The Big Blue

There was an incredible Obituary in the Guardian this week on Loic Leferme, the recently deceased freediver.

To freedive, you wear no breathing equipment. You take a VERY deep breath and ride down on a "sled" to a depth of over 170 metres. This takes just over two breathless minutes, so you are in no shape for sightseeing. You activate an air balloon that drags you the heck to the surface. "The Bends" is not an issue as you are not submerged long enough, so maximum speed is what you are looking for.

Most mere mortals battle Gravity; freedivers battle Boyle's Law. This states that at constant temperature, the volume of a gas decreases in proportion to pressure. Double the pressure of the air in your lungs, halve the volume of your lungs.

Imagine breathing out until you physically can't breathe out any more. Now measure the size of your lungs. This is the Residual Volume and as recently as 30 years ago it was assumed that no-one could survive a dive beyond the point that their lungs were crushed to Residual Volume.

They reckoned without a wonderful mammalian survival technique known as Bradycardia.

Literally it means low (brady) heart beat (cardia). As soon as the brain detects that certain reflexes are immersed in water, it slows the heart rate. As the heart is made of muscular tissue, this reduction in heart rate in turn results in a reduction in the amount of oxygen required by the body, which means that you can go deeper than Boyle's Law would seem to suggest.

Additionally there is a "blood shift" mechanism where blood starts to "shift" from other parts of the bodies and begins to flood the lungs to equalize the outside water pressure.

If this isn't a clue that humans have evolved from something that was comfortable in a hundred metres of water, I don't know what is. There is no reason for an Intelligent Designer to build this feature for a land-based biped.

At 170 metres, your lungs are compressed to the size of an orange and your heart-rate slows to 20 beats per minute. Your lungs are filling with blood, your sinuses full of water and you are being crushed from all sides. Normal atmospheric pressure is 1 bar, car tyres have 2 bars of pressure in them - at 170 metres you have more then 18 bars pressure on your body.

I've experienced Bronchopneumonia and this sounds to have the same kind of entertainment value. I can honestly say that I have never had an inclination to push myself to any kind of physical limit. I'm delighted that there are others in the world who are less sane and less boring by far, otherwise we'd never learn anything about where the limits actually are - not just where science thinks they are.