In the blood circulation, there are several compounds that are working to promote coagulation (= pro-coagulants) and other compounds that inhibit coagulation (= anti-coagulants).
In the circulation, there is a delicate balance between pro- and anti-coagulants. Too much pro-coagulation and you induce intravascular clotting, to little and you may have bleeding that does not stop.
To much coagulants -> intra-vascular clotting -> in brain or heart vessels -> infarcts -> dead tissue -> dead person!?
Not enough coagulants -> risk of bleeding -> in the brain or the heart -> causes cell and tissue death -> dead person!?
Either way, the outcome of a disturbance in the coagulation system is dramatic. Therefore, there must be a (delicate) balance between pro- and anti-coagulants.
The coagulation system consists of two branches coming together in a final common pathway:
a) Intrinsic pathway (inside the blood)
b) Extrinsic pathway (outside the blood; i.e. in tissues)
c) Common pathway
All three pathways consist of many different components (= coagulation factors) that are present and dissolved in plasma.
What you seen happening during coagulation is a complicated (but well-regulated) chain reaction of these compounds.
The ultimate goal of this coagulation chain-reaction is to produce fibrin; a non-dissolved compound that fixates the platelet plug that was formed during the hemostatic process.
Let’s start with the intrinsic pathway. “Intrinsic” means that this chain reaction starts and develops within the blood.
Inside the blood, several compounds (called “factors”) are present and dissolved in the plasma. These are factors XII, XI, IX, VII and X.
(remember your Roman numerals?)
Roman numerals: I = 1; V = 5; VII = 7; X = 10; XI = 11, XII = 12
If a serious disturbance, or a trauma, occurs inside the blood, or exposure to something from outside the endothelium such as collagen, then factor XII will be activated. This is shown as XIIa (a = activated).
Once XIIa is present, it will, in turn, activate factor XI. Then, XIa will activate factor IX. Meanwhile, due to the trauma, factor VIII has also been activated. This will, together with IXa also activate factor X into Xa.
This Xa, the product of this intrinsic pathway will now activate the Prothrombin-activator; the beginning of the Common Coagulation Pathway.
As I said before, calcium ion (Ca2+) is very important in enabling these reactions.
This is usually no problem at all in the body (without calcium ions, many things would immediately stop!), so that’s not the issue.
But this makes it possible to store blood, as a fluid, in a container outside the body!
For example, in the case of a vena puncture, when you draw the blood outside the body into syringe, you don’t want the blood to coagulate.
In that situation, you can avoid coagulation by adding EDTA to the blood. This EDTA removes all the Ca2+ from the plasma and stops the coagulation process.
Another important point; many coagulation factors are produced by the liver. Therefore, if there is a liver problem (for example liver cirrhosis), you may also develop a coagulation problem.
In addition, many factors also require the vitamin K.
In fact, the presence of the coagulation factors is so important that we have given a special name for blood in which the blood factors have been inactivated (by EDTA for example).
This plasma without coagulation factors is called: serum.
Finally, this chain reaction is one of the few examples in the body of a positive feedback reaction in the body!
Link: A.1.2. Physiological Concepts
It is really a cascade that is waiting to happen in the blood and where the blood, hopefully at the right spot, coagulates quickly (1-2 min) to stop the bleeding!
As you can imagine, looking at the complex coagulation chain reactions, it has taken a long time and the work of many investigators to unravel all its components and describe this chain of reactions.
Therefore, many of these factors have been given different names, sometimes dependent on the country of the investigators, sometimes because of certain diseases involved in these reactions.
Here is a list of some important synonyms in the coagulation system:
Note the synonyms of factors VIII and IX; anti-hemophilic factor A and B.
Apparently, these factors are absent in people suffering from these two diseases!