What prevents clotting of blood inside the blood vessels?

Options

A. Heparin
B. Serotonin
C. Fibrinogen
D. Fibrin

Answer

To answer the question "What prevents clotting of blood inside the blood vessels?", we can follow these steps:

### Step-by-Step Solution

1. **Understanding Blood Vessels**: Blood vessels are the channels through which blood circulates in the body. They need to remain unobstructed for proper blood flow.

2. **Identifying Anticoagulants**: The primary substances that prevent blood clotting in the vessels are known as anticoagulants. These substances inhibit the coagulation process, allowing blood to flow freely.

3. **Evaluating Options**:
- **Option 1: Heparin**: Heparin is an anticoagulant that prevents the formation of blood clots. It is produced naturally in the body and is often used in medical settings to prevent clotting during surgeries or in patients at risk of clot formation. This option is correct.
- **Option 2: Serotonin**: Serotonin is released by blood platelets and plays a role in constricting blood vessels and promoting clotting. Therefore, this option is incorrect.
- **Option 3: Fibrinogen**: Fibrinogen is a protein that is converted into fibrin during the clotting process. It is essential for clot formation, making this option incorrect.
- **Option 4: Fibrin**: Fibrin is a protein that forms the mesh of a blood clot. Since it is involved in the clotting process, this option is also incorrect.

4. **Conclusion**: The correct answer to the question is Heparin, as it is the substance that prevents clotting of blood inside the blood vessels.

### Final Answer
Heparin prevents clotting of blood inside the blood vessels.

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