At the tissue site where partial pressure of CO2, is high due to catabolism, CO2, diffuses into blood (RBCs and plasma) and forms HCO2 and IT. At the alveolar site where pCO2, is low, the reaction proceeds in the opposite direction leading to the formation of CO2, and CO2. Thus, CO2, trapped as bicarbonate at the tissue level and transported to the alveoli is released out as CO2

Answer:

The majority of carbon dioxide (CO2) released from body tissues into the blood is present as bicarbonate in blood plasma and red blood cells (RBCs). This process is part of the bicarbonate buffering system, which helps regulate the pH of the blood.

1. The Bicarbonate Buffering System:
- The bicarbonate buffering system is an important mechanism that helps maintain the pH of the blood within a narrow range.
- It involves the conversion of carbon dioxide into bicarbonate ions (HCO3-) and hydrogen ions (H+) in the red blood cells.

2. Carbon Dioxide Transport:
- When tissues produce carbon dioxide as a byproduct of metabolism, it diffuses into the bloodstream.
- Some of the carbon dioxide binds to hemoglobin in the red blood cells, forming carbamino-hemoglobin.
- The remaining carbon dioxide is converted into bicarbonate ions through a series of reactions.

3. Conversion to Bicarbonate:
- Carbon dioxide diffuses into the red blood cells and combines with water to form carbonic acid (H2CO3) through the action of the enzyme carbonic anhydrase.
- Carbonic acid then dissociates into bicarbonate ions (HCO3-) and hydrogen ions (H+).
- The bicarbonate ions are transported out of the red blood cells into the blood plasma in exchange for chloride ions (Cl-), a process known as the chloride shift.

4. Transport of Bicarbonate:
- Bicarbonate ions are highly soluble and can easily move through the plasma.
- They are transported in the blood plasma to the lungs, where they can be converted back into carbon dioxide for elimination.

5. Reverse Reaction in the Lungs:
- In the lungs, carbon dioxide diffuses from the blood into the alveoli and is exhaled.
- At the same time, the reverse reaction occurs, where bicarbonate ions combine with hydrogen ions to form carbonic acid.
- Carbonic acid then dissociates into carbon dioxide and water, which can be exhaled.

6. Overall Distribution:
- The majority of carbon dioxide released from body tissues is transported as bicarbonate ions in the blood plasma.
- Some of the carbon dioxide binds to hemoglobin as carbamino-hemoglobin.
- Only a small fraction of carbon dioxide is present as free CO2 in the blood plasma.

Therefore, the correct answer is option A) bicarbonate in blood plasma and RBCs.