Composition of Blood > Cellular Elements: Blood Platelets (Thrombocytes)

The enzyme released by injured tissues and disintegrating platelets that initiates clotting is called thrombokinase (also known as Thromboplastin or Factor X or Stuart factor).

The solid mass left behind after clotting, formed by fibrin and trapped blood cells, is called clot or thrombus.

Platelets are extremely small and have a disc-like shape. They are much smaller than red or white blood cells, yet play a crucial role in the body's healing process. 

• Platelets, also known as thrombocytes, are produced in the bone marrow, which is the spongy tissue inside the bones. Along with red and white blood cells, platelets are formed there. Once platelets enter the bloodstream, they have a lifespan of about 8 to 10 days.
• Platelets are approximately 2.5 to 4 lakh (250,000 to 400,000) platelets per cubic millimetre (mm³) of blood. This high number is necessary to ensure quick and effective blood clotting in case of injury. 
• Platelets contain granules filled with proteins and enzymes that are released to help in blood clotting and healing wounds.

1. Help in blood clotting to prevent excessive bleeding.
2. Form a temporary plug at the site of an injury by clumping together.
3. Release chemicals that activate the clotting process and strengthen the clot.
4. Work with proteins like fibrin to create a stable and durable clot.
5. Repair damaged blood vessels by promoting tissue healing and regeneration.
6. Maintain the integrity of the circulatory system by sealing small breaks in blood vessel walls.

• Platelets are oval-shaped, enucleated cell fragments found in mammals only. Normal count = 2.5–4.5 lakh/mm³. Lifespan = 3–5 days, destroyed mainly in the spleen.
• At injury site, platelets disintegrate → release thrombokinase (thromboplastin/Factor X) → initiates clotting cascade. Vitamin K is essential for the synthesis of prothrombin in the liver.
• Thrombokinase + Ca²⁺ ions → converts inactive prothrombin → active thrombin.
• Thrombin converts soluble fibrinogen → insoluble fibrin threads → mesh traps blood cells → forms a clot sealing the wound.
• Clot contracts → squeezes out serum → leaves behind a solid mass called thrombus. Low platelet count = Thrombocytopenia (excessive bleeding).