The word “hemophilia” is a combination of the Greek words for “blood” and “love”, a way of saying that people with hemophilia “love to bleed”, or rather that it’s hard to stop bleeding. This is because the process called hemostasis, literally meaning to stop the flow of blood, is impaired.
Normally, after a cut and damage to the endothelium, or the inner lining of blood vessel walls, there’s an immediate vasoconstriction or narrowing of the blood vessel which limits the amount of blood flow. After that some platelets adhere to the damaged vessel wall, and become activated and then recruit additional platelets to form a plug. The formation of the platelet plug is called primary hemostasis.
After that, the coagulation cascade is activated. First off in the blood there’s a set of clotting factors, most of which are proteins synthesized by the liver, and usually these are inactive and just floating around the blood. The coagulation cascade starts when one of these proteins gets proteolytically cleaved. This active protein then proteolytically cleaves and activates the next clotting factor, and so on. This cascade has a great degree of amplification and takes only a few minutes from injury to clot formation. The final step is activation of the protein fibrinogen to fibrin, which deposits and polymerizes to form a mesh around the platelets. So these steps leading up to fibrin reinforcement of the platelet plug make up the process called secondary hemostasis and results in a hard clot at the site of the injury.
In most cases of hemophilia there is a decrease in the amount or function of one or more of the clotting factors which makes secondary hemostasis less effective and allows more bleeding to happen.