The swim bladder of fish...

This story is about the swim bladder that many fish have, but as is true with most good stories, it will end up being about much more than swimming.

A problem associated with swimming, particularly in deep water, is that you want your body to be buoyancy neutral: it should have the same density as the water so you don't have to work to keep from floating to the top or sinking to the bottom. Since the density of water in the ocean varies with location and time the best solution is to be able to adjust the effective density of your body to match the water. The adjustment should be rapid if you are a fast swimmer since you are going to move though water of different density.
The solution to maintaining neutral buoyancy is a bladder containing gas (the red oval) which can be filled or emptied to achieve the desired density for the whole fish.
Fish "gulp" water, most of which then exits through the gills (the black crescents behind the mouth, on the light blue background). The gills are the functional equivalent of our lungs; blood adsorbs oxygen from the water and releases carbon dioxide. If the fish has swallowed food it passes into the digestive system (yellow). The swim bladder in the adult is not connected to the mouth or digestive system. However, during embryonic development it was formed from a pouch of the digestive system.

Moving gas into and out of the swim bladder

The gas that fills the bladder is oxygen, carried by hemoglobin in the blood of the fish. In all animals most of the oxygen in blood is bound to the protein hemoglobin, which is inside the red blood cells. A unique property of the hemoglobin in fish with swim bladders enables the fish to fill or empty its swim bladder.

When the blood surrounding the bladder becomes slightly acidic the hemoglobin releases oxygen into the bladder. If the blood becomes less acidic the oxygen is reads orbed by hemoglobin. The response of hemoglobin to acidity can be summarized by oxygen binding curves. A binding curve shows how the amount of oxygen bound to hemoglobin increases with an increase in the free oxygen concentration. The black curve on the right shows the binding of oxygen to hemoglobin under normal conditions.

The hemoglobin of fish with bladders is extremely sensitive to acidic conditions, releasing about half of bound oxygen even at high oxygen concentrations. This property is called the Root effect, after the scientist that characterized it. As seen by the blue curve to the right, approximately half of the oxygen is released, even if the free oxygen concentration is very high.

The hemoglobin of most animals, including humans, is sensitive to acidic conditions, but to a much reduced degree. The red curve represents this effect, which is called the Bohr effect.

Why (or how) does muscle and other tissue become acidic

glucose + oxygen => carbon dioxide + water + energy

The conversion of glucose provides most of the energy for animal activity. If oxygen is available the glucose is oxidized (burned), producing carbon dioxide and water as waste products. This is called aerobic metabolism. The actual implementation of the above reaction requires many sequential chemical reactions, each catalyzed by enzymes.

glucose => lactic acid + energy

If there is no oxygen the glucose is converted to lactic acid. In this series of reactions less energy is produced and the tissue in which the reaction occurs becomes acidic. This is called anaerobic metabolism.
For most animals, including humans, the goal is to produce the greatest energy using the least glucose. When the oxygen concentration becomes low and lactic acid is produced it causes hemoglobin to release more oxygen (the Bohr effect). The released oxygen allows glucose to again be burned and thus extends the aerobic phase of metabolism. Of course if the demand for oxygen continues to exceed the supply eventually the tissue must use the anaerobic reaction to produce energy, and the tissue becomes steady acidic until eventually it is unable to function.

The fish uses lactic acid to force oxygen into the bladder

Some of the cells (the little orange ovals) that make up the wall of the swim bladder convert glucose to lactic acid. The lactic acid diffuses into blood circulating over the outer surface of the bladder. Due to the Root effect oxygen is released from hemoglobin and then diffuses into the bladder which expands and gives the fish greater buoyancy. The acidic blood is shown as red in the Figure. Due to the Root effect red also indicates high oxygen concentration.
As seen in the Figure the region of acidic blood and high oxygen concentration is mainly restricted to vessels on the surface of the bladder. The advantage of this localization is three fold:

1. Acidic blood flowing into the main circulation of the fish would depress the performance of the swimming muscles (remember how your muscles ache after a hard race).
2. Keeping the acid in the region of the bladder means the acid cells of the bladder don't have to produce as much lactic acid.
3. Recycling the free oxygen back to the bladder increases its concentration and forces it into the bladder.

The advantages are clear, how does the fish do it?

Blood entering and leaving the bladder surface pass through the rete mirable (Latin for wondrous network). In this network of capillaries, represented symbolically in the above Figure, the two blood flows are separated by only thin walls of capillaries. This is a counter-current exchange system, in which acid and free oxygen in the blood leaving the bladder diffuses back into the less acidic blood entering the bladder.

A similar system operates in the eyes of some fish

Many fish have a choroid rete (the choroid is the vascular space between the retina and the sclera). As in the swim bladder the blood flowing into the choroid is made more acidic which then displaces oxygen from hemoglobin. The oxygen diffuses into the eye to support the metabolic needs of the retinal cells and associated neurons.

Just as in the swim bladder, the rete isolates both acidity and high oxygen concentration to the circulation around the eye.
Evolution

A recent study of about 50 fishes has revealed a great deal about the evolution of these systems. The Bohr effect (decrease in oxygen affinity in acidic conditions) appears to have been present some 500 million years ago (when fish first appeared in the oceans). The Root effect (release of oxygen in acidic conditions) evolved 350 million years ago and 100 million years later the choroid rete appeared. However, evolution of a swim bladder rete occurred independently at least four times (but shown only twice in the simplified time line on the left). In recent times (if you call the last 100 millions years recent) the swim bladder rete, the choroid rete, and even the Root effect has been lost several times by various groups of fish as they evolved. However, if either rete is present, the Root effect is always present. This is probably because a rete allows some oxygen from arterial blood to directly "leak" into the venous blood, bypassing the organ associated with the rete. If there were not a Root effect to facilitate oxygen supply, the rete would only be a selective disadvantage.

 

References for this essay

 

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