Mechanism of Blood Clotting
Introduction
Blood clotting, or coagulation, is a natural defense process that prevents excessive bleeding when a blood vessel is injured.
It is a complex chain reaction involving platelets, clotting factors, calcium ions, and fibrin.
This process converts liquid blood into a gel-like clot, sealing the wound until healing occurs.
Definition
Blood clotting (Coagulation) is defined as:
“The process by which blood changes from a liquid to a solid state to prevent blood loss after injury.”
Original Notes:
Importance of Blood Clotting
- Prevents excessive blood loss from injuries.
- Provides a temporary seal for damaged vessels.
- Starts the healing process by stabilizing the wound.
- Protects the body from infection through exposed tissues.
Main Steps in Blood Clotting Mechanism
The process of coagulation occurs in three major stages:
1. Formation of Prothrombin Activator
When a blood vessel is injured:
- Platelets stick to the damaged site and release thromboplastin (or prothrombin activator).
- Calcium ions (Ca²⁺) and clotting factors (especially Factor X and V) are required.
- The thromboplastin helps convert prothrombin into thrombin in the next step.
Pathways involved:
- Intrinsic Pathway: Initiated by damage to blood vessels and exposure of collagen.
- Extrinsic Pathway: Initiated by external trauma that causes blood to escape the vascular system.
- Both pathways converge to form prothrombin activator.
2. Conversion of Prothrombin to Thrombin
- Prothrombin, a plasma protein made in the liver, is converted into thrombin, an active enzyme.
- This step requires calcium ions and prothrombin activator.
- Reaction:
Prothrombin + Thromboplastin + Ca²⁺ → Thrombin
3. Conversion of Fibrinogen to Fibrin
- Thrombin acts on fibrinogen (a soluble plasma protein) and converts it into fibrin (an insoluble protein).
- Fibrin threads form a mesh-like network that traps blood cells and platelets, creating a solid clot.
- Reaction:
Fibrinogen → (Thrombin) → Fibrin
The fibrin network then contracts and hardens, pulling the edges of the wound closer to speed up healing.
Clot Retraction and Repair
After the clot is formed:
- Platelets contract, shrinking the clot (clot retraction).
- This draws the damaged vessel edges together.
- Fibroblasts and endothelial cells begin tissue repair.
Fibrinolysis (Clot Removal)
Once the vessel heals, the clot is no longer needed.
- Plasminogen (inactive form) is converted to plasmin (active enzyme).
- Plasmin dissolves fibrin and removes the clot.
- This process is called fibrinolysis.
Essential Substances Involved in Clotting
- Calcium ions (Ca²⁺) – Required at multiple steps of clotting.
- Vitamin K – Needed for synthesis of prothrombin and other clotting factors in the liver.
- Clotting Factors (I–XIII) – Specialized proteins that act in sequence (cascade).
- Platelets – Initiate clot formation by releasing thromboplastin.
Clotting Time
- Normal clotting time: 3 to 8 minutes.
- Prolonged clotting time indicates deficiency of clotting factors, calcium, or vitamin K.
Disorders Related to Blood Clotting
- Hemophilia: Genetic disorder where one or more clotting factors are missing.
- Thrombosis: Formation of an unwanted clot inside a blood vessel.
- Embolism: Dislodged clot blocking blood flow elsewhere in the body.
- Vitamin K Deficiency: Leads to poor synthesis of clotting factors.
