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On the posterior wall of a closed swimbladder lies a network of capillary beds. Next to these capillaries is a muscular valve, the oval, which has the job of controlling gases exiting the swimbladder. Within the oval there are two types of muscles-- circular and radial. When the circular muscles are relaxed, the radial muscles are contracted in order to stretch the swimbladder out, which in turn creates a greater capillary surface area for diffusion. Gases from within the bladder diffuse out to the blood through these capillaries. When sufficient levels of gases have been removed to create a new level of buoyancy, the circular muscles contract. Now the oval is no longer in contact with the bladder and so the gases are not able to diffuse from one area to another. A fish "knows" its buoyancy through the responses of nerve endings to the stretching and shrinking of the bladder wall. So with any change of pressure within the swimbladder as a result of diving or surfacing, a message can be relayed to the brain to initiate the deflation or inflation of the swimbladder so that the fish can maintain neutral buoyancy (Marshall 1966). The movement of gases from the bladder through the oval is a slow process however, taking over several hours to adjust to just a small pressure change (Jones 1957). |
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