Haemostasis

Question 1

Define haemostasis

Answer 1

The arrest of bleeding and maintenance of vascular patency

Question 2

Name the four key components of haemostasis

Answer 2

1. Primary haemostasis e.g platelet plug
2. Secondary haemostasis e.g fibrin clot
3. Fibrinolysis
4. Anticoagulant Defences

Question 3

Where are platelets formed?

Answer 3

Bone marrow by budding from megakaryocytes cytoplasm

Question 4

How long does a platelet live for?

Answer 4

7-10 days

Question 5

Describe the platelet cascade

Answer 5

1. Endothelial/vessel damage exposes collagen and releases Von Willebrand Factors and other proteins
2. Platelet adhesion occurs at the site of injury
3. Secretion of chemicals which send signals to others and leads to platelet aggregation

Question 6

What can cause failure of primary haemostasis?

Answer 6

Vascular - loss of collagen (scurvy, old age)
Platelets - reduced number e.g thrombocytopenia
Von Willebrand factor - inherited

Question 7

How will problems in primary haemostasis present?

Answer 7

Spontaneous bruising/purpura
Mucosal bleeding
Intracranial haemorrhage
Retinal haemorrhage

Question 8

What is used to assess primary haemostasis?

Answer 8

Platelet count

Question 9

Describe how a fibrin clot is formed

Answer 9

Platelet phospholipid is positive and attracts negative clotting factors - TF and VIIa
TF and VIIa activate clotting factors Xa and V
This allows the conversion of prothrombin to thrombin and thus fibrinogen to fibrin

Question 10

How does thrombin cause positive feedback?

Answer 10

Activates clotting factors VII and IXa which in turn activate more Xa and V

Question 11

What causes failure of fibrin clot formation?

Answer 11

Single clotting factor defence
Multiple clotting factor deficiency
Liver disease
Increased fibrinolysis

Question 12

Describe fibrinolysis

Answer 12

Plasminogen is converted to plasmin by tissue plasminogen activator
Plasmin converts fibrin to fibrin degradation products

Question 13

How can fibrinolysis be measured?

Answer 13

FDP cross links form D-dimers and this can be measured

Question 14

What screening tests are used to assess coagulation?

Answer 14

Prothrombin time

Activated partial thromboplastin time

Question 15

What are the two anticoagulants within the body?

Answer 15

Serine protease inhibitor - anti-thrombin
Protein C and S inhibit clotting factors VII/IXa and Xa/V
Thrombin activates Protein C and S

Question 16

What is thrombophilia?

Answer 16

Deficiency of naturally occurring anticoagulants