Which substance initiate clotting of blood ?

Options

A. Prothrombin
B. Antithrombin
C. Prothrombinase
D. Thromboplastin

Answer

To answer the question "Which substance initiates clotting of blood?", we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Blood Clotting**: Blood clotting, also known as coagulation, is a complex process that prevents excessive bleeding when a blood vessel is injured. It involves a series of reactions that lead to the formation of a fibrin clot.

2. **Identifying Key Substances**: There are several substances involved in the clotting process, including prothrombin, antithrombin, prothrombinase, and thromboplastin. Each of these plays a different role in coagulation.

3. **Role of Tissue Factor**: The initiation of blood clotting is primarily triggered by the presence of a protein known as tissue factor (also called thromboplastin). This protein is released from damaged tissues and interacts with blood components to start the coagulation cascade.

4. **Understanding the Pathway**: The pathway activated by tissue factor is known as the extrinsic pathway of coagulation. This pathway is crucial for the rapid response to vascular injury.

5. **Conclusion**: Based on the above understanding, the substance that initiates the clotting of blood is **thromboplastin**.

### Final Answer:
The substance that initiates the clotting of blood is **thromboplastin**.

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