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Discovery Magazine 5/1/2000

Under Pressure

by  Trevor Major, M.Sc., M.A.

Pressure is something we all have to deal with. Just walking around on dry, solid ground pits us against the weight of the gases in the atmosphere above us. Sure, air doesn't seem to weigh much, but we sit under thousands of feet of the stuff. All those gas molecules add up to a force of about 15 pounds on every square inch of our bodies. We call this measure "1 atmosphere."

Water is much more dense than air, so the pressures get worse as we dive under water. You can feel this pressure on your ears when you swim to the bottom of a deep pool. Roughly, for every 33 feet we go down, we're going to experience an extra atmosphere of pressure. So at the deepest spot in the world's oceans-in the murky depths of the almost seven mile Challenger Deep—the pressure would be 1,100 atmospheres. That's pretty rare, but still, lots of animals have to cope with hundreds of atmospheres of pressure. How can they survive these tremendous pressures?

One approach is to do without gas compartments, like lungs. That's easy for fish: they have gills instead. But lots of fish have a gas compartment called a swim bladder. By letting some of the gas out of this bladder, a fish becomes sink lower; by taking up more gas, a fish becomes more buoy­ ant, which makes it float higher. That's fine, as long as the fish stays in shallow waters. As the fish swims into deeper waters, pressure will build up on the gas inside the bladder. If it goes too deep, the pressure could force the gas into places where it doesn't belong. This would not be good for the fish. To get around this problem, deep-sea animals often use oil instead of gas, or they don't have a swim bladder

Sea animals that breathe air, like whales and dolphins, use a different tactic. Their lungs can shrink as they dive. The extra gases are moved into temporary holding areas where they cannot harm the surrounding body.

But crushing body parts is not the only problem. At mind­ boggling depths, high pressures stop proteins from forming into the right shape. If they don't have the right shape, they can't take part in the body's chemical reactions. It would be like trying to unlock a door with a bent key. Deep-sea animals seem to get around this problem by using an extra high dose of a special chemical called TMAO. This chemical allows proteins to take their proper shape, even under high pressure (it also gives sea creatures that "fishy'' smell we know and love).

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