The floor of the bucopharyngeal cavity is compressed by the contraction of the hypobranchial (hypoglossal) muscles, due to which the visceral arches expand the pharyngeal wall, so that seawater containing dissolved oxygen flows in through the open mouth. Water entering the external branch holes is prevented by an anterior fold of skin on each gill pouch.
During exhalation, the mouth is closed by the action of the adductors. Now the buccopharyngeal floor is raised and the contraction of the pharyngeal wall pushes the water into the internal branch openings, the esophagus is closed, and further into the gill slits, where it washes the branchial lamellae and exits through the external branch openings. Respiratory movements are caused by the pharyngeal muscles, which are innervated by cranial nerves V, VII, IX, and X and the hypoglossal spinal nerve.
In the branches of the lamellae, the blood flows from the tip to the base, i.e. in the opposite direction to the direction of the water flow, so that just before leaving the lamellae, the blood encounters the highest concentration of oxygen and the lowest concentration of carbon dioxide, so there is an efficient exchange of oxygen and carbon dioxide between the blood and sea water.
Fresh 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 the thin capillary walls into the blood through endosmosis, 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 the carbon dioxide supplied to the gills in the venous blood is eliminated by the water of the outgoing respiratory stream. Considering that the blood creates a complete circulation in the capillaries of the gills in a very short time, it is obvious that the gas exchange also takes place very quickly.
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