Respiratory System

The simplest respiratory system is found in organisms with a gastro-vascular cavity. They simply diffuse oxygen into their cells. Respiratory systems have gotten more complex as the animals need for oxygen increased. Early aquatic organisms developed gills in order to extract the disolved oxygen in the water. These external gills are feather like extensions that are specially designed to be as effecient as possible in extracting oxygen. The oxygen simply diffuses into capillaries while the Carbon Dioxide diffuses out the other way. Although gills can be suitable for terrestrial animals awell, they are usually housed in special chambers that keep them moist and prevent them from collapsing. This lead to the development of lungs, and internal system of gaining oxygen. Lungs branch extensively throughout the cavity and are lined with a moist epithelium. Like gills, the oxygen diffuses into capilaries, and is carried throughout the body through the circulatory system. The other system that developed for the minority of terrestrial animals was the tracheal system. Found mostly in insects, the tracheal system consists of holes along the side of the body. Like lungs, trachea branch extensively, but instead of being carried throughout the body in the blood stream, the oxygen diffuses straight into the cells.