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The horribly complicated clotting cascade.


Coagulation is the process by which blood clots, turning from liquid to solid. It is very important, since if it doesn't work, you get bleeding disorders and if it works too well you get thrombosis.

How it works

The whole process of clotting is based on many parts, each component performing a small but vital role. The diagram on the right shows the Clotting Cascade, which details this, but in essence, damaged vessel walls cause platelets to get joined together by fibrin, forming a tight clot.

Thus the three components of the coagulation process are: platelets, fibrin, and the damaged endothelium. Whilst platelets are naturally present in blood, fibrin is formed through the clotting cascade...

Clotting Cascade Made Easy

Frankly, I don't think its worth putting in the time to memorise each of the 30+ factors and chemicals involved in clotting. However, there are some key salient points:

  • There are two pathways: the contact activation pathway (also known as the intrinsic pathway) and the tissue factor pathway (known as the extrinsic pathway).
  • Contact activation pathway is disrupted by heparin, not working properly in Haemophilia, and can be measured using the APTT clotting test.
  • Tissue factor pathway is interrupted by warfarin, and is measured using INR or PT (technically, warfarin disrupts APTT too, but the key is that INR is used to measure it's efficacy).