The Respiratory System Functions and Varieties Across the Animal Kingdom

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Updated: Mar 28, 2022
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There are various bodily functions that are staples for all animal life on earth, whether the organism is advanced or primitive, all need a way to obtain oxygen, dispose of waste, and break down food to create energy. One of the main systems that perform a life-giving function is the respiratory; the respiratory system comes in many varieties and can cater to many different organism’s needs. The respiratory system in primitive animals is simple, as many of these animals are not advanced enough for lungs or gills, they simply diffuse oxygen through their skin.

However as animals became more advanced, their respiratory systems improved with them, this is seen especially with the diversity of respiratory system types in vertebrates. The beginnings of the primitive respiratory system can be first found in protozoa, which use diffusion to collect the oxygen they need; however, diffusion is also used in some more advanced animals, such as salamanders. Diffusion works by taking in oxygen through layers in the skin, but this of course has its drawbacks, as the skin has to be extremely porous.

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Diffusion can also be found in aceolomates, ceolomates, cnidarians, and some annelids. Another primitive way of obtaining oxygen, which is found in sponges, is the use of ostia. Ostia are small holes found in sponges that allow water to pass into chambers in the body of the sponge, similar to diffusion. In these chambers, the sponge uses small filters that help them pick up food particles and absorb oxygen. Diffusion and the use of ostia are both methods that simply draw oxygen out of the water, rather than use a more complex structure, like a gill.

The next big respiratory system in the animal kingdom is gills, which can be found in various forms in molluscs, in most annelids, in fish, in amphibian larval stages, and in some adult stages.The most common form of gill is the internal gill, which is found most often in fish and molluscs. Gills operate by taking in water through a mouth or another hole and filtering it through gill filaments– structures that diffuse the water into the organism’s lungs. Gills are more effective than diffusion because they allow the organism to have more protective skin and better specialized breathing. In molluscs, the intake of water is different than fish, they use valves to take in and expel water; whereas, fish employ the use of a mouth and gill slits.

However both molluscs and fish still use internal gills. These valves are also sometimes used to aid the organism in movement. Although fish may be associated with only gills by some individuals, a few fish do posses lungs- some examples being lungfish or mudskippers, both of which can survive on land providing their bodies are kept damp. One variation of the gill is the external gill, which is found on some fish species, on amphibian larvae, and on nudibranchs. One of the most popular animals with external gills is the axolotl.

These cute amphibians have become increasingly popular in the pet trade and one of their most distinctive features is their external gills. External gills work identical to internal gills, with the exception of finding them unprotected on the outside of the body on stalks. These gills are typically on the head of the organism; however with nudibranchs, they are typically found near the tail. One of the rarest gill forms is the book gill, which is a type of external gill that is only found on horseshoe crab. Book gills are similar to book lungs (which will be discussed later) but book gills are external whereas book lungs are internal.

Both book lungs and book gills are found in arthropods. The book gills are located on the underside of the horseshoe crab- above the telson but below the genital operculum. The final respiratory advancement in the animal kingdom are the lungs which can be found in arthropods, fish, amphibians, reptiles, birds, and mammals. Lungs come in many different varieties- including book lungs and closely related tracheae. Basic lungs resemble large air sacs. These basic lungs take in air that travels to the alveoli, where eventually puts the oxygen into the bloodstream.

Birds have a variation of this basic lung; however, birds have multiple air sacs connected to their lungs. This helps keep the airflow unidirectional, which then helps them receive air with higher oxygen content. Mammals, unlike birds, have a bidirectional flow of air- meaning mammal’s lungs get more old air, which contains less oxygen. Closely related to lungs, book lungs are found in arachnids such as spiders and scorpions; book lungs seem like gills in appearance and structure, but they have leaf-like structures that air passes through in order to reach the bloodstream.

The book lungs are located on the underside of The organism’s abdomen. Another lung-like respiratory system found in arthropods is the tracheae, which are tubes that are found inside the body of insects that connect to spiracles- small holes on the outside of the organism’s body. The tubes connected to the spiracles are interconnected, and they lead to the bloodstream, thus allowing the insects to breathe. The respiratory system, however, is not only used to acquire life giving oxygen, it is also used to display different behaviors, such as flaring gill in fish or puffing out the chest in animals with lungs. Animals may do these things to make themselves look larger than they really are, or to signal that they are angry or agitated.

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Betta fish in particular flare their gills out when they see another male, as a sign of agitation. Sometimes they also do this when they see their own reflection, as they do not recognize themselves. Some mammals, such as kangaroos, puff up their chests when they are angry or ready to fight.

These are just a couple of examples of how animals use their respiratory systems to not only breathe but also to communicate. Overall the respiratory system is extremely important, as it provides life giving oxygen to organisms through complex and primitive means that work in various different environments. As well as helping animals adapt to different environments and communicate with each other through body language.

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The Respiratory System Functions and Varieties Across the Animal Kingdom. (2020, May 13). Retrieved from