The respiratory system is responsible for performing gas exchange within an animal’s body at the respiratory surface and between tissues. External respiration refers to gas exchange which occurs at the respiratory surface. Conversely, internal respiration is used to describe gas exchange which takes place between tissues.
Among animals, the components which play a role in gas exchange and make up the respiratory system differ considerably. For example, in fish respiration takes place using their gills in most cases, in amphibians such as frogs, their skin plays a role in gas exchange, and in humans, their lungs are the only organ contributing to gas exchange.
Basically, respiration allows for oxygen and carbon dioxide to be exchanged between the outside/external environment and mitochondria. The mitochondrion requires oxygen in order to produce ATP during cellular respiration and a by-product of this process is carbon dioxide. This carbon dioxide needs to be exchanged so that more oxygen can enter the body.
In simple unicellular and small multicellular organisms, no respiratory system is present. Instead, these organisms rely on simple diffusion mechanisms to perform gas exchange. For example, sponges rely on the bulk flow of water to perform diffusion and exchange gases. In other invertebrates, such as arthropods (insects), a trachea is used for diffusion. Oxygen is able to diffuse out of the tracheoles of this system, so that oxygen can enter the body cavity.
Vertebrate respiratory systems are much more complex and generally, are based on breathing air. The more sophisticated systems, such as those in mammals, depend on the circulatory system in combination with the respiratory system. There is an interaction between respiratory gases and blood flow. Oxygenated blood travels towards the heart and other tissues, whereas deoxygenated blood travels to lungs to perform gas exchange.
All terrestrial vertebrates depend on some sort of lung structure to perform gas exchange with the air. However, the lungs among different vertebrates differ significantly because of their diverse metabolic demands. For example, the anatomy of bird lungs is very unique and provides them with a very efficient respiratory system suited for flight. In comparison to mammal lungs, birds are able to transfer more oxygen with one breath.