A Balancing Act

Competetive free divers use a weighted sled to descend quickly, then inflate an airbag to shoot back to the surface

The dive response also helps combat the effects of high pressure on a diver's body. Though the water pressure compresses the lungs, the extra blood shunted to the heart and lungs expands the blood vessels in the chest, balancing the forces. "A diver will be all right," says Lundgren, "as long as the amount of pressure inside keeps in step with the pressure outside."

Scientists do not yet know how far down the body can maintain this balancing act. "How much blood can move in before you start bleeding into your lungs?" Lundgren asks. "It's not unheard of for deep divers to cough up blood when they get to the surface. I think these divers are very close to the limit."

Pushing the Limits

Though physiologists have yet to determine this "limit,"—which likely varies among individuals—they do know what could happen to the unlucky diver who passes it. At the critical depth, bleeding into the lungs could become excessive. According to Lundgren, it's unlikely that such bleeding would result in immediate death, but it could cause irreversible damage. Similarly, lung edema, the leakage of clear blood plasma into the lungs, is a serious occurrence.

"You can tell it has happened when a diver coughs up foam, often tinged with blood," says Lundgren. "This is most often observed in spear fishers, who repeatedly dive quite deeply, are really close to their limits."

More seriously, the redistribution of blood from the limbs to the body's core puts an unhealthy load on the heart muscle. The muscle tissue could become overly distended, resulting in permanent damage. Or, the strain of all that extra blood could cause the heart to beat irregularly, which can be fatal.

"The diver could die right there and then," says Lundgren.

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