1. Blood is always flowing at high speed.
2. Blood contains 50 types of procoagulant factor (blood clotting) and anticoagulant factor (prevents clotting).
3. Because the blood vessel walls are very smooth.
4. Anticoagulant factors include heparin and thrombomodulin, which prevent blood clotting.
5. Because active coagulant factors are always removed by the liver.