Here's why:

* Carbon dioxide (CO2) is a gas produced as a waste product of cellular respiration.

* Plasma is the liquid component of blood, making up about 55% of its volume. It contains water, proteins, electrolytes, and other dissolved substances.

* About 70% of the CO2 in the blood is transported as bicarbonate ions (HCO3-). This happens through a series of reactions in the red blood cells, where CO2 combines with water to form carbonic acid (H2CO3), which then dissociates into H+ and HCO3-.

* About 20% of CO2 binds directly to hemoglobin in red blood cells. This is called carbaminohemoglobin.

* The remaining 10% of CO2 is dissolved directly in the plasma.

So, while hemoglobin plays a significant role in oxygen transport, it's primarily plasma that carries the majority of CO2 in the blood.