Haemostasis recap

Question 1

What is haemostasis?

Answer 1

Haemostasis is the arrest of bleeding in the body

Question 2

What are the main characteristics of the haemastatic system?

Answer 2

• Permanent state of readiness
• Prompt response
• Localised response
• Protection against unwanted thrombosis

Question 3

What are the main components of the haemostatic system?

Answer 3

• Platelets - Primary haemostasis (Formation of platelet plug)
• Blood clotting factors - Secondary haemostasis (Formation of fibrin clot)
• Fibrinolysis (Breakdown of blood clots)
• Anticoagulant defences against blood clotting factors

Question 4

What is primary haemostasis?

Answer 4

Primary haemostasis is the formation of a platelet plug in order to prevent bleeding

Question 5

What is the mean lifespan of a platelet?

Answer 5

7-10 days

Question 6

Describe the physiology of primary haemostasis?

Answer 6

Endothelial damage exposes collagen underneath, which releases Von Willebrand Factor (VWF) and other proteins, complementary to platelet receptors

This allows platelets to adhere to the site of injury via glycoproteins

The platelets then release chemicals such as ADP and Thromboxane A2 which recruit other platelets for platelet aggregation

This forms a platelet plug

Question 7

What is secondary haemostasis?

Answer 7

Secondary haemostasis is the formation of a strong fibrin clot, formed by the activation of the coagulation cascade

Question 8

Describe the physiology of secondary haemostasis

Answer 8

Platelets are full of phospholipids and calcium

They release high levels of calcium which sit on their cell surface membrane

Clotting factors such as Clotting factors VII, X, IX and prothrombin are negatively charged and are attracted to the Ca2+ on the surface

These are enzymes that need to be close to be activated, so when they bind, they get closer and so activate a cascade

There are 2 coagulation cascades, the intrinsic and the extrinsic cascade

Question 9

What is the intrinsic cascade of haemostasis?

Answer 9

Question 10

What is the extrinsic cascade of haemostasis?

Answer 10

Question 11

How is the extrinsic cascade (TF, 7) measured?

Answer 11

Prothrombin time (PT)

Question 12

How is the intrinsic cascade (12, 11, 8, 9) measured?

Answer 12

Activated Partial Thromboplastin Time (APTT)

Question 13

What is fibrinolysis?

Answer 13

The breakdown of a fibrin clot

Question 14

Describe the physiology of fibrinolysis

Answer 14

Tissue plasminogen activator (tPA) converts plasminogen to plasmin

Plasmin then breaks down fibrin into FDPs (Fibrin Degradation Products), also known as D-dimers

Question 15

What are the 3 main natural anti-coagulants?

Answer 15

• Anti-thrombin III
• Protein C
• Proteins S

Question 16

What is the function of anti-thrombin III?

Answer 16

Antithrombin III binds to and inactivates thrombin

This stops fibrin formation and prevents the re-activation of factor 8 and 9, therefore preventing excess clotting

Question 17

What is the function of protein C?

Answer 17

Inactivates factors 8 and 9

Question 18

What is the function of protein S?

Answer 18

Inactivates factors 5 and 10

Question 19

How is thrombus formation modulated?

Answer 19

By thrombomodulin, which binds to thrombin and, when the clot is achieved, activates protein C and S

Question 20

How is the common pathway (5, 10, thrombin) of haemostasis measured?

Answer 20

Both APTT and PT

Question 21

How can thrombin activity be measured?

Answer 21

TCT (Thrombin correct time)