Diffusion is a process that occurs during respiration, in which gasses pass between the walls of the alveoli and the capillaries. According to the National Institutes of Health, oxygen moves from the lungs into the blood, and carbon dioxide is removed from the blood an exhaled.
Diffusion is an effortless process that occurs naturally within the lungs. After the transfer of gasses, oxygen which has traveled through the thin walls of both the alveoli and capillaries is picked up by hemoglobin and carried to cells to be utilized for glycolysis, the process of turning glucose into usable energy. Cellular waste is picked up and carbon dioxide is expelled back into the alveoli where it gets exhaled.