Why does blood clot when removed?

[scilearner]

Hello everyone,

 

If blood is taken to a tube it clots after a while. If the tube is glass, since it is a wettable surface the process is hastened, but my question is normally in the body when blood is flowing, anticoagulents are dominant over coagulents. Now if the blood is moving slow inside the body or out, why do coagulents become more dominant.

 

Also in capillaries there is only endothelium right no collagen. So how do platelets bind if a capillary wall is damaged, platelets bind to collagen right?

 

Thanks a lot

Edited June 30, 2010 by scilearner

[cypress]

There are two pathways for blood clotting cascade. One has, as an initial step, surface contact with collagen as you have indicated, which occurs for example with an open wound. The second does not involve contact with collagen rather it begins with tissue/cell trauma which exposes blood to cells that express thrombokinase, or coagulation factor III. Also known as Tissue Factor (TF), cells that express TF, a surface glycoprotein, are not normally exposed to blood except when tissue trauma occurs. TF binds with VIIa and X and the cascade proceeds. There are a number of good articles on this process on the web I am sure, have a look at them and let us know if you have further questions.

[ewmon]

And part of the beauty is that the blood clots only "just enough" to close the wound. Imagine if you cut your fingertip and all the blood in your finger or hand clotted!

[scilearner]

Thanks for all the answers I read the paths you have described however I'm still little bit off in my thinking. Ok so if you get some blood into a test tube, where does the trauma occur. I mean the surrounding vessel is not damaged, I just put some blood into a test tube, what creates the trauma and make platelets release tissue factor.

 

Also a capillary is made only of endothelium, so how can platelets bind to sub endothelium collagen creating intrinsic pathway, in capillaries does extrinisic pathway take place. Thanks again

[cypress]

Thanks for all the answers I read the paths you have described however I'm still little bit off in my thinking. Ok so if you get some blood into a test tube, where does the trauma occur. I mean the surrounding vessel is not damaged, I just put some blood into a test tube, what creates the trauma and make platelets release tissue factor.

 

Some research has indicated that platelets switch on expression of TF when stimulated. There is clear evidence of TF in Apoptotic platelet cell fragments. When I read about this a couple years ago, it was not clear when or how the stimulation occurs, but it is likely that this may be the source of TF and the cause of coagulation you describe in test tubes.

 

Also a capillary is made only of endothelium, so how can platelets bind to sub endothelium collagen creating intrinsic pathway, in capillaries does extrinisic pathway take place. Thanks again

 

As far as I understand, in capillaries, minor trauma to the endothelium generally exposes blood to cells that contain TF in the membrane surfaces as opposed to exposure to collagen proteins. If the trauma is severe it is likely that both mechanisms are involved.

[scilearner]

Some research has indicated that platelets switch on expression of TF when stimulated. There is clear evidence of TF in Apoptotic platelet cell fragments. When I read about this a couple years ago, it was not clear when or how the stimulation occurs, but it is likely that this may be the source of TF and the cause of coagulation you describe in test tubes.

 

 

 

As far as I understand, in capillaries, minor trauma to the endothelium generally exposes blood to cells that contain TF in the membrane surfaces as opposed to exposure to collagen proteins. If the trauma is severe it is likely that both mechanisms are involved.

 

Thanks again So in the test tube platelets are damaged and release tissue factor. According to wiki Tissue factor, also called platelet tissue factor, factor III, thrombokinase, or CD142 is a protein present in subendothelial tissue, platelets, and leukocyte. So capillaries only have an endothelium, so if only the endothelium is damaged, what releases tissue factor?What are the cells that contain TF in the membran surface, I can only think of platelets in this case.