Biomedical Engineering Reference
In-Depth Information
Fig. 7.4
Sketch of amplification phase
2.
Amplification
(Fig.
7.4
)
Thrombin produced in the previous stage exerts many actions both at this and at
later stages. We list them as (Thr. n):
•
(Thr. 1). It dissociates the complex FVIII-vWF, at the same time activating FVIII,
•
(Thr. 2). It activates FV, stimulating platelets to produce more of FV,
•
(Thr. 3). It activates FXI.
These actions have important consequences. FXIa is a fast activator of FIX, so
that the amplification stage makes both FIXa and FVIIIa available. Moreover vWF
can promote further platelets aggregation (if stress conditions are favorable). Finally,
platelets become fully activated, releasing the granules content.
3.
Propagation
(Fig.
7.6
)
The coagulation machine is now ready to produce the
thrombin burst
(during
which 95 % of thrombin is produced [
160
]). The surface of activated platelets
provides the ideal site for the combination of FVIIIa, FIXa into the complex
tenase
,
activating FX very rapidly. FXa combines with the available FVa (still on activated
platelets surface) yielding the
prothrombinase
complex. From now on activation
of prothrombin occurs at a large speed and the processes already described in the
previous stage trigger an enormously effective positive feedback. The onset of the
propagation phase is indeed recognizable by the sudden and marked increase of
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