Coagulation of Blood 

The coagulation of blood is a vital biological process that prevents excessive bleeding when blood vessels are injured. This mechanism, also known as blood clotting, helps minimise blood loss and promote effective wound healing. The process involves plasma proteins, platelets, and enzymes working together in a precise sequence to form a stable clot.

1.0What Is Coagulation of Blood?

• Blood coagulation is the transformation of liquid blood into a gel-like clot at the site of vascular injury. 
• It is a protective mechanism that seals damaged blood vessels to prevent blood loss.
• The clot is mainly composed of fibrin threads, which trap blood cells and platelets, forming a stable mesh that closes the wound.

2.0Composition of Blood 

3.0Mechanism of Blood Coagulation

The coagulation process occurs in a series of well-regulated enzymatic steps. These reactions convert inactive clotting factors into their active forms, leading to fibrin clot formation.

1. Injury and Vascular Response

When a blood vessel is damaged:

• The vessel constricts to reduce blood flow (vasoconstriction).
• Platelets adhere to the exposed collagen fibres at the injury site.

2. Platelet Plug Formation

• Platelets release serotonin, ADP, and thromboplastin.
• These substances attract more platelets, forming a temporary platelet plug.

3. Activation of Clotting Factors

• Thromboplastin (tissue factor) combines with Ca²⁺ ions and prothrombin to form thrombin.
• This step requires several clotting factors, such as I (Fibrinogen), II (Prothrombin), V, VII, IX, and X.

4. Conversion of Fibrinogen to Fibrin

• Thrombin acts as an enzyme to convert soluble fibrinogen (a plasma protein) into insoluble fibrin threads.
• Fibrin forms a mesh-like network that traps RBCs and platelets, creating a stable clot.

5. Clot Retraction and Repair

• The clot contracts to reduce bleeding and draw the wound edges together.
• Later, tissue repair begins, and the clot is dissolved by plasmin during fibrinolysis.

4.0Clotting Factors Involved in Blood Coagulation

There are 13 clotting factors, most of which are plasma proteins synthesised in the liver. Here’s a summary of key clotting factors:

5.0Role of Platelets in Coagulation

Platelets, though forming less than 1% of total blood volume, play a central role in blood coagulation.

Functions of Platelets in Clotting:

1. Detect Injury: Platelets stick to exposed collagen fibres.
2. Release Chemicals: Secreted factors that activate the coagulation cascade.
3. Form Platelet Plug: Acts as a temporary barrier to prevent bleeding.
4. Support Fibrin Formation: Strengthen and stabilise the clot through fibrin linkage.

Thus, in both composition and function, platelets are indispensable for blood clot formation and vascular repair.

6.0Importance of Plasma in Coagulation

Plasma makes up about 55% of total blood volume — as indicated in the composition of blood percentages — and serves as the medium for clotting factor reactions.

• Fibrinogen: Converted to fibrin for clot structure.
• Prothrombin: Precursor of the thrombin enzyme.
• Albumin & Globulins: Maintain osmotic balance and support transport of clotting elements.

These proteins highlight the composition of blood and function in preventing haemorrhage and maintaining internal stability.

7.0Disorders Related to Blood Coagulation