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Describe the mechanism of gaseous exchange in the gills of a bony fish.

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The mouth opens and the floor of the mouth is lowered ;hence volume of the buccal cavity increases and pressure falls; this causes water to rush into the mouth;
opercula bulge outwards raising the volume; and a fall in the pressure of the buccal cavity.
Water is forced to flow over the gills at high pressure; This is enabled by the higher external pressure; which presses both opercula initially before opening the mouth and the opercula.;
The water entering the mouth has more oxygen; and low in carbon (iv) oxide; which creates a steep diffusion gradient;
Oxygen diffuses from the water to the blood in the capillaries and combines with hemoglobin;
Carbon (iv) oxide diffuses from the blood capillaries into the water
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