In the intrinsic pathway, the reaction begins with clotting factors from the _.

Clotting factors are circulating in the blood.

Explanation:

In the intrinsic pathway, the body is responding intravascular damage through binding of platelets, exposure of chemicals or even collagen from endothelial membranes. The most of the clotting factors are synthesized by the liver and circulate around the blood stream until they are needed or until they are degraded based off their half lives.

When the platelets start to aggregate they release chemicals and have proteins that can attract the clotting factors. In the intrinsic pathway, clotting Factor XII starts off the cascade. Hope that helped :)