Which of the following substances are required for the start of coagulation o f blood in mammals ?

Options

A. Thrombin and sodium ions
B. Fibrin and Iron ions
C. Thromboplastin and calcium ions
D. Heparin and fibrinogen

Answer

To answer the question regarding the substances required for the start of coagulation of blood in mammals, we can break down the process of coagulation into specific steps and identify the necessary components.

### Step-by-Step Solution:

1. **Understanding Coagulation**: Coagulation, or blood clotting, is a complex process that prevents excessive bleeding when blood vessels are injured. It involves a series of biochemical reactions leading to the formation of a blood clot.

2. **Initial Trigger**: The coagulation process begins when there is an injury to a blood vessel. This triggers the release of certain substances that are essential for the clotting process.

3. **Role of Thromboplastin**: The first key substance required for the initiation of coagulation is thromboplastin (also known as tissue factor). Thromboplastin is released from damaged tissues and plays a crucial role in the conversion of prothrombin to thrombin.

4. **Calcium Ions**: Along with thromboplastin, calcium ions (Ca²⁺) are also essential for the activation of the coagulation cascade. Calcium ions facilitate various steps in the coagulation pathway, including the conversion of prothrombin to thrombin.

5. **Conversion of Prothrombin**: Prothrombinase, an enzyme complex formed in the presence of thromboplastin and calcium ions, converts prothrombin (an inactive protein) into thrombin (an active enzyme).

6. **Formation of Fibrin**: Thrombin then acts on fibrinogen (a soluble plasma protein) to convert it into fibrin (an insoluble protein), which forms the meshwork of the blood clot.

7. **Conclusion**: Therefore, the substances required for the start of coagulation in mammals are thromboplastin and calcium ions.

### Final Answer:
The correct answer is: **Thromboplastin and Calcium ions.**

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