Biology and Physical Factors #7: Gas Exchange

We humans have lungs. But ants don’t. Why do some living things need lungs while others don’t?

 

In So Simple a Beginning, Raghuveer Parthasarathy starts from the basics. All creatures need a way to exchange gases, usually to take in oxygen and remove carbon dioxide. The easiest way is for the surface of the creature to do the gas exchange. A tiny creature like an ant does exactly this – the surface areas of its internal tubing is sufficient for gas exchange of its tissues.

 

Next, take a larger creature. Simple physics kicks in. The surface area of the living thing increases as a square of the increase in its length whereas its volume increases as the cube. If you increase the length by a factor of 3, the area increases by a factor of 32 = 9 times while its volume increases by 33 = 27 times. The volume, as you see, increases much faster than the area. The larger volume means the creature has a lot more cells, which in turn means, the creature needs a lot more gas exchange to occur. But the surface area, as we just saw, didn’t increase proportionally. Thus, the ant’s mechanism cannot work for larger creature. This is why larger living things evolved elaborate systems for gas exchange. Like lungs.

 

The above assumes the current oxygen levels in the air, i.e., 21%. But the oxygen levels were much higher (around 35%) during the Carboniferous period 300 million years back. In that case, yes, insects could be much, much larger, and their existing mechanism (gas exchange without lungs) would still work. The fossil record of that period shows giant insects, confirming the theory.

 

Biology is always intertwined with multiple variables, which is what makes it so complex to analyze, why rules are rarely universal.

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