## **Core Concept**
The transport of carbon dioxide (CO2) in the blood involves several chemical forms. CO2 is a byproduct of cellular metabolism and must be efficiently transported from tissues back to the lungs for exhalation. The major forms of CO2 transport in the blood are bicarbonate ions (HCO3-), dissolved CO2, and carbaminohemoglobin.

## **Why the Correct Answer is Right**
The largest fraction of CO2 in the blood is transported as **bicarbonate ions (HCO3-)**, which accounts for approximately 70% of CO2 transport. This process occurs mainly in red blood cells. The enzyme **carbonic anhydrase** catalyzes the reaction of CO2 with water to form carbonic acid (H2CO3), which then quickly dissociates into bicarbonate ions (HCO3-) and protons (H+). The bicarbonate ions can then move out of the red blood cells into the plasma.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because while some CO2 does bind to hemoglobin forming **carbaminohemoglobin**, this is not the largest fraction of CO2 transport in the blood.
- **Option B:** This option is incorrect as it represents **dissolved CO2**, which is the smallest fraction of CO2 transport, accounting for only about 5-10% of the total CO2 in the blood.
- **Option D:** This option seems to represent another form or context not directly relevant to the primary methods of CO2 transport in blood.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the bicarbonate buffering system is crucial for maintaining acid-base balance in the body. The reaction of CO2 with water to form H2CO3, and then HCO3-, is vital for the transport of CO2 and for buffering pH changes in the blood.

## **Correct Answer:** B. HCO3-