Respiration in Animals :
Single-celled organisms exchange gases directly across their cell membrane. However, the slow diffusion rate of oxygen relative to carbon dioxide limits the size of single-celled organisms.
Respiratory Surfaces - Terrestrial animals which cannot maintain gaseous exchange by diffusion across their outer surface, have developed a variety of respiratory surfaces covered with thin, moist epithelial cells that allow oxygen and carbon dioxide to exchange.
Methods of Respiration - Sponges and jellyfish lack specialized organs. The gaseous exchange takes place directly from the surrounding water. Flatworms and annelids use their outer surfaces for gas exchange. Some arthropods, some annelids, and fish use gills; terrestrial vertebrates use internal lungs. Amphibians use their skin as a respiratory surface. Frogs eliminate carbon dioxide 2.5 times as fast through their skin as they do through their lungs. Eel (a fish) obtains 60% of its oxygen through the skin. Humans exchange only 1% of their carbon dioxide through their skin.
Typically gills are organized into a series of plates and may be internal (as in crabs and fish) or external to the body (as in some amphibians). Gills are very efficient in removing oxygen from water. There is only 1/20th amount of oxygen present in water as compared to the same volume of air. Water flows over gills in one direction while blood flows in the opposite direction through gill capillaries. This counter current flow maximizes oxygen transfer. Many terrestrial animals have their respiratory surfaces inside the body and are connected to the outside by a series of tubes called tracheae. Reptiles, birds and mammals respire through lungs as an adaptation for terrestrial life.