Which gas molecules move from the region of their higher concentration to the region of their lower concentration inside the body of animals in the process of diffusion in animals?

During Respiration, Diffusion occurs through which oxygen gas is taken inside the body and Carbon dioxide gas is released outside the body.

As cells use up glucose and oxygen, they produce waste chemicals and carbon dioxide. 

If these were to build up in the cells they would poison them, therefore they diffuse out of the cells into the blood.