As we just learned in the previous section, oxygen is necessary for the efficient respiratory processes in most organisms. When oxygen is absorbed and circulated to the cells for aerobic respiration, carbon dioxide will be released as a waste by-product. This exchange of gases, oxygen – inward and carbon dioxide – outward, from the environment must occur through moist membranes in the organism by the process of diffusion. Diffusion is the movement of molecules from an area of high concentration to an area of low concentration without the use of cellular energy. Since respiration must occur continuously in living things, the more complex the organism, the more complex the organs needed in the design of the respiratory system.