In which main form, `CO_2` comes in blood?

Options

A. Bicarbonate
B. Carbon mono-oxide
C. Carbonic acid
D. Carbonate

Explanation

To answer the question "In which main form does CO2 come in blood?", we can follow these steps:

### Step-by-Step Solution:

1. **Understanding CO2 Transport in Blood**:
- Carbon dioxide (CO2) produced by cellular metabolism needs to be transported from the tissues to the lungs for exhalation.

2. **Formation of Bicarbonate Ion**:
- In the blood, CO2 primarily reacts with water (H2O) to form carbonic acid (H2CO3). This reaction is facilitated by the enzyme carbonic anhydrase.
- The carbonic acid then dissociates into bicarbonate ions (HCO3-) and hydrogen ions (H+).

3. **Main Form of CO2 in Blood**:
- The majority of CO2 in the blood is present in the form of bicarbonate ions (HCO3-). This is the main form in which CO2 is transported in the bloodstream.

4. **Role of Bicarbonate**:
- The bicarbonate ions are then released into the plasma, helping to maintain pH balance in the blood and transporting CO2 to the lungs.

5. **Conclusion**:
- Therefore, the main form in which CO2 is transported in the blood is as bicarbonate ions (HCO3-).

### Final Answer:
CO2 comes in blood mainly in the form of bicarbonate ions (HCO3-).

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