Water is caused to flow through the gills by the expansion and contraction of the buccal and the opercular cavities. The mouth and opercula donít have to entirely close for their valves to operate but, to ensure that the efficiency of the one way pumping action is maintained, they have to close sufficiently to allow their valves to be able to fully close and prevent the back flow of water during the respiration cycle.
There is no muscular action involved in opening and closing the valves, they are just flaps and, as the buccal and opercular cavities expand, the negative water pressure in the two cavities causes the oral valve to open and the opercular valves to close. This allows water to be drawn into the two cavities but only via the mouth.
When the buccal and opercular cavities contract, the resultant positive pressure inside the chambers causes the oral valve to close and the opercular valves to open. In this way water can only be expelled by passing through the gill filaments and then through the opercular valves and not back out through the mouth.
With these two cycles, water flows almost continuously across the gills in the same direction. When the buccal and opercular expand and contract, they are almost simultaneous but the opercular cavity lags slightly behind the buccal cavity. This slight lagging of the movement of the opercular cavity behind the buccal cavity makes the process a little more efficient.
There are two phases:
This could roughly be described as breathing in through the mouth.
The buccal cavity begins to expand with the opercular cavity following very slightly later. The resulting negative pressure that is created opens the oral valve and closes the opercular valves which causes water to be drawn, first into the buccal cavity, and then through the gill filaments into the opercular cavity. As the water passes through the gill filaments, it is only separated from the blood flowing through them by just two layers of epithelial cells.
This thin permeable membrane allows the excess carbon dioxide in the blood to diffuse out and into the water and for dissolved oxygen in the water to diffuse into the blood where it can be carried around the body.
It is often assumed that the only way that oxygen can be carried around the body is when itís attached to haemoglobin in red blood cells but only about 95% of the oxygen is carried by haemoglobin, the remainder is dissolved directly into the blood.
This could roughly be described as breathing out through the gills.
As the buccal cavity begins to contract, the oral valve is closed by the positive pressure so water in the buccal cavity is forced through the gill filaments. As the opercular cavity begins to contract, slightly later, the combined positive pressure opens the opercular valves and the oxygen depleted water is expelled through them.
The process may seem to be unnecessary elaborate but, even in well aerated water, there is far less oxygen available to fish than we humans find available in the atmosphere we breathe and so itís important that the process of extracting the maximum amount of oxygen must be as efficient as possible.