vitamin K to Gamma-carboxyglutamic acid
Once this extra carboxyl group is added, calcium can facilitate the binding of gamma carboxyglutamic acid to the activated platelet membrane phospholipid

32
Q

How does warfarin actually inhibit the post-translational modification of these factors?

A

Warfarin inhibits vitamin K epoxide reductase thus inhibiting the gamma carboxylation of factors 2, 7, 9 and 10

33
Q

Which factors are inhibited by anti-thrombin?

A

2, 9, 10, 11

34
Q

What effect does heparin have on anti-thrombin?

A

Heparin potentiates the action of anti-thrombin by 3000x

35
Q

In what situation is heparin used?

A

Heparin is used for immediate anticoagulation in deep venous thrombosis and pulmonary embolism

36
Q

Describe how anti-thrombin inhibits the clotting factors.

A

Anti-thrombin has a reactive loop that irreversibly inhibits the active site on the clotting factors
So anti-thrombin acts as a scavenger in stopping inappropriate action of clotting factors

37
Q

How does heparin potentiate the action of anti-thrombin and why is the chain length important?

A

Heparin is a linear negatively charged polysaccharide
Once bound to anti-thrombin it changes the position of the reactive loop and makes the inhibition occur faster
When inhibiting factor 10a, a relatively SHORT chain of heparin is sufficient (low molecular weight heparin)
When inhibiting thrombin, you require a LARGE chain (standard/unfractionated heparin)
NOTE: standard/unfractionated heparin inhibits either Factor 10a or thrombin

38
Q

State three laboratory tests for blood coagulation.

A

Activated Partial Thromboplastin Time (APTT)
Prothrombin Time (PT)
Thrombin Clotting Time (TCT)

39
Q

What do each of these laboratory tests represent?

A

APTT – detects abnormalities in the INTRINSIC and COMMON pathways (coagulation is triggered by activation of factor 12)
PT – detects abnormalities in the EXTRINSIC and COMMON pathways (tissue factor is added to trigger the extrinsic pathway)
TCT – shows abnormality in the fibrinogen  fibrin conversion (not important any more)

40
Q

What are the main uses of these laboratory tests?

A

APTT and PT are used together for screening causes of bleeding disorders
APTT is used to monitor heparin therapy for thrombosis
PT is used to monitor warfarin treatment

41
Q

What two proteins assemble on the surface of a clot to allow fibrinolysis to take place? Where are these proteins made?

A

Plasminogen
Tissue Plasminogen Activator (tPA)
Plasminogen is a plasma protein
tPA is produced by endothelial cells

42
Q

What is produced from the break down of the fibrin clot and how does this level change in disseminated intravascular coagulation (DIC)?

A

Fibrin degradation products (FDP)

This is elevated in DIC

43
Q

What factors are used in therapeutic thrombolysis of myocardial infarction?

A

tPA and bacterial activator streptokinase

44
Q

Describe the function of the Protein C anticoagulant pathway.

A

Thrombin also has a role in anticoagulation
It binds to thrombomodulin on the surface of endothelial cells and by binding to thrombomodulin it activates protein C, which, along with co-factor protein S, INACTIVATES FACTOR Va and FACTOR VIIIa

45
Q

How does the’ vessel constriction’ step contribute to haemostasis?

A

Contraction of vascular smooth cell muscles limits blood flow to injured vessel

46
Q

Hows does the ‘formation of an unstable platelet plug’ step contribute to haemostasis?

A

Platelet adhesion and aggregation

Limits blood loss and provides surface for coagulation

47
Q

How does the ‘stabilisation of the plug with fibrin’ step contribute to haemostasis?

A

Blood coagulation

Stops blood loss

48
Q

How does the ‘vessel repair and dissolution of clot’ step contribute to haemostasis?

A

Fibrinolysis

Restores vessel integrity

49
Q

Where would you find elastin and collagen in a blood vessel?

A

Basement membrane

also tunica intima, media, and adventitia

50
Q

Where would you find TF in a blood vessel?

A

VSMC

Fibroblasts

51
Q

What is the lifespan of a platelet?

A

10 days

52
Q

Where are platelets stored?

A

Spleen

53
Q

What is the pathway of the production of platelets?

A

Haematopoietic stem cell
Promegakaryocyte
Megakaryocyte
Platelet

54
Q

How many platelets are made each day?

A

10^11

55
Q

What are some ultrastructural features of a platelet?

A

Phospholipid membrane- coagulation
Dense granules- ADP, ATP, serotonin, Ca2+
Alpha granules- growth factors, fibrinogen, FV, vWF
Surface glycoproteins (Glp1a,Glp1b, Glb2b/3a)
Mitochondria
Enzymes- TXA2 synthase, COX 2

56
Q

Outline the pathway in which platelets bind to collagen via the Glp1b receptor

A
  1. normally vWF circulated blood in globular conformation so binding sites are hidden from platelet Glp1b
  2. vascular injury damages endothelium and exposes sub-endothelial collagen
  3. exposed sub-endothelial collagen binds globular vWF
  4. vWF unravels due to shear forces of blood
  5. unravelling of vWF exposes platelet binding sites (Glp1b), they get tethered
  6. this binding of vWF to platelet Glp1b recruits platelets to site of vessel damage
57
Q

Outline the pathway in which platelets bind to collagen via the GpVI and Integrinalpha2beta1 receptor

A

only at low shear so not in arteries or capillaries

  1. platelets bind directly to sub-endothelia matrix containing collagen via these receptors
  2. platelets become activated and recruit more platelets
58
Q

Where is the site of production of most coagulation proteins?

A

Liver

59
Q

At what platelet count do you get spontaneous bleeding?

A

Less than 40 x10^9

60
Q

What is thrombin and what does it do?

A

Serine protease

Converts fibrinogen to fibrin

61
Q

By what fold do Factor 8a and 5a accelerate thrombin production?

A

x10,000

62
Q

What are the two forms of heparin and which works with which clotting factor?

A

Low molecular weight
High molecular weight
Both LW and HW work with Factor 10a
HW works best with Factor 2a (thrombin) as it anchors thrombin to antithrombin with oppositely charged molecules

Haematology (11 decks)

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