Carbon dioxide is a molecular gas made up of two atoms of oxygen and one of carbon. It is a natural by-product (along with water) of the metabolism of glucose and is produced in all living cells in the body. It is removed by the blood through the veins to the lungs where it is exhaled in respiration.
The automatic response that causes respiration is based on the level of carbon dioxide in the lungs.
In high concentrations, carbon dioxide will cause fatigue and disorientation. However, such levels are generally only found in very enclosed spaces with poor ventilation.