## **Core Concept**
The transport of carbon dioxide (CO2) in the body is a critical process that occurs mainly through the bloodstream. CO2 is a byproduct of cellular metabolism and must be efficiently transported from tissues back to the lungs for exhalation. This process involves various mechanisms to ensure effective CO2 removal.

## **Why the Correct Answer is Right**
The major form of CO2 transport in the body is as **bicarbonate ions (HCO3-)**, which accounts for approximately 70% of CO2 transport. This process begins when CO2 enters the bloodstream from tissues and reacts with water (H2O) in the presence of the enzyme **carbonic anhydrase** within red blood cells to form **carbonic acid (H2CO3)**. Carbonic acid then quickly dissociates into **bicarbonate ions (HCO3-)** and **hydrogen ions (H+)**. The bicarbonate ions are transported out of the red blood cells into the plasma, where they are carried to the lungs. In the lungs, the process reverses, and CO2 is exhaled.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because, although some CO2 does bind to hemoglobin as carbaminohemoglobin, this is not the major form of CO2 transport.
- **Option C:** This option is incorrect because dissolved CO2 in plasma is only a small fraction of the total CO2 transported and not the major form.
- **Option D:** This option is incorrect because, while CO2 does bind to plasma proteins, this is not the primary mechanism of CO2 transport.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the enzyme **carbonic anhydrase** plays a crucial role in CO2 transport by facilitating the conversion of CO2 and H2O into H2CO3, which then dissociates into HCO3- and H+. This process is vital for maintaining acid-base balance and ensuring efficient CO2 removal.

## **Correct Answer:** B. Bicarbonate ions.