Haemostasis Flashcards
(6 cards)
What is haemostasis
Complex multistage process where body prevents blood loss following tissue / blood vessel damage.
What are the 4 stages of haemostasis
Stage 1 : Vasoconstriction
Stage 2 : Activation of platelets and formation of plug (primary plug)
Stage 3 : Coagulation, fibrin formation and clot retraction (secondary plug)
Stage 4 : Fibrinolysis
Stage 1 : Vasoconstriction
- Reflex response to causing local constriction of blood vessels
- Triggered by direct injury to vascular smooth muscle cells, signalling molecules released from injured endothelial cells, signalling molecules released from activated platelets, or neuronal signals from local pain receptors
- It reduces the blood flow to injured vessels, which reduces blood loss. It also increases likelihood of contact activation of platelets and coagulation factors by slowing blood flow
Stage 2 : Platelet activation and platelet plug
Endothelial injury exposes collagen and vWf (glycoprotein receptor), which platelets bind to, activateing the platelets.
They change shape and release granules (alpha and gamma), which reinforces platelet activation, activates coagulation cascade and drives vessel wall / tissue repair.
Platelets become sticky and bind to fibrinogen causing aggregation of platelets, (primary haemostatic plug)
Stage 3 : Coagulation and fibrin formation
Coagulation cascade activates clotting factors, causing :
- Thrombin to covert soluble fibrinogen to insoluble fibrin
- Fibrin threads cause cross-linked mesh strengthening the plug
Key parts of coagulation cascade :
- Proteolysis cascade converts precursor factors to active factors
- Each step in pathway amplifies signal
- End result is generation of thrombin
- Also has negative feedback loops to ensure local and limited thrombin generation
Stage 4 : Fibrinolysis
Once healing starts the clot is broken down by plasmin which dissolves fibrin and prevents clotting / blockage of vessel.
