For survival an organism must exchange materials effectively with its environment. Examples of materials that need to be interchanged between an organism and its environment include: Respiratory gases Heat Nutrients Excretory Products Exchanges take place at the surface of an organism, and the materials absorbed are used by the cell. The cells or cells of the organism makeup its volume. However, the surface area to volume ratio gets smaller as the objects get larger.
Single Celled Organisms Oxygen is required to produce ATP during respiration and carbon dioxide is produced as a waste product during this process. This produces concentration gradients of these gases in opposite directions. All organisms rely on diffusion for exchange of oxygen and carbon dioxide down their concentration gradients. Single celled organisms and some small multi cellular organisms have large enough surface area to volume ratio to meet their gas exchange needs by diffusion across their surface. However, larger organisms have relatively small surface area to volume ratio. They cannot rely on diffusion across their surface alone to supply sufficient amounts of oxygen to all of their cells. So larger organisms have developed specialised gas exchange systems which have adaptations to ensure the rapid diffusion of gases.