Session 5 - Haemostasis Flashcards
Why don’t you bleed to death from a minor injuet?
Haemostasis
What are the four factors upon which sucessful homeostasis depends?
vessel wall
platelets
coagulation system
fibrinolytic system
What are three types of blood vessels?
arteries, veins, capillaries
What do blood vessels do to prevent blood loss?
constrict to limit blood loss
What are the four roles of platelets?
adhere to damaged vessel wall
adhere to each other
form a platelet plug
platelet release reaction
Outline the platelet release reaction
Requires ATP
ADP, thromboxane A2 causes platelet aggregation. Serotonin and platelet factor 3 also released. PF3 important in coagulation.
Platelets coalesce after aggregation.
What kind of reaction is coagulation?
A cascade reaction, in which a series of inactive components are converted to active components.
What happens to prothrombin in the coagulation cascade, and what is its purpose?
Prothrombin –> Thrombin –> Fribrinogen –> Fibrin
Thrombin positively feeds forward on factors V, VIII and XI
Why is tight regulation of coagulation required?
1 ml of blood can generate enough thrombin to convert all the fibrinogen in the body to fibrin
Define haemostasis
The process which spontaneously arrests bleeding or haemorrhag
Why does there need to be a balance between coagulant and anti-coagulant factors?
To ensure clotting cascade initiates and terminates
What happens in the body if coagulant > anti-coagulant
Prolonged clotting, unstable clots
What happens in the body if coagulant levels < anti-coagulant
Brief coagulation and quick termination, excess bleeding
What is the extrinsic pathway in haemostasis?
endothelium is damaged causing the release of tissue factor
What is the intrinsic pathway in haemostasis?
activated by thrombin in a feedforward mechanism
What factor do both the extrinsic and intrinsic pathways meet?
At factor Xa
What are the 6 stages of fibrinolysis in haemostasis
- Thrombin binds to receptors on endothelium > protein C released
- Protein C degrades factors V and VIII in the liver
- Protein C causes urokinase-like plasminogen activator to be released
- ULPA converts plasminogen to plasmin
- Plasmin aids fibrinolysis
- tPG also aids fibrinolysis by the activation of plasminogen
Define thrombosis
The formation of a solid mass of blood within the circulatory system during life
Name two anti-thrombotic chemicals
prostacyclin
nitric oxide
Why does thrombosis occur ?
Abnormalities of vessel wall
Abnormalities of blood flow
Abnormalities of blood components
Give two possible abnormalities of blood flow
Stagnation
Venous stasis via the slow flowing blood in veins
Turbulence
Laminar flow disturbed > Cells hit vessel wall > damage can occur when vessels branch
Give three possible abnormalities of blood components
Smokers
Nicotine causes sticky platelets
Post partum
Liver compensates for blood loss by synthesizing more clotting factors
Post op
Liver compensates for blood loss by synthesizing more clotting factors
Give three possible abnormalities of vessel wall and how they could predispose someone to thrombosis
• Atheroma > cracks vessel wall > clotting cascade initiation
• Direct injury
• Inflammation (in vessel wall)
o E.g. primary vasculitis: thrombosis in temporal artery > blindess
What is the appearance of arterial
thrombi?
- Pale
- Granular – due to lots of platelets and fibrin
- Lines of Zahn – ‘waves’ of thrombi
- Lower cell content e.g. RBCs and WBCs
What is the appearance of venous thrombi?
- Soft
- Gelatinous
- Deep red – lots of RBCs
- Higher cell content – low fibrin & platelets
How can post mortem plots be distinguished from those that cause death?
Post mortem clots wash out easily, whereas DVT adher to vessel wall
What are the effects of arterial thrombosis?
- Ischaemia
- Infarction
- Depends on site and collateral circulation
What are the effects of venous thrombosis?
- Congestion
- Oedema
- Ischaemia
- Infarction
Give four outcomes of a thrombosis
Lysis Propagation Organisation Recanalisation Embolism
Outline the process of lysis of a thrombosis
- Complete dissolution of thrombus
- Firbrinolytic system active
- Bloodflow re-established
- Most likely when thrombi are small
Outline propagation of a thrombosis
- Progressive spread of thrombosis
- Goes distally in arteries, proximally in veins
Outline the process of organisation of a thrombosis
- reparative process
- Involves the ingrowth of fibroblasts and capillaries (similar to granulation tissue)
- Lumen remains obstructed