Platelets

Question 1

Phases of thrombosis

Answer 1

1. Initiation phase: injury of vessel walls leads to contact between blood and subendothelial cels. TF is exposed and binds to FVIIa or FVII which in subsequently converted to FVII. The complex between TF and FVIIa activates FIX and FX. FXa binds to FVa on the cell surface.
2. Amplification phase: the FXa, FVa complex converts small amounts of prothrombin into thrombin. The small amount of thrombin generated activates FVIII, FV, FCI and platelets locally. FXIa converts FIX to FIXa. Activated platelets bind FVa, FVIIIa and FIXa.
3. Propagation phase: the FVIIIa/FIXa complex activates FX on the surfaces of activated platelets. FXa in association with FVa converts large amounts of prothrombin into thrombin creating a ‘thrombin burst’. The ‘thrombin burst’ leads to the formation of a stable fibrin clot.

Question 2

Bleeding disorder classification

Answer 2

Can be disorders of:

• Platelets (decreased number o