Re: Why don't diving animals suffer from the bends?

Hi Sylvia,

Good question! Before I try to answer it I should warn you that I'm not a biologist and I don't know very much about marine animals. I study physics and I'm a scuba diver, though, and I think I can give your question a good guess.

First we need to discuss a fact from physics and chemistry. Liquids which are under high pressure can dissolve (that is, hold) more gas than liquids which are under low pressure. A familiar example is a soft drink: when you open the bottle, you quickly lower the pressure on the liquid, and the liquid can no longer hold all the carbon dioxide it was holding under pressure. So much of the carbon dioxide has to come out of the liquid fast, and it does--as bubbles. That's what all the fizz is.

Since 80% of the air we breathe is nitrogen, our blood is full of dissolved nitrogen. The bends happens when the pressure on a person is lowered quickly. Suddenly at lower pressure, the blood can no longer hold all the nitrogen it's holding, so some of the nitrogen comes out of the blood as little bubbles. These bubbles can block the capillaries in the body and stop the flow of blood, which is why the bends are so dangerous.

What causes the bends to happen to human divers is this. Say a diver starts at the surface with a full load of nitrogen in her blood. Then she goes underwater, where the pressure is greater. Her blood can now hold more than it could before, so there's room for more. As she breathes from her tank, more nitrogen gets into the blood, which becomes saturated (filled) at the higher, high-pressure level. If she returns to the surface too quickly, then the pressure is lowered quickly, and all that extra nitrogen has to come out of the blood as bubbles. (That's why no one should try scuba diving without proper training! In a scuba course, you learn how long you can stay down and how slow you should ascend to avoid the bends.)

Diving animals, of course, don't have scuba tanks. When they're underwater, they're not taking in any air, so they're not taking on extra nitrogen to fill up their blood beyond its surface-pressure capacity. They come up with the same amount of nitrogen they started with, which is no problem. That's why they don't get the bends. It's also why people can free-dive (i.e. dive without tanks) without worrying about getting the bends.

Hope that answers your question!

Dan Goldner
goldner@mit.edu
MIT/Woods Hole Joint Program in Oceanography