Haemostasis Flashcards
What is haemostasis
The arrest of bleeding and
the maintenance of vascular patency
List the key steps of the haemostatic system
Primary haemostasis - formation of platelet plug
Secondary haemostasis - formation of fibrin clot
Fibrinolysis
Anticoagulant defence - switch off the mechanism
What is fibrinolysis
Breaking down of a blood clot
Why is fibrinolysis vital to the haemostatic system
Need to keep the clot an appropriate size so that the vessel isn’t blocked
Balance between haemostasis and this
How do platelets form
Platelets bud off from the megakaryocyte in the bone marrow
Describe the structure of a platelet
Small discs without a nucleus
Have a lifespan of 7-10 days
How does endothelial damage lead to platelet aggregation at the site
The damage exposes collagen and releases Von Willebrand Factor (VWF), and other proteins to which platelets have receptors – platelet adhesion at the site of injury.
There is then secretion of various chemicals from the platelets, which leads to aggregation of platelets at the site of injury (more are signalled to come to site)
What can cause a failure of formation of the platelet plug
Thrombocytopaenia - reduced number of platelets
Reduced platelet function
Lack of collagen in vessels - age or Vit C deficiency
Collagen disorders - SLE, anti-phospholipid
Aspirin use
VWF diseases
What happens if the platelet plug fails to form at a damaged site
Spontaneous Bruising and Purpura ( mainly on lower limbs)
Mucosal Bleeding - epistaxis, GI, conjunctival and menorrhagia
Intracranial haemorrhage
Retinal haemorrhages
How can you screen for primary haemostasis
Platelet count
Part of the FBC
How does the fibrin clot form
Platelets release calcium – it is positive charged so sits on the negative phospholipid layer
This attracts negatively charged clotting factors
Tissue factors is released when tissue is damaged and activates factor 7 – starts haemostasis
This activates 5 and 10 which cleave prothrombin to its active form – thrombin (2a)
Thrombin then cleaves fibrinogen to fibrin which forms the clot
Thrombin activates 8 and 9 which amplify the 5/10 which acts as positive feedback to produce more fibrin
What can cause a failure of fibrin clot formation
Single clotting factor deficiency - haemophilia
Multiple clotting factor deficiency - DIC
Increased fibrinolysis
What causes haemophilia
Inherited defect
Haemophilia A – clotting factor 8 is missing
In B, clotting factor 9 is missing
What is Disseminated Intravascular Coagulation
Occurs when there is massive tissue damage
You release lots of TF which causes lots of blood clots
The clotting factors are used up and eventually haemorrhage occurs
Why can liver disease affect the clotting mechansms
Clotting factors are made in the liver
If production is affected then the system doesn’t work as well
How can you measure fibrinolysis
Measure degradation products in a blood test
D-dimers are the ones used
Can screen for DVT and PE
What is the pattern of bleeding in haemophilia
More likely to bleed into muscles and joints - leads to pain and swelling
Delayed bleeds
Knees and ankles most often affected
How do you screen for fibrin clot formation (secondary haemostasis)
Prothrombin time - measures extrinsic path
Activated partial thromboplastin time - APTT
This measures intrinsic
Which clotting time is affected in haemophilia
The APTT would be prolonged in haemophilia
It measures the action of factors 8 and 9
What are the naturally occurring anticoagulants
Anti-thrombin = neutralizes thrombin (binds to it)
Protein C and S = switch off factor 8 and 5 to stop haemostasis
How does heparin function
Heparin helps anti-thrombin do its job to neutralise thrombin – stabilizes the complex
How does a failure of primary haemostasis present
Usually problems with superficial bleeds
Easy bruising (petechiae)
Prolonged bleeding after dental work/surgery
Mucosal bleeding
Menorrhagia
How long before surgery should aspirin be stopped
7 days
Platelets live for around 7 days so need to be sure you have outlived the anti-platelet effect
What is the function of secondary haemostasis?
It reinforces the plug through the formation of a fibrin clot
