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Coagulation, is factor IX a required part of external path?

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Coagulation, is factor IX a required part of external path?

Postby north3rner » Sun Oct 25, 2015 8:58 am

Been reading "Clinical Laboratory Science" - Linne & Ringsrud. Scanned relevant paragraphs. Cannot grasp this, the book seems contradictory regarding factor IX in the external path.

I've scanned two paragraphs and two images from the book chronologically (se bottom). The part I don't grasp says:
Factor VII is activated to its VIIa form in the presence of ionized calcium (factor IV) and tissue thromboplastin (factor III). Factor VIIa activates coagulation factor IX to IXa, which in turn activates factor X to Xa. Thromboplastin is released from the injured wall of the blood vessel. Only activated factor VII is needed in the extrinsic pathway, bypassing factors XII, XI, IX, and VIII ...
So first it states factor IX is part of the external path ("Factor VIIa activates coagulation factor IX to IXa") and then immediately afterward it states that factor IX is bypassed in the external path ? Wut?

Factor VII is the activator of factor X in the external pathway (as made clear by second image) but does it depend on factor IX or not ? The image doesn't indicate that but why does the text say so ?

~

Second quandary. What does the arrow in the first image from activated factor VIIa connecting to (intrinsic pathway)factor IX activation mean. Does it mean that activated factor VII (VIIa) is required to activate factor IX (along with calcium Ca++ and factor XIa)? or does it mean something else ?

Begin chronologically scanned textbook text and images:
Image
Image
Image
Image
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Postby Hodag » Sat Dec 05, 2015 7:52 pm

The answer is yes, Factor IX is required. Let me see if I can explain it better than the book did. There are two ways to activate X to Xa, after which all the steps are the same, hence the name "Common Cascade".

The contact activation cascade needs four components to activate Factor X.

Factor VIIIa
Factor IXa
Phospholipids
Ionized calcium (formerly Factor IV)

The tissue factor cascade also needs four components to activate Factor X.

Factor III (tissue factor)
Factor VIIa
Phospholipids
Ionized calcium (formerly Factor IV)

Since Factor III is required to activate VII to VIIa, is must already be present (by implication) but you really do need all four components to get the job done. To say Factor VIIa alone is enough is an oversimplification.

I think the main reason you are confused is that diagram is not very clear and, frankly speaking, all the coagulation cascade diagrams that you can find on Google or Bing are a mess of arrows and boxes, because it really is a complicated part of biology. However, the diagram I refer back to time and time again can be downloaded from this link:

http://www.uab.edu/medicine/coag/images ... iagram.pdf

It is no less messy than the others, but it is very comprehensive and better labeled than most. I hope that helps.
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