In which of the following forms `CO_2` is mostly carried in blood?

Options

A. Hydrogencarbonate ions
B. Carbonic acid
C. Carbonate ions
D. Carbaminohaemoglobin

Explanation

To determine how carbon dioxide (CO2) is mostly carried in the blood, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Role of Blood in Gas Transport:**
- Blood is a connective tissue that plays a crucial role in transporting gases, nutrients, and waste products throughout the body.

2. **Identify the Main Components Involved in Gas Exchange:**
- Hemoglobin, an iron-containing protein in red blood cells, is essential for transporting oxygen (O2) and carbon dioxide (CO2).

3. **Recognize the Forms of CO2 Transport:**
- CO2 is transported in three main forms:
- **Carbaminohemoglobin:** About 20-25% of CO2 binds to hemoglobin to form carbaminohemoglobin.
- **Dissolved CO2:** A small percentage (5-7%) of CO2 is dissolved in the blood plasma.
- **Bicarbonate Ions:** The majority (approximately 70%) of CO2 is transported in the form of bicarbonate ions (HCO3-).

4. **Determine the Most Common Form of CO2 Transport:**
- Since the question asks for the form in which CO2 is mostly carried, we focus on the largest percentage, which is the bicarbonate ions.

5. **Conclude the Answer:**
- Therefore, the correct answer is that CO2 is mostly carried in the blood in the form of hydrogen carbonate ions (bicarbonate ions).

### Final Answer:
CO2 is mostly carried in the blood in the form of hydrogen carbonate ions (bicarbonate ions).

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