In most fish, the gas bladder is a hydrostatic organ. It evolved from a primitive lung and still has respiratory function in lungfish, gars, and bowfins. Fish have a specific gravity of about 1.06 to 1.09. The specific gravity of freshwater is 1.00, and salt water is slightly more dense because of dissolved salts, about 1.03. Most fish would sink if they didn't have some kind of hydrostatic compensation. It is advantageous to be neutrally buoyant, that is, have a specific gravity of 1.00, so that it is not necessary for the fish to expend energy to keep itself up in the water column. The gas bladder lies just below the spinal column at the top of the body cavity in most fish. This puts it a little bit above the midline, so the fish has more of its weight below the air bladder and tends to float right side up. There is a South American catfish that normally swims upside down. The gas bladder in this fish is located more ventrally.
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Since it evolved from a lung, the primitive condition [An Example] is where a direct connection exists between the gas bladder and the esophagus, and is termed physostomous. This tube is termed the pneumatic duct. In the phylogenetic hierarchy of the bony fishes, all groups up through the salmonids are physostomous. This includes the eels and herrings.
It is unclear how much the duct is routinely used to move gas in and out of the gas bladder. Phyosotomes like gars and bowfins that use the gas bladder for respiration obviously use the duct to move air in and out. It appears that when physostomous fish fry, such as trout, "swim up", or become free-living after absorbing their yolk sac, they gulp air to fill the gas bladder. It also seems likely that when a physostome swims rapidly upward in the water column, the duct is used to vent the expanding gas. If a physostome were to stay neutrally buoyant below 3 or 4 meters, however, it would need to fill the gas bladder to above one atmosphere (See Ch. IIA), which could not be accomplished through the duct, but only with a gas gland and rete mirabile. The degree of gas gland and rete development in physotomes is variable, with some advanced types (e.g. eel) having them well developed, while others show less anatomical evidence of gas gland function. The physostomes with a well developed gas gland and rete also have a resorptive area for the removal of gas. The ability to fill and empty the gas bladder through the blood calls into question the routine use of the pneumatic duct by these advanced physotomous fish.
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There is no duct between the bladder and the esophagus in physoclistous fishes (all teleosts above salmonids), so there must be a well developed gas gland/rete and resorptive area in order to fill and empty the gas bladder. The function of the gas gland and rete are described below. Resorption of gas is more straightforward because the pressure in the gas bladder will never be lower than one atmosphere and, therefore, will always be higher than the blood. In order to reduce pressure in the gas bladder, the fish needs only to allow the gas to flow into the blood and then out into the water at the gill. At low pressures, this is easily accomplished by simply increasing the blood flow to the capillary bed in the resorptive area to decrease pressure or decreasing blood flow to allow the gas gland to increase pressure. When pressure in the gas bladder rises, however, and it may rise to 10 A or more in moderately deep dwelling ocean fishes, a more effective barrier must be created to prevent outgassing. Physoclists that maintain high gas pressure in the gas bladder have an out-pocketing of the bladder, called the oval, with a muscular sphincter at its neck that physically prevents the gas from entering the blood stream when it is contracted. When gas is to be resorbed, the sphincter is relaxed and gas enters the pocket to be taken up by blood circulating through the capillaries of the resorptive area.
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The gas gland is a highly vascular area on the inside of the air bladder, where capillaries allow for exchange between the blood and lumen of the gas bladder. The rete mirabile feeds the gas gland and the countercurrent system in the rete maintains a pressure differential between the blood and gas bladder. The rete has a barrier effect that keeps high pressure in the bladder and prevents the bladder from losing gas into the blood, and then into the water at the gill. The barrier is maintained by the long parallel capillaries which are arranged so that gas can diffuse from higher pressure in the efferent capillary to lower pressure in the afferent capillary. Over a long enough distance, this will result in blood exiting the rete with the same low gas pressure as the blood entering.
Pressure is increased in the gas bladder by aerobic and anaerobic metabolism in the gas gland cells that result in the excretion carbon dioxide and lactic acid. The CO2 induces the Root effect and the lactic acid induces the Bohr effect (See Ch. IIE). These shifts cause the hemoglobin to release bound O2 which increases the partial pressure of the blood. Moreover the presence of lactic acid in the blood causes a "salting out" effect which reduces the solubility of the plasma water for all gases, thus increasing the partial pressure of both O2 and N2. All of these increases in pressure are small, but they may occur in blood that already has very high gas pressure due the high pressure in the bladder. These incremental elevations in gas pressure are preserved by the rete and, slowly, the fish is able to pump up pressures in the gas bladder to remarkable levels. This is termed the multiplier effect. Because the hemoglobin shifts are more important in the multiplier effect than salting out, the predominant gas in the gas bladder of fish with high bladder pressure is O2.
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