Haemostasis

Question 1

What is haemostasis?

Answer 1

the arrest of bleeding and the maintenance of vascular patency

Question 2

What are the requirements of haemostasis?

Answer 2

permanent state of readiness, prompt response, localised response, protection against unwanted thrombosis

Question 3

What are the components of haemostasis?

Answer 3

ARREST OF BLEEDING:

• formation of platelet plug/ primary haemostasis
• formation of fibrin clot/ secondary haemostasis

SWITCH OFF CLOTTING:

• fibrinolysis
• anticoagulant defences

Question 4

How are platelets formed?

Answer 4

they are formed in the bone marrow from budding from megakaryocytic (lots of platelets come off these massive cells)

Question 5

Platelets are small _______ with a mean life span of ______

Answer 5

anucleate discs with a mean life span of 7-10 days

Question 6

Describe how the platelet plug is formed?

Answer 6

endothelial (vessel wall) damage exposes collagen and releases Von Willebrand Factor (VWF) and other proteins to which platelets have receptors
There is then secretion of various chemicals from platelets which leads to aggregation of platelets at the site of injury

Question 7

When may the platelet plug be sufficient on its own?

Answer 7

if there is only a small cut it may be sufficient, however if cut is bigger then flowing blood can wash away the platelet plug, hence the fibrin clot is needed

Question 8

Describe the process of fibrin clot/ secondary haemostasis?

Answer 8

tissue factor is released from damaged endothelium
tissue factor binds to factor VII and activates it initiating haemostasis
there is then activation of factor V and X
factor V and X activate pro-thrombin, a precursor to thrombin
thrombin then activates fibrinogen which forms the end fibrin clot
there is only a small amount of factor VII and tissue factor at the initial stage so this is increased through amplification
once thrombin is activated it activates factor VIII and IX which activates more V and X generating more thrombin in a positive feedback loop

Question 9

Describe the process of fibrinolysis?

Answer 9

as soon as bleeding is stopped, start breaking down the clot
fibrin clot is broken down by plasmin (formed from plasminogen)
plasminogen formed by action of tissue plasminogen activator (TPA)
when fibrin is broken down it is broken into fibrin degradation products which when cross linked are known as D dimers

Question 10

Describe anticoagulant defences?

Answer 10

when bleeding stops anti-thrombin binds to thrombin and switches it off
protein c and s bind to clotting factor and switch them off
thrombin can also bind to thrombo-modulin which then can go onto switch on protein C and S