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24th July 2024
Physiology
Haemostasis is achieved through several main steps:
- Formation of a platelet plug
- Coagulation cascade: amplification of the clotting process by sequential enzyme activation
- Termination of the clotting process
- Clot breakdown by fibrinolysis
The traditional model of the coagulation cascade contains two converging pathways. This is a good way to interpret coagulation tests (APTT and PT) but is no longer thought to represent in vivo coagulation.
| Pathway | Initiated by | Measurement |
|---|---|---|
| Intrinsic | Exposure to negatively charged ions activates factor XII | APTT |
| Extrinsic | Damage to a blood vessel exposes tissue factor, which activates factor VII | PT |
| The pathways converge to activate factor X, which stimulates thrombin production from prothrombin
Thrombin catalyses the conversion of fibrinogen to fibrin, forming a cross-linked fibrin clot |
||
Image Source: Wikimedia Commons (2007). Coagulation full., Licensed under a creative commons licence. Available for free at: https://en.m.wikipedia.org/wiki/File:Coagulation_full.svg (Accessed: 16 January 2024)
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