I love connections. To me, they are what learning and understanding are all about. Plus they’re really cool. For instance, yesterday I learned something cool and amazing about fish and swim bladders.

OK, a little background. I was reading something about discrepant events – you know, those science demonstrations that make you go, whoa! I believe they are the key to creating disequilibrium in learners’ minds, forcing them to accommodate their world views . . . I’m losing you, aren’t I?

Anyway, while reading a list of discrepant events to discuss with learners, I came across one item that struck me as just wrong. The author was claiming that a fish’s swim bladder is a discrepant event. Most learners will think that a fish adds air to its swim bladder in order to float higher. In fact, claims the author, just the opposite is true. The fish expels air to swim higher, because the vacuum created has less mass than the air. This struck me as almost certainly wrong, so I did some research.

Sure enough, the author was mistaken. Fish do add air to the swim bladders to increase their buoyancy. But . . . how?

Think about it for a moment and it’s a great puzzle. Fish can’t have had that air inside them to begin with (unless it was compressed, and I couldn’t see how a fish could be holding compressed air in). Do they “breathe” in a bunch of air very quickly to rise? This seems impractical, as often fish need to change their buoyancy quite quickly. So what do they do? We’ll come back to it.

Have you ever been exercising and felt that painful burning in your muscles? That good ache that lets you know you’re working hard? That pain is from lactic acid. When you exercise, your muscles burn lots of glucose by combining it with oxygen, thereby releasing its stored-up energy (energy that came from the Sun via photosynthesis of the plant that made the glucose, but that’s another connection story). However, if you run low on oxygen, your muscles start to convert glucose to lactic acid. This releases energy, too, but not as efficiently as the glucose plus oxygen reaction. And the side-effect is that the lactic acid starts to make your muscles ache as it turns the tissue acidic.

Fortunately, your body has a built-in defense mechanism against lactic acid damage. When tissue starts to turn acidic, the blood feeding that tissue becomes acidic, too. And when blood becomes more acidic, hemoglobin (red blood cells) start to release more dissolved oxygen. Oxygen, of course, is exactly what your muscles are screaming for, and so everyone is happy again.

Fish, with whom we share a common ancestor (we are, in fact, highly-modified, bicycle-riding fish – apologies to Gloria Steinem), have this same physiological response, but in fish the response is even stronger. Fish blood is extremely sensitive to changes in pH, so that a little lactic acid can cause a large release of oxygen. And fish use this response in an amazing way.

Lining the fish’s swim bladder are cells that are specially adapted to produce lactic acid. When they do, instantly the blood near these cells dumps lots and lots of dissolved oxygen. Much of that oxygen goes into the bladder as gas, and that gas makes the swim bladder expand. The fish carefully controls the amount of gas going into and out of the swim bladder so that as a whole the fish remains neutrally buoyant in the water. Lactic acid as a buoyancy control! Amazing!

But it gets even better. Why do we produce lactic acid at all? Because we all evolved from bacteria that used this method to eat! Before there was much free oxygen in the atmosphere (which, after all, came from plants), all the creatures on the Earth used this non-oxygen (anaerobic) method of eating. Many bacteria still do, of course, and they can be found anywhere food is abundant but oxygen is not. It was only when the plants “poisoned” the atmosphere with this volatile, fire-supporting waste gas that evolution found the more efficient pathway of burning glucose with oxygen to release energy. We might get annoyed at this scar of evolution every time our muscles start to ache, but for fish, it’s the very scar that keeps them afloat!

And that, dear readers, is what makes life cool.