Mark the pair of substances among the following which is essential for coagulation of blood.

Options

A. Heparin and calcium ions
B. Calcium ions and platelet factors
C. Oxalates and citrates
D. Platelet factors and heparin

Answer

To solve the question regarding which pair of substances is essential for the coagulation of blood, we will analyze each option step by step.

### Step-by-Step Solution:

1. **Understanding Blood Coagulation**:
- Blood coagulation is a complex process that involves various factors, including proteins, ions, and cells that work together to form a clot and prevent bleeding.

2. **Analyzing the Options**:
- **Option A: Heparin and Calcium Ions**
- Heparin is an anticoagulant, meaning it prevents blood clotting. Therefore, this pair cannot be essential for coagulation.

- **Option B: Calcium Ions and Platelet Factors**
- Calcium ions are crucial for the activation of several clotting factors (like Factor II, VII, IX, and X) and play a vital role in the coagulation cascade.
- Platelet factors are also essential as they help in the activation of prothrombin and the conversion of fibrinogen to fibrin, which is crucial for forming a stable clot.
- This pair is essential for blood coagulation.

- **Option C: Oxalate and Citrates**
- Both oxalate and citrate are known to bind calcium ions, effectively inhibiting their availability for the clotting process. Thus, they do not support coagulation.

- **Option D: Platelet Factors and Heparin**
- As mentioned earlier, heparin is an anticoagulant, which means it inhibits coagulation. Therefore, this pair cannot be essential for coagulation.

3. **Conclusion**:
- The only pair that is essential for the coagulation of blood is **Calcium Ions and Platelet Factors** (Option B).

### Final Answer:
**The correct pair of substances essential for coagulation of blood is Calcium Ions and Platelet Factors (Option B).**