Carbon dioxide is transported from tissues to respiratory surface by only:

Options

A. Plasma and erythrocytes
B. Plasma
C. Erythrocytes
D. Erythrocytes and leucocytes

Answer

To solve the question regarding the transport of carbon dioxide from tissues to the respiratory surface, we can break it down into the following steps:

### Step-by-Step Solution:

1. **Understanding Carbon Dioxide Transport**:
- Carbon dioxide (CO2) is produced as a waste product during cellular respiration in tissues. It needs to be transported to the lungs for exhalation.

2. **Main Transport Mechanisms**:
- CO2 is transported in the blood primarily in three forms:
- Dissolved in plasma (about 7-10%)
- Chemically bound to hemoglobin (about 20-25%)
- As bicarbonate ions (HCO3-) formed in red blood cells (RBCs) (about 70%).

3. **Role of Red Blood Cells (RBCs)**:
- The majority of CO2 (approximately 70%) is transported in the form of bicarbonate ions. This conversion occurs in the RBCs, where CO2 reacts with water to form carbonic acid, which then dissociates into bicarbonate and hydrogen ions.

4. **Role of Plasma**:
- A smaller portion of CO2 (about 7-10%) is transported dissolved directly in the plasma. Additionally, around 20-25% of CO2 is bound to hemoglobin, forming carbaminohemoglobin.

5. **Conclusion**:
- The primary transport of CO2 from tissues to the respiratory surface is carried out by red blood cells (RBCs), while a smaller amount is transported in plasma. Therefore, the answer to the question is that carbon dioxide is transported from tissues to the respiratory surface mainly by RBCs.

### Final Answer:
Carbon dioxide is transported from tissues to the respiratory surface primarily by red blood cells (RBCs).