Factors Preventing Intravascular Coagulation
Which of the following factor(s) is/are responsible for preventing intravascular coagulation in the body?
- Smoothness of endothelium
- Thrombomodulin binds with thrombin
- Negatively charged endothelium repels platelets & clotting factors
- All the above — Correct Answer
Correct Answer: All the above
Three major mechanisms prevent intravascular (inside blood vessel) coagulation under normal conditions:
Mechanism 1: Smoothness of Endothelium
The inner lining of blood vessels (endothelium) is extremely smooth. Platelets and clotting factors are only activated when they contact rough or damaged surfaces (e.g., exposed collagen after injury). Smooth endothelium does not trigger platelet adhesion or the intrinsic pathway.
Mechanism 2: Thrombomodulin binds with Thrombin
Thrombomodulin is a receptor protein on endothelial cells. When thrombin is produced in small amounts in normal circulation, thrombomodulin binds and inactivates it, preventing uncontrolled clot formation inside vessels. Thrombomodulin-thrombin complex also activates Protein C, which destroys Factors Va and VIIIa (anticoagulant effect).
Mechanism 3: Negatively Charged Endothelium
The endothelial surface carries a net negative charge. Since platelets and clotting factors also carry negative charges, electrostatic repulsion prevents them from adhering to the vessel wall under normal conditions. Damage to the endothelium neutralizes this charge and allows clot initiation.
Additional Anticoagulant Mechanisms
- Prostacyclin (PGIâ‚‚): secreted by endothelium; inhibits platelet aggregation
- Heparin: produced by mast cells & basophils; natural anticoagulant
- Antithrombin III: neutralizes thrombin in plasma
