ABSTRACT
Ventilation, or gas exchange, is the exchange of Oxygen and Carbon dioxide between the body fluids of an animal and the environment in which that animal lives. Gas exchange occurring across the general body surface area is the means by which single cell animals and relatively simple multicellular animals achieve respiration. Gas exchange across the general body surface area would be inadequate to meet these demands. By and large, animals tend to be either air breathers or water breathers. The exchange of gases between animals and their environments occurs by the process of simple diffusion. The movement of gases, across the general body surface or from alveoli to pulmonary capillary blood in the human lung occurs by the process of diffusion. Bearing in mind all the requirement of gas exchange organs, evolution has provided animals with three possibilities: gills, lungs and tracheal systems.
