Cellular respiration, which generates ATP in your cells, requires oxygen to take place, also, it generates CO2 as a waste product.
The respiratory system allows oxygen to diffuse into your blood, and carbon dioxide to diffuse out. The blood then carries the oxygen to the cells, so cellular respiration to continue. When the blood reaches the cells, it also takes the carbon dioxide out of the cells, and carries it to the lungs to get rid of it.
