Carbon Dioxide is a by-product of the process of cell respiration, as is water. In this process, energy is produced in the mitochondria of cells. This energy is our physical "life force". Both oxygen and glucose are required for this.
The way our body expels it is by breathing it out either through our noses or our mouths and by replacing it with Oxygen. Oxygen helps keep our lungs balanced so the Carbon Dioxide doesn't overpower our lungs.