10. THE SWIM BLADDER AS A HYDROSTATIC ORGAN
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creased hydrostatic pressure. This is well demonstrated when a herring
filled purse seine is hauled up through the water. An ascending school of
herring can also be identified on an echo sounder screen by a “cloud
above it. The cloud is reflections from rising gas bubbles, the school
itself is detected by reflection from their bladders.
In physoclists, and also in physostomes exposed to mild reduction in
external pressure, gas is removed by reabsorption into the blood circulating in the resorbent area. The mechanism in this case is identical to that
in respiratory organs: Gases diffuse according to their partial pressure,
and the amount of reabsorption is equal to the arteriovenous difference
in gas content times the blood flow (Steen, 1963b). Reabsorption is
regulated partly by varying the proportion of this area exposed to the
gas phase and partly by variation of the blood flow through the resorbent
area. In a few physostome fishes (the common eel, Anguilla vtilgaris, is an
example) the pneumatic duct acts as an organ of gas reabsorption in
addition to being a direct route for gas release.
The oxygen reabsorbed for hydrostatic purposes is most likely utilized
metabolically. However, since the blood from the reabsorbent regionbe it a pneumatic duct or an oval-drains directly to the gills, the possibility exists that some of the reabsorbed 0,, under certain conditions,
may be lost to the ambient water.
B. The Maintenance of a Gas-Filled Swim Bladder
The swim bladder is in principle in the same situation as a gas bubble
submerged in water which is equilibrated with air: The gas is compressed, and diffusion will ceaselessly cause reduction in the amount of
gas. This is directly limited by a supposedly low gas permeability of
the swim bladder wall. However, the most important mechanism to
reduce gas loss is the barrier function created by arteriovenous exchange
in the rete. Since this organ also plays the crucial role in the deposition
of gas, we shall discuss the general features of countercurrent exchange.
COUNTERCURRENT EXCHANGE
Consider Fig. 10. The arterial blood has a Po, of 0.2 atm, and the
bladder is filled with gas containing 0, at 100 atm. Let us assume that
blood from the bladder enters the rete with a Po, of 100 atm. In the
rete 0, will therefore diffuse into the arterial blood, thereby reducing
the venous Po, and increasing the arterial Po, in the two flow directions.
Thus, as arterial blood enters the bladder, its Po, will approach that of
the venous blood leaving the rete. In the same way the venous Po, will
429
creased hydrostatic pressure. This is well demonstrated when a herring
filled purse seine is hauled up through the water. An ascending school of
herring can also be identified on an echo sounder screen by a “cloud
above it. The cloud is reflections from rising gas bubbles, the school
itself is detected by reflection from their bladders.
In physoclists, and also in physostomes exposed to mild reduction in
external pressure, gas is removed by reabsorption into the blood circulating in the resorbent area. The mechanism in this case is identical to that
in respiratory organs: Gases diffuse according to their partial pressure,
and the amount of reabsorption is equal to the arteriovenous difference
in gas content times the blood flow (Steen, 1963b). Reabsorption is
regulated partly by varying the proportion of this area exposed to the
gas phase and partly by variation of the blood flow through the resorbent
area. In a few physostome fishes (the common eel, Anguilla vtilgaris, is an
example) the pneumatic duct acts as an organ of gas reabsorption in
addition to being a direct route for gas release.
The oxygen reabsorbed for hydrostatic purposes is most likely utilized
metabolically. However, since the blood from the reabsorbent regionbe it a pneumatic duct or an oval-drains directly to the gills, the possibility exists that some of the reabsorbed 0,, under certain conditions,
may be lost to the ambient water.
B. The Maintenance of a Gas-Filled Swim Bladder
The swim bladder is in principle in the same situation as a gas bubble
submerged in water which is equilibrated with air: The gas is compressed, and diffusion will ceaselessly cause reduction in the amount of
gas. This is directly limited by a supposedly low gas permeability of
the swim bladder wall. However, the most important mechanism to
reduce gas loss is the barrier function created by arteriovenous exchange
in the rete. Since this organ also plays the crucial role in the deposition
of gas, we shall discuss the general features of countercurrent exchange.
COUNTERCURRENT EXCHANGE
Consider Fig. 10. The arterial blood has a Po, of 0.2 atm, and the
bladder is filled with gas containing 0, at 100 atm. Let us assume that
blood from the bladder enters the rete with a Po, of 100 atm. In the
rete 0, will therefore diffuse into the arterial blood, thereby reducing
the venous Po, and increasing the arterial Po, in the two flow directions.
Thus, as arterial blood enters the bladder, its Po, will approach that of
the venous blood leaving the rete. In the same way the venous Po, will