A simplified scheme of the important steps

The blood clotting reaction is an enzyme cascade involving Factors XII, XI, IX, X and II. Each of these enzymes are proteases that cut (or cleave) the next protein in line thus activating it. Other factors including proteins like Factor VIII are essential as coenzymes.

Heamophilia

About 85% of haemophiliacs suffer from classic haemophilia (1 male in 10 000). they cannot produce factor VIII. The rest show Christmas disease where they cannot make factor IX.

Haemophiliacs do clot their blood slowly because there is an alternative pathway via thromboplastin.

The genes for both forms of haemophilia are sex linked.