September 21, 2009 - Constant weight free diving is a sport in which highly evolved land mammals, humans, swim deep underwater without supplemental air and return to the surface with the same weight they started with. In competitions tags are placed at various depths along a chain and the diver proves how deep he or she has gone by bringing back a tag. The men's record is held by Martin Stepanek who earlier this year swam down to and returned from 122 meters (400 feet) in 3:36 minutes with a single breath. Two women, Sara Campbell and Natlia Molchanova, have approached 100 meters (Molchanova set a world record by holding her breath underwater for 8 minutes). It's surprising that men and women can dive and return from these depths without anything more than a monofin, that no one has died in competitions and that there are no known long term effects (a few burst eardrums aside). But that's the case. Humans can dive to great depths because they posess the mammalian diving reflex. Alec Wilkinson explains in his August 24th New Yorker Article The Deepest Dive:The mammalian diving reflex is activated when the nerves in the face come into contact with water, most effectively with cold water... First, the heartbeat slows; if you were to put your face into a sink of cold water for thirty seconds, your heart would slow too. Under pressures of depth, blood withdraws from the arms and legs and concentrates in the chest. This is called the blood shift. Meanwhile, the lungs compress, halving themselves after ten metres, then reducing by degrees until, by a hundred metres, they are something like the size of a fist; free diving is the only sport in which the lungs shrink and the heart slows. The blood shift prevents the chest from collapsing. In theory, however, a depth could be reached after which the chest became so compressed that the heart could no longer function... Shrunken lungs give a diver a sense of having plenty of oxygen... As a diver returns toward the surface, his lungs expand, more oxygen is consumed, and suddenly he feels as if he hasn't got enough to reach the surface. He might feel that he needs to breath, then suffer convulsions and black out... For someone holding his breath, the urge to breathe comes not from a lack of oxygen but from the accumulation of carbon dioxide in the blood as oxygen is used.
Here's a video of Martin Stepanek starting and returning from his record setting dive to 122 meters:
