Can any doc explain why are smoking, oral contraceptives and dehydration risk factors for thromboembolism?

Blood clotting: All of these things increase the coagulability of blood by either making platelets clump together more easily or increasing clotting factors in blood from the liver. They also reduce the clot-busting activity in blood (fibrinolysis) which is always in some balance with clotting (thrombosis). The combination of oral contraceptives and smoking are particularly nasty in terms of increasing clotting.