Gill

Description

The gills are tissue évaginations vascularized which form a respiratory Heat-transferring surface. They constitute the principal gas heat-transferring surface for the aquatic animals.

The gills of the Invertébré S (Mollusc S, Annélide S, certain larvae of Insects) result from foldings up of the Ectoderme while gills of the Cordé S (of which the Vertébré S) are made up mainly of Endoderme and are thus not homologous structures. Certain Vertebrate external gills resulting exclusively from the Ectoderme have however (it is the case of the first generation of gills at the Têtard S of Amphibien S).

Role

In addition to their fundamental role in breathing, the gills are used to trap food in particular in the animals Microphage S filterers (mould, Ascidie…) and to control the ionic concentration of the interior medium (in fish).

One finds four gills under each cover in a fish. A gill is composed of a bone on which two blades branchiales made up of fine filaments fit (approximately 200 by blades, therefore 400 by gills). Each filament is very richly irrigated and blood is separated from water only by a few millimetres.

Operation

The gills have an operation comparable with the Poumon S, insofar as they constitute a heat-transferring surface, very wide in a volume restricted thanks to their form; the membrane is used as filter letting pass oxygen (towards the interior of the organization) and carbon dioxide (towards outside). Water returns by the mouth and leaves by hearing, a sufficient current of water being created by the displacement of the animal, and movements of pumping of the mouth or gills (Note:: at the species with external gills, only the movement of the body allows the creation of the water movement). In the passing, blood collected dioxygene dissolved in water.

One can illustrate this analogy between lungs and gills by the case of a man who drowns: the lungs can filter the oxygen of water, but our respiratory system cannot pump water rather quickly to renew the supply oxygen effectively. The animals equipped with gills have bodies which function, not like a cul-de-sac, but like a tunnel, even like a tree in the wind (external gills), which makes it possible to make circulate water rather quickly. From the point of view of the evolution of the life, the lungs are regarded an evolution of the swim bladder and not as gills become interns and invaginated (only one opening). The opening branchial, as for him, became one of the bodies of hearing.

Water, like the air, is a fluid where oxygen is, but in less quantity: a pulmonary membrane must filter approximately 25 liters of air to extract one liter from oxygen (the output is modest, the air including/understanding oxygen 21%), but gills must see passing from 300 to 500 liters of water to obtain the same quantity from it. Water is in addition 800 times denser than the air, and 60 times more viscous. The system of the gills thus makes it possible to make pass in one way a maximum of liquid with a muscular minimum of effort (requiring oxygen).

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