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.