Which of the following proteins is present in the circulating blood of man ?

Options

A. thromboplastin
B. thrombin
C. fibrinogen
D. fibrinogen to fibrin

Answer

To determine which protein is present in the circulating blood of humans, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Proteins**: The question provides several proteins: thromboplastin, thrombin, and fibrinogen. We need to analyze which of these is present in the blood.

2. **Understand Blood Proteins**: In human blood, there are major proteins that include serum albumin, globulins, and fibrinogen. Each of these proteins has a specific role in the blood.

3. **Percentage Composition**:
- Serum albumin constitutes about 55% of blood proteins.
- Globulins make up approximately 38%.
- Fibrinogen accounts for about 7%.

4. **Activation of Fibrinogen**: When blood clotting occurs (for example, due to a cut), fibrinogen is activated to form fibrin. However, in its inactive form, fibrinogen is present in the circulating blood.

5. **Presence of Other Proteins**: Thrombin and thromboplastin are involved in the clotting process but are not present in the circulating blood in their active forms. They are produced during the clotting cascade.

6. **Conclusion**: Since fibrinogen is the only protein listed that is present in the circulating blood in its inactive form, the correct answer is fibrinogen.

### Final Answer:
**Fibrinogen** is the protein present in the circulating blood of man.

---