Which of the following statement is true about RBCs in humans?

Options

A. They do not carry `CO_(2)` at all
B. They carry about 20-25 per cent of `CO_(2)`
C. They transport 99.5 per cent of `O_(2)`
D. They transport about 80 per cent oxygen only and the rest of 20 per cent of it is transported in dissolved state in blood plasma

Answer

To determine which statement is true about red blood cells (RBCs) in humans, we will analyze each of the provided options step by step.

### Step-by-Step Solution:

1. **Option 1: They do not carry carbon dioxide at all.**
- This statement is incorrect. RBCs do carry carbon dioxide. They contain hemoglobin, which is responsible for transporting both oxygen and carbon dioxide. Therefore, this option is false.

2. **Option 2: They carry about 20 to 25 percent of the carbon dioxide.**
- This statement is correct. RBCs are known to carry approximately 20 to 25 percent of the carbon dioxide produced in the body. The majority of carbon dioxide (about 70 percent) is transported in the form of bicarbonates in the blood. Hence, this option is true.

3. **Option 3: They transport 99.5% of oxygen.**
- This statement is incorrect. RBCs transport about 97% of the oxygen in the blood, while the remaining 3% is dissolved in the plasma. Therefore, this option is false.

4. **Option 4: They transport about 80% of the oxygen and only the rest of the 20% is transported in the dissolved state in plasma.**
- This statement is also incorrect. As mentioned previously, RBCs transport approximately 97% of the oxygen, and only about 3% is carried in a dissolved state in the plasma. Thus, this option is false.

### Conclusion:
The only true statement about RBCs in humans is **Option 2: They carry about 20 to 25 percent of the carbon dioxide.**

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