Oxygen is critical to the survival of most organisms. Cells use this important element in the process of cellular respiration, in which glucose molecules are converted into energy. The oxygen used in cellular respiration comes from the environment. However, even though most organisms acquire oxygen from their environment (a process commonly referred to simply as respiration), relatively few of these organisms have lungs.

Some of the smallest and simplest life forms -- the single-celled organisms -- respire directly through their cell membranes. Oxygen passes readily from the environment into these cells by simple diffusion. And because molecules always move from where they are most concentrated to where they are least concentrated, new oxygen from the environment constantly moves into the cell to replace oxygen that is used up.

Many multicellular organisms also acquire oxygen through simple diffusion. Earthworms, for example, respire exclusively through their skin. Frogs, too, get much of their oxygen directly through their skin. However, because these animals have many layers of cells, many of which are far from the surface, they must have circulatory systems that pump blood through capillaries that are just one cell layer below the surface and then on to the innermost cells.

Despite its simplicity, a respiratory system that relies on diffusion through the skin is very limiting. For example, diffusion works well only when oxygen is dissolved in water, which is why earthworms and frogs are confined to moist environments. Skin respiration also limits an animal's size. A hippopotamus, for example, could not possibly acquire enough oxygen through its skin because the ratio of its surface area to its total volume is quite small. This means that most of the hippo's cells are internal rather than on the surface. Skin respiration is simply too inefficient to supply oxygen to all of those internal cells.

Lungs and gills improve the efficiency of respiration by maximizing the surface area through which oxygen can diffuse. The human respiratory system, for example, branches again and again, with each tube leading to several smaller tubes and ultimately to a group of tiny air sacs called alveoli. The 300 million alveoli in a pair of human lungs provide a surface area of about 750 square feet -- about 40 times the body's surface area. In addition, capillaries surround each alveolus, just as they surround each filament in a fish's gills. This increases the efficiency of respiration by minimizing the distance across which oxygen must diffuse.