Taking the Ultimate Plunge

Tanya Streeter takes a last breath. A moment later, the water closes over her head. Her destination is 160 meters below—a depth no human has ever reached on a single breath.

How deep can a “free diver” go? No one really knows the human limit. “Sometimes I think I don't want it to be me who finds out,” says Tanya. “On the other hand, I believe so much in our body's responses.” Recently, a free diver drowned during a record attempt. “There are a lot of good free divers who are very safe,” says Dr. Richard Vann, a dive physiologist at Duke University Medical Center. “But it takes training and practice.”

It's All in the Training

Our bodies are well adapted to sea level or moderate altitudes. by studying free divers like Tanya, scientists have discovered that we also have a remarkable ability to survive below sea level, even under pressures many times greater than at the surface.

When Tanya is at sea level, she's at the bottom of an “ocean of air” 800 kilometers deep. Scientists call the weight of the air pressing down upon her “one atmosphere” of pressure. Because water is heavier than air, just 10 meters of seawater equals one atmosphere of pressure. At 160 meters, Tanya faces 17 atmospheres of pressure (16 atmospheres of water pressure, plus one atmosphere of air pressure).

The Test

It's August 17, 2002. Tanya, wearing a nose clip, steps onto a heavy sled attached to a weighted cable. She wraps her arms across a T-shaped bar. She takes rapid, strong breaths to flush carbon dioxide from her lungs and replace it with oxygen. Then she packs—takes small gulps of extra air. Tanya gives a nod to her husband, Paul, who releases the sled. She slips quickly into warm Caribbean water.

As Tanya moves down, water pressure increases. Most of her body isn't noticeably affected. A human body contains a lot of liquid, and liquids aren't very compressible. Gases, however, are easily compressed. The air spaces in Tanya's body—her lungs, ear canals, and sinuses—soon feel pressure.

Water presses on Tanya's sensitive eardrums. She blows into her nose, which is blocked by the nose clip. Air is forced from her lungs into her ear canals. This makes the pressure inside her ears equal to the water pressure outside (divers call this equalization).

Tanya is now 20 meters down. . .then 30 meters. . .then 40 meters, past the depth limit of recreational scuba divers. Her heart rate slows. Her body pushes more blood into her heart, lungs, and brain. Less blood reaches her arms and legs. Scientists call this the “dive reflex.” All life began in the sea, and this reflex may be an evolutionary relic of our watery past.

The water pressure compresses Tanya's lungs, and she pushes more air from her lungs into her ears. She never exhales, because she needs every bit of the air in her body to protect her ears. With the air spaces in her body tightly compressed, Tanya is much less buoyant. The sled speeds up. If it goes too fast, Tanya won't be able to push air into her ears fast enough to offset the increasing water pressure. Her eardrums would burst. Tanya reaches up and opens the valve on a lift bag attached to the sled. The sled slows. Her depth: 60 meters and dropping. At 80 meters, Tanya's lungs are the size of her fists. Blood pulled from her arms and legs fills the space left by her compressed lungs. The blood keeps Tanya's chest wall from collapsing.

One hundred meters. Little sunlight reaches the diver at this depth. Tanya's heart rate is slow, perhaps only 40 beats per minute, compared to 60 at the surface. She pushes air from her mouth and throat into her ears. 110 meters…130 meters. Tanya's heart rate drops to 30 beats per minute. She is surrounded by absolute darkness. With a clunk, the sled hits a plate at the bottom of the cable. One hundred sixty meters!

A Record Dive!

“During a dive, I am only thinking about where I am, whether I should slow down or speed up,” says Tanya. “On that dive, I stopped to take it all in and think about the fact that I may never again be that deep…that nobody may ever be that deep. It was very special, very peaceful.”

After a two-minute trip down, Tanya spends 15 seconds at the bottom of the cable. She blows a kiss to a camera hanging below her (for proof of her world record). There really isn't much to see. The sea floor lies in eternal darkness 2,100 meters below.

Tanya reaches up and opens a valve on a lift bag from the sled. She inflates it and begins a quick trip up. As she rises, water pressure decreases. Air moves back down into her lungs. Blood is pushed out of her chest cavity. Her eardrums equalize by themselves as the air in her body expands.

At 30 meters, Tanya releases her air bag and swims slowly up. Although the dive is almost over, the last few meters are in some ways the most dangerous. During her dive, Tanya's body has been using oxygen. She has very little left in her body. If she doesn't have enough oxygen when her lungs re-expand, her lungs will suck oxygen out of her blood and away from her brain. Tanya could lose consciousness. Divers call this “shallow water blackout.”

Shallow water blackout usually happens in the last 10 meters of a dive. That's because pressure changes are most intense near the surface. When Tanya comes up from 10 meters to the surface, the pressure is suddenly cut in half, from two atmospheres to one atmosphere. (When Tanya rose from 160 meters to 150 meters, she was going the same distance, but the change from 17 to 16 atmospheres of pressure was slight.)

Thankfully, Tanya has enough air left to make it back. She breaks the surface to cheers from the dive boat crew. She's just broken both the women's and the men's world record!

As a child, Tanya says that she believed she could breathe underwater. Perhaps she can't, but this dive was surely the next best thing!

Vocabulary

  • free dive: The act of diving underwater while holding one's breath.
  • lift bag: A bag with its own air tank.

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Activity

  1. After reading Lesson 2, what did you learn about the purpose of the body's circulatory system?
    [anno: The circulatory system transports oxygen, nutrients, water, and waste inside of the body.]
  2. What happened to Tanya Streeter's heartbeat during her free dive?
    [anno: It slowed down.]
  3. Write a few sentences explaining why you think Tanya Streeter's heart rate changed during her dive. Think about the resources that blood carries to the cells of the body. What benefit might there have been to her circulatory system as her heart rate changed?
    [anno: Answers may vary. Students should indicate that Streeter's heart rate slowed because the body was conserving resources, particularly oxygen.]