Well the cascade is very complex indeed as the many factors interact with those down the line and also with those before them in the cascade. This seems very complex until you realize that many of the factors are very similar in build. Perhaps they are the result of duplicate genes which have diverged? No matter lets continue.
The only required protein is of course fibrinogen and the protease which works on it. In this case its thrombin. The thrombin breaks the fibrinogen apart and allows the resulting proteins to stick together.
Another thing to keep in mind is that there are two pathways the extrinsic and the intrinsic pathway. However the extrinsic pathway is much faster compared to the reaction time of the intrinsic pathway. So why have the intrinsic pathway in the first place? Well the intrinsic system responds to damage to cell surfaces as in a cut. While the extrinsic system responds to proteins only found in the bodies tissues not in the bloodstream. This pathway would be activated in the case lets say that your kidney is hemorrhaging.
The intrinsic pathway also creates in the cascading enzymatic action, an anti-thrombin which ceases the action of the protease thrombin (ie. stops clotting). This gives us a clue into why there are so many steps in the intrinsic pathway.
Now we need an analogy to simplify this discussion. Don't worry we will always refer back to the actual chemistry and not depend on the analogy so much. It is only to help us get a picture in our minds to work with.
Lets use a management ladder for the intrinsic pathway, and for the extrinsic one we will use, the emergency response system, like a firehouse.
Now before we go into the next section we need a basic understanding of what a protease is.
It is an enzyme which acts on proteins.
And how do enzymes work?
hardhttp://academic.brooklyn.cuny.edu/biolo ... ow_enz.htm
mediumhttp://science.howstuffworks.com/cell2.htm
easyhttp://www.tvdsb.on.ca/westmin/science/ ... nzymes.htm
Great I'm sure everyone had fun reading that material.
=)
Ok cool, now we can continue. Imagine that each protease can convert 4 molecules a minute.
Now in a thromin fibrinogen system the only enzymatic activity is the thrombin cutting the fibrinogen.
In an organism such as this, like the modern lobster the thrombin material is stored intracellularly. The thrombin is realeased and the process is very slow. So we have
Minute 1 4 fibrinogens cut
Minute 2 4 fibrinogens cut
Minute 3 4 fibrinogens cut
Minute 4 4 fibrinogens cut
Result 16 fibrinogens cut. In a vertebrate with a high pressure vascular system I would have bled to death.
Now lets add a cascade level.
We now have prothrombin in the blood and factor X to cut it is now residing intracellularly. The cell is now damaged releasing factor X. Each factor X cuts 4 molecules a minute, and what is it cutting? Prothrombin.
Each Prothrombin becomes a Thrombin protease which in turn cuts 4 Fibrinogen's a minute. So we have
Minute 1 4 prothrombins cut
Minute 1 4 prothrombins cut 16 fibrinogens cut
Minute 1 4 prothrombins cut 32 fibrinogens cut
Minute 1 4 prothrombins cut 48 fibrinogens cut
Result 96 fibrinogens cut. Increddible I add one step and the process is now 6 times faster!
Now lets take a look at the intrinsic system where there is only one protease acting on factor X. In this system factor X is in an inactive form and requires an enzyme to activate it. Lets call this enzyme factor IX.
When a cell is damaged there are proteins on the surface of a cell which will activate factor IX. So we have.
Minute 1 4 factor IX cut
Minute 2 4 factor IX cut 16 Factor X cut
Minute 3 4 factor IX cut 32 Factor X cut 64 prothrombins cut
Minute 4 4 factor IX cut 48 Factor X cut 128 prothrombins cut 256 fibrinogens cut
So now we can see why there is an advantage to having multiple steps. A small stimulus gets amplified greatly!
One should note that adding more cascading steps will delay the point where actual Thrombine is active. but at this point it will be a large amount of proteins working.
Ok great we have answered the innitial questions. What now?
Well next we will examine the factors more closely and see how they interact.
And we will answer this question, why not add one more step to make the process even faster?