i'm feeling a little gassy

Fancy me this.

Let’s say you were to fill a Ziploc baggie partly with air and a little bit of solid mass. What would happen if you put it in a tank of water? Depending on the air:solid ratio, it would settle at one particular depth and stay there. Try to force it down, it will float back up. Pull it up, it will sink back down. To make it rest at a different depth, you have to alter the air:solid ratio. But in a closed system like a Ziploc- or a fish- how can buoyancy be adjusted to change depth?

In most fish, there exist buoyancy-control organs called the swim bladder. In some fish, the swim bladder is connected to the digestive tract and the fish can gulp air at the surface to expand the bladder, expel it to contract the bladder. So in that case, the fish is not exactly a “closed system”. The more air the fish gulps, the higher it settles in the water column. If it wants to descend, it burps some out.

In other fish, the swim bladder is unconnected to any other system. No amount of burping or gulping air will affect the fish’s buoyancy. Instead, a gland moderates how much gas fills the swim bladder. Fittingly, it is called the gas gland and excretes both lactic acid and carbon dioxide. The resulting reactions cause hemoglobin to release its oxygen from the blood stream, which then inflates the swim bladder. Up the fishy goes.

Oxygen diffuses back into the bloodstream and goes home to its hemoglobin at another little structure called the oval window. Down the fishy goes. And then the cycle is free to start again.

Although the sea kind of freaks me out, I do think it’s pretty neat that its residents must locomote in one more dimension than us. Our physiology does not need to accommodate “floatability.” Otherwise, we would have to evolve some sort of regulator like a swim bladder.