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haematology > Introduction to Haemostasis > Flashcards

Introduction to Haemostasis Flashcards

1
Q

What is haemostasis?

A

Arrest of bleeding and maintenance of vascular patency

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2
Q

What is required of haemostasis so that it may be an effective response?

A

Must be in permanent state of readiness
Prompt and localised response
Give protection against unwanted thromboses

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3
Q

What are the stages of normal haemostasis?

A 
Formation of platelet plug = primary haemostasis
Formation of fibrin clot on top of platelets = secondary haemostasis
Fibrinolysis and anticoagulant defences
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4
Q

Where are platelets formed?

A

In the bone marrow by budding from megakaryocytes

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5
Q

What are platelets?

A

Small anucleated discs with a lifespan of 7-10 days

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6
Q

What causes platelet adhesion to the site of an injury?

A

Endothelial damage exposes collagen and releases VWF and other proteins to which platelets have receptors

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7
Q

What causes platelet aggregation at the site of the injury?

A

Platelets already adhered to the injury secret various chemicals that attract other platelets

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8
Q

What are the causes of failure to form a platelet plug?

A

Vascular = lack of collagen in vessel wall

Thrombocytopenia, reduced platelet function and VWF deficiency

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9
Q

What does failure of platelet plug formation cause?

A

Spontaneous bruising and bleeding
Mucosal bleeding and retinal haemorrhage = epistaxis, GI/conjunctival bleeding, menorrhagia
Intracranial haemorrhage if severe

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10
Q

How are disorders in primary haemostasis screened for?

A

Using platelet count

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11
Q

Why are clotting factors attracted to platelets?

A

Platelets contain positively charged phospholipids = attracts negatively charged clotting factors

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12
Q

What causes release of tissue factor?

A

Endothelial damage

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13
Q

What is the first step in fibrin clot formation?

A

Tissue factor binds to clotting factor VIIa = this then activates factors V and Xa

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14
Q

What does the activation of factors V and Xa in the clotting cascade cause?

A

Activate prothrombin to form thrombin = this causes fibrinogen to form fibrin

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15
Q

Why is an alternative simultaneous pathway needed to produce fibrin?

A

Only limited amount of tissue factor released which wouldn’t produce enough fibrin by the main pathway to stop bleeding

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16
Q

What is the alternative pathway that produces fibrin via a positive feedback loop?

A

Thrombin activates factors VIII and IXa = these activate further factors V and Xa

17
Q

What are the different phases of fibrin clot formation?

A 
Initiation = tissue factor release
Amplification = activation of factors VIII and IXa
Propagation = fibrin formation
18
Q

What are some causes of failure to form fibrin clot?

A

Single clotting factor deficiency = usually hereditary
Multiple clotting factor deficiencies = usually acquired
Increased fibrinolysis = usually part of complex coagulopathy

19
Q

What occurs in fibrinolysis?

A

tPA activates plasminogen to form plasmin

Plasmin causes fibrin to break down to form FDPs

20
Q

What can be measured to show excessive fibrinolysis?

A

D-dimers = formed by cross-linkage of FDPs

21
Q

What can be used to screen for fibrin clot formation abnormalities?

A

Prothrombin time and activated partial thromboplastin time

22
Q

What is the initial step in both prothrombin time and activated partial thromboplastin time?

A

Blood is added to citrate and centrifuged to separate platelets

23
Q

How is prothrombin time measured?

A

Add tissue factor and phospholipid = measure time taken for blood to clot

24
Q

How is activated partial thromboplastin time measured?

A

Add phospholipid and contact activator and measure clotting time = tests factors VIII and IXa

25
Q

What is the clinical approach to a patient with abnormal bleeding?

A

History = bleeding or bruising, duration (lifelong?), previous surgery or dental extractions
Drug history = aspirin, NSAIDs, anticoagulants

26
Q

What are some naturally occurring anticoagulants?

A

Serine protease inhibitors, proteins C and S, thrombin

27
Q

How do serine protease inhibitors cause anticoagulation?

A

Anti-thrombin binds to thrombin to deactivate it

28
Q

How do proteins C and S cause anticoagulation?

A

Bind to factors V and VIII to deactivate them

29
Q

How does thrombin cause anticoagulation?

A

Switches function once adequate clotting has occurred = binds to thrombomodulin to activate proteins C and S

30
Q

What is thrombophilia?

A

Deficiency of naturally occurring anticoagulants

31
Q

What do thrombophilias cause?

A

Increased tendency to develop venous thrombosis

haematology (24 decks)

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