In mammals, birds, and other terrestrial animals the respiratory pump is bidirectional. Air moves in and out of the same opening into and out of the lungs. This is not the most efficient system because there is some mixing of fresh and respired air and there are pockets in the lungs that may never or hardly ever get fresh air. Such an inefficient system is not a problem for terrestrial animals because of the rich concentration of oxygen in the air and the consistently high PO2.
Fish need a more efficient method, due to lower amount of oxygen in water and variable PO2 . In most fish, the system is unidirectional; water always moves through the mouth and one way across the gills, then out through the operculum. There is no mixing of fresh and respired water, maintaining as high a PO2 at the gill surface as possible. When the fish is moving rapidly or in a strong current it may ram ventilate. This is an effective and energy-efficient system of ventilation, but is limited in its application. [An Example] This problem is solved by respiratory pumping.
Water is drawn through the mouth into the buccal cavity by expansion of the buccal and opercular cavities while the opercula are closed. Then the mouth and oral valve close, the opercular valve opens, the buccal and opercular cavities contract, forcing the water across the gills and out the opercula.
The respiratory pump is sometimes called the dual pump because the pumping takes place in two phases.
Phase I: [ha!] mouth open, opercula closed, buccal cavity expanding, opercula cavity expanding. (Buccal and opercular expansion are almost simultaneous, but buccal expansion may occur slightly ahead of opercular expansion).
Phase II: mouth closed, opercula open, buccal cavity contracting, opercula cavity contracting. (Again, the contractions are almost simultaneous with the buccal cavity contracting perhaps slightly ahead of the opercular contraction).
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