In single-celled organisms, diffusion is sufficient to meet their oxygen requirements. Why do multicellular organisms with the same requirement have a respiratory system?

 Single-celled organisms have a relatively large surface area to volume ratio, which allows for sufficient diffusion of gases to meet their oxygen requirements. However, larger multicellular organisms have smaller surface area to volume ratios, so they have evolved exchange surfaces to exchange molecules with their surroundings. In many animals, the exchange of gases by diffusion between the internal and external environments of the organism is facilitated by the structure and function of the respiratory system. The respiratory system is a body system adapted for efficient gas exchange. The body contains so many different types of cells and organs that work together and need oxygen to function. Most animals are complex multicellular organisms that require a mechanism for transporting nutrients throughout their bodies and removing waste products. In organisms that possess lungs and gills, oxygen must be transported from these specialized respiratory organs to the body tissues via a circulatory system.