Carbon dioxide is the waste product of the respiritory system, and of several other chemical reactions in the body, such as the creation of ATP. Pure carbon cannot be transported in the body, so CO2 is one form it takes that is water soluble. Levels of CO2 also tell the body when it needs more oxygen.
In the human body, the oxygen is absorbed by the blood stream in the lungs, being then transported to the cells where an elaborated change process takes place.
Oxygen plays a vital role in the breathing processes and in the metabolism of the living organisms.
Probably, the only living cells that do not need oxygen are some anaerobic bacteria that obtain energy from other metabolic processes.