Here's why:

* Cellular Respiration: Our cells constantly use oxygen to produce energy, generating CO2 as a byproduct.

* Transport: CO2 is transported in the blood in three main ways:

* Dissolved in the plasma

* Bound to hemoglobin (forming carbaminohemoglobin)

* As bicarbonate ions (HCO3-)

* Excretion: The lungs are responsible for removing CO2 from the blood and exhaling it.

When CO2 builds up in the blood, it leads to a condition called hypercapnia, which can be dangerous if left untreated. This can cause:

* Acidosis: CO2 reacts with water to form carbonic acid (H2CO3), which lowers the blood pH, making it acidic.

* Respiratory Distress: Elevated CO2 levels stimulate the respiratory center in the brain, leading to increased breathing rate and depth.

* Neurological Effects: High CO2 levels can affect brain function, potentially causing confusion, headache, and even coma.

It's important to note that other gases, such as nitrogen, can also build up in the blood in certain situations, such as diving. However, CO2 is the primary gas that accumulates in the blood under normal physiological conditions.