Primary versus secondary haemostasis
Primary haemostasis is the platelet plug that forms first after a vessel is injured, and secondary haemostasis is the fibrin that reinforces it into a firm clot.
Remember
Platelets form the first plug within seconds to minutes, and fibrin then turns that fragile plug into a firm clot.
Memory aid
Platelets lay the cork first, and fibrin adds the cement afterwards
The cork goes in first and the cement sets it afterwards, just as the platelet plug comes first and fibrin reinforces it.
Haemostasis (the body's way of stopping bleeding) happens in overlapping steps. The injured vessel contracts first. This is not enough to stop the bleeding, but it reduces the pressure downstream of the injury and helps to control the bleeding.
Primary haemostasis is the platelet plug. Platelets (small cell fragments with no nucleus) stick (adhere) to collagen that is exposed when the vessel wall is damaged, helped by von Willebrand factor, a sticky protein that tethers platelets to the vessel wall. The activated platelets release ADP and thromboxane A2, which recruit more platelets and activate them, and the platelets aggregate (clump together) as fibrinogen bridges receptors on their surfaces. The plug forms within seconds to minutes, but on its own it is fragile.
Secondary haemostasis is the fibrin reinforcement. The clotting cascade (a chain of steps in which clotting factors switch each other on) ends with thrombin, an enzyme that turns soluble fibrinogen into fibrin. Fibrin threads weave through the platelet plug and bind the platelets together. Activated factor XIII (a clotting factor) then cross-links (chemically joins) the fibrin strands, strengthening the clot.
The two stages overlap, with no fixed delay before fibrin forms, but the platelet plug still forms first. Together they make a firm clot that can hold until the vessel wall has healed.
Key facts
- Sequence
- the vessel contracts, then the primary platelet plug forms, then fibrin reinforces it in a secondary plug, although the steps overlap.
- Speed
- the primary plug forms within seconds to minutes of the vessel being cut.
- Platelet steps
- platelets adhere to exposed collagen with the help of von Willebrand factor, are activated and release ADP and thromboxane A2, then aggregate through fibrinogen bridges.
- Secondary haemostasis
- thrombin from the clotting cascade turns fibrinogen into fibrin, and the fibrin makes a mesh around the platelets.
- Cross-linking
- activated factor XIII cross-links the fibrin strands, which strengthens the clot.
- Platelets alone
- a plug of platelets alone is fragile and needs fibrin to become a firm clot.
Exam traps
- Platelet adhesion and aggregation make the primary plug, but on its own that plug is fragile, so they do not stabilise it: a fibrin mesh around the platelets is needed.
- Vasoconstriction (narrowing of the injured vessel) helps to control bleeding, but it does not form a plug, so it is not what builds the first platelet plug.
- Fibrin polymerisation, the linking of fibrin strands into a mesh, belongs to secondary haemostasis, so it reinforces a platelet plug that is already in place rather than starting one.
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