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There are two major cascades that can activate thrombin.

The INTRINSIC PATHWAY, like platelet adherence, is started when subendothelial tissue is exposed. INTRINSIC means 'part of'; the clotting factors in this pathway attach to the injury and become part of it.

Clotting proteins from the blood begin to adhere to the exposed tissue; as they adhere, they activate one another until a complex of activated proteins is formed, all stuck to the injury. Calcium is necessary for the activation of many of these proteins.

The EXTRINSIC pathway involves a different set of clotting factors which are activated by TISSUE FACTOR, a chemical released from damaged cells. This pathway happens out in the blood, not in the injury (King and Marchesini, 2004)

So how do all these activated proteins activate thrombin and form a clot? And how do they make sure that the clot forms in the right place?

Here's where platelets come in.

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