Respiratory System
All fish have gills instead of lungs as they live in water. These allow for gas exchange where diffusion cannot occur. In bony fish the gills are covered by an operculum while in cartilaginous fish they are exposed and visible. In bony fish there are specialized muscles that are responsible for pumping water through the gills. Since cartilaginous fish do not have that they rely on constant movement/swimming (called ranjet ventilation system) or the opening and closing their mouth for gas exchange to occur by forcing the water through their gill slits.
Amphibians then evolved their respiration in such a way that their larvae, which initially lived in the water to respire through gills and their skin, and as they grew into adults they would acquire lungs, through their modified pouches changing into them, and get rid of the gills because they became air-breathing. This metamorphosis occurs due to the fact that their habitat changes and they become mainly land living. Amphibians are considered the first animals to bridge the gap between water and land and are accordingly the first animals that have evolved lungs that we have seen so far. They require positive pressure breathing to respire because they are less complex animals compared to mammals or even reptiles and do not have a diaphragm or ribs to make negative pressure breathing even possible. Most reptiles possess large lungs since they are terrestrial, which are divided into chambers. Even though they live both on land and in the water no gills evolve at any point in their life. A reason why most reptiles have large lungs is because the majority is quite large and consequently requires larger oxygen supply. They use negative pressure breathing because they are more advanced than amphibians and have ribs. Crocodilians even possess a diaphragm, but their breathing is slightly different to that of mammals, as they use a hepatic piston, a procedure requiring the muscles of the pelvis and liver to move, to expand the lungs, rather than the movement of the rib cage. Reptiles have alveoli because it allows for increased gas exchange through higher surface area available, faster as well as more efficient diffusion of gases because they are very thin and moist, and they are surrounded by a large network of capillaries which also allows gas exchange to occur much more efficiently because the blood is almost directly in contact with many alveoli. Birds have lungs just like all other land living vertebrates, but in addition they possess air sacs, which first of all allow for extra supplies of oxygen to be stored, and to be at the body’s disposal at any time as high amounts of oxygen are always present. They also result in the bird to become less dense or lighter due to the additional amount of air that they are carrying which then results in easier flight. Since no other vertebrate animals require flying such long distances this special adaptation in the respiratory system can only be found in birds. Birds use negative pressure breathing like reptiles and mammals, but they do not utilise a diaphragm to do this as it will deflate their air sacs, which they require to keep filled. They use avian respiration which has a unidirectional flow of air, meaning that the air flowing through will be largely fresh and not mixed with “old air”, while in mammals there is a bidirectional flow. This makes sure that the oxygen supply in the body of a bird is always high, which allows them to breath much more efficiently at much higher elevations than mammals. |
Respiration in mammals is exactly the way we know it to be from humans, with lungs and a diaphragm, which allows for negative pressure breathing. Mammals also utilize this breathing because it is more efficient than positive pressure breathing, as air has to be forced into the body rather than creating a pressure gradient where it naturally flows into and out of the lungs when required. Mammals require an efficient supply of oxygen as they are considered the most advanced animals, and therefore require a lot of processes to occur in their body. As the diaphragm contracts the volume in the lungs increases, resulting in a lower pressure to be present in the lungs than outside, which results in the air to flow in. When the diaphragm relaxes the volume decreases and therefore the pressure increases leading to the air to flow out.
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