Mechanism of Respiration of Scoliodon.

By burn-it-down | Educational Contents | 26 Aug 2022

During respiration, water is drawn into the mouth and passes through the internal gill slits of the bathing gill lamella and emerges from the outer gill slits.

The floor of the buccopharyngeal cavity is dented contraction of the hypobranchial (hypoglossal) muscles, thanks to which visceral arches expand the pharyngeal wall so that seawater containing dissolved oxygen enters through the open mouth. The entry of water into the external branch holes is prevented by an anterior fold of skin on each gill pouch. Since the outer gill slits are tightly closed, water enters the expanded buccopharyngeal cavity through the open mouth.

Respiratory System of Scoliodon | Dog Fish | Diagram

During exhalation, the mouth is closed by the action of the adductor muscle. At the same time, the retractor and interbranchial muscles are contracted
to increase the floor of the pharynx and reduce its volume. As a result, water is forced into the gill pouches, through the gill lamellae, and out through open external gill slits. Spiracles are occasionally used as auxiliary routes of entry water to breathe instead of the mouth.

In the branches of the lamellae, blood flows from the tip to the base, i.e. in the opposite direction to the direction of the water current, so the blood just before leaving the lamellae encounters the highest concentration of oxygen and the lowest concentration of carbon dioxide, i.e., an
there is an efficient exchange of oxygen and carbon dioxide between the blood and the seawater.
Seawater entering the gill pouches with the respiratory stream contains dissolved oxygen. This water is separated from the blood
contained in the capillaries of the gill lamellae only by the thin and permeable membrane walls of the capillaries. The oxygen of the water passes through endosmosis through the thin capillary walls into the blood and at the same time, carbon dioxide from the blood passes into the water through the process of exosmosis. Oxygen is carried by the blood to all parts of the body, while carbon dioxide supplied to the gills in the venous blood is eliminated by the water of the outgoing respiratory stream. Since the blood makes a complete circuit in the capillaries of the gills in a very short time, it is obvious that gas exchange also happens very quickly.

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