快猫短视频

IT鈥橲 DEATHLY dark, wet, and you鈥檙e chilled right through. You haven鈥檛 drawn a
breath for a couple of minutes now, and your heart is barely beating. Your lungs
have been crushed until they take up little more space than a Coke can, and
although your spleen has splurged out a mass of extra blood cells, your veins
have collapsed and the blood forced out of your limbs into the space where your
lungs should be. What little oxygen you have left is devoted only to keeping
your heart and brain ticking over, and there鈥檚 an intolerable pain as your
eardrums feel about to burst.

This is what your body would be going through 150 metres below the surface of
the sea. It sounds like a living hell. Yet, believe it or not, people do this
for sport. Last June, Frenchman Lo茂c Leferme became the world record holder
for 鈥渘o limits鈥� freediving when he took a deep breath and was dragged 152 metres
down on a weighted sled without any breathing apparatus. He went down further
than the doomed Kursk submarine. But whereas the Norwegian rescue divers who cut
into the sub spent the next five days recovering in a decompression chamber,
Leferme was talking to the press just moments after he broke the surface.

Competitive freediving pushes the body to the absolute limits of endurance.
And for as long as divers have been willing to test the body鈥檚 limits,
scientists have come along for the ride. Researchers are studying some of the
world鈥檚 most talented freedivers, scanning their brains, monitoring their hearts
and lungs, even simulating deep dives in the lab. They鈥檙e busy trying to figure
out how the body copes with such unnatural challenges, and trying to hazard a
guess at just how much further you can push the human body and live.

From the very first moment cold water hits the face, your body starts
behaving differently. There is a huge shift in the pattern of blood circulation.
Blood vessels in the muscles, skin and internal organs contract, channelling
pretty much all of the available oxygen to our two most vital organs鈥攖he
heart and brain. 鈥淭he other tissues can make do for a while,鈥� says Claes
Lundgren, Director of the Center for Research and Education in Special
Environments at the State University of New York at Buffalo.

At the same time, the heart slows right down. 鈥淲hen you shut off a big part
of the circulation, you can survive with a much lower heart rate,鈥� says Erika
Schagatay, a physiologist at Mid Sweden University in H盲rn枚sand. And
by reducing the heart rate, you also reduce the amount of oxygen your big,
energetic heart muscle needs.

These effects are known as the physiological diving response. 鈥淚t is
something that we share with all other vertebrates, even the fish,鈥� says
Lundgren, although in fish, the oxygen-preserving reflex kicks in when
they鈥檙e out of the water. The human diving response is so efficient that the
world record for breath-holding is a phenomenal 7 minutes 35 seconds
(see diagram).

To survive a deep dive you need this efficient diving response, and as big a
lungful of air as you can manage. Some divers have even learned to swallow air
into their already full lungs. It鈥檚 not just to keep up the oxygen supply, but
also to cope with the increasing pressure. For every 10 metres you descend, the
pressure rises by 1 atmosphere, and the air in the lungs is compressed to match.
When the chest cavity has compressed as much as it can and the pressure is still
rising, the space has to be filled up somehow, so blood is sucked into the blood
vessels around the collapsing airways and alveoli.

Tanya Streeter, who holds the women鈥檚 record for 鈥渃onstant ballast鈥�
freediving, explains that she feels this as a real crushing sensation in her
chest down to about 60 metres. Deeper down, the chest pain eases and another
problem takes over. As the pressure on the outside of her eardrums mounts, the
biggest thing on her mind is finding enough air left to force into the
Eustachian tubes between the throat and middle ear to equalise the pressure.
鈥淭he limits are about finding one more equalisation,鈥� she says.

Getting back to the surface can be just as gruelling. Paradoxically, the
problems increase as divers near the surface. When the lungs expand again, the
oxygen pressure drops, most dramatically in the final 10 metres where the lungs
double in volume. Now, with less oxygen passing into the blood, there鈥檚 a big
risk of fainting鈥攚hat鈥檚 commonly known as shallow water blackout.

Pushing the limits

With all these hazards and discomfort, as well as such major physiological
changes going on, what is it that makes a record-breaking diver? Can the body
adapt to the pressure and lack of air through sheer hard work and training, or
are the best divers born rather than made?

To try to find out, Schagatay measured the strength of people鈥檚 diving
response, using the drop in heart rate as an index. When trained divers from the
Swedish national freediving team submerged their faces and held their breath,
their heart rates dropped by half鈥攁 diving response comparable to otters
and beavers. And the colder the water, the stronger the response. In a sample
group who had never been diving, the drop was less impressive, more like 20 or
30 per cent.

Schagatay and her colleagues also compared the breath-holding ability of the
Swedish freedivers to populations with a long history of diving. She studied Ama
divers from Japan鈥攁 population that has been using breath-holding
techniques to harvest shellfish and seaweed for more than 2000 years. And she
also tested members of a tribe called the Suku laut, or the 鈥淪ea People鈥�, who
live a semi-aquatic existence in the Indonesian archipelago. The Sea People
spend up to 10 hours every day in the water, they give birth in the water, the
children dive before they walk and the people harvest all their food from the
sea.

Despite the long history of diving in these two populations, Schagatay and
her colleagues found no obvious differences between them and the freediving team
in their breath-holding skills and diving responses. In another study, Swedish
non-divers practised breath-holding for two weeks, and both their breath-hold
times and diving response increased. This suggests that skilled divers don鈥檛
have specialised genes, she believes. 鈥淧ractice can bring out these effects in
anyone,鈥� says Schagatay. 鈥淭he diving response is clearly trainable.鈥�

She also found that divers could improve breath-hold times by half as much
again after just five successive attempts in a single day. Most divers turn this
to their advantage by performing a series of breath-holds and shallow warm-up
dives in the run-up to a record attempt. Schagatay鈥檚 latest work shows how the
spleen, an organ that rarely gets a mention, might be the key to this successive
improvement. In a study published in next month鈥檚 issue of the Journal of
Applied Physiology, she describes how breath-holding seems to cause the
spleen to release a store of oxygen-carrying red blood cells.

Animals such as racehorses and seals were known to do this, but 鈥渋t is quite
new that this mechanism is important in humans in any situation,鈥� says
Schagatay. Diving or breath-hold training might help this reservoir get bigger
or, more likely, empty more efficiently, she adds.

The body seems to cope with unusually long breath-holds in other ways, too.
Paul Gabbott and his colleagues from the University of Oxford have some
preliminary evidence that the blood flow in divers鈥� brains rises substantially
while they are holding their breath. The researchers used a technique called
transcranial Doppler ultrasonography on freediving instructor Marie-Teresa
Sol-omons. The technique used changes in reflected sound waves from moving red
blood cells to measure the speed of blood flow in a major blood vessel, the
middle cerebral artery.

The velocity of blood flow was normal until Solomons held her breath. Then
the flow began to increase dramatically. 鈥淭here seemed to be a 100 per cent
increase in the flow of blood through the middle cerebral artery after 4 minutes
of apnoea,鈥� says Gabbott. He is planning to follow up this study with functional
brain imaging. He鈥檚 particularly interested in how conscious control and the
divers鈥� psychological state blend with the physiological processes to help them
cope during a dive.

Fighting the urge to breathe is just one of the psychological challenges.
Streeter admits that a deep dive can be a frightening experience. It鈥檚
completely dark, you鈥檙e all alone, intensely cold as the blood drains from your
limbs, and you have to brake the sled to keep from plummeting quicker and
quicker. 鈥淵ou need balls of steel,鈥� she says.

And she鈥檚 right to take the risks seriously. Several divers have died during
training. 鈥淭here are frequent deaths in snorkellers, swimmers and breath-hold
divers that can鈥檛 be explained,鈥� says Lundgren. He speculates that abnormal
heart rhythms triggered by the diving response could be to blame in some cases.
He found that even fit, trained divers experience abnormal heart rhythms during
simulated dives in a pressure chamber.

Indeed, Lundgren thinks an inappropriate diving response, perhaps sparked by
an icy blast of wind to the face, might explain some mysterious wintertime
deaths out of the water. An overactive diving response might even be behind some
cases of sudden infant death syndrome. When a warm baby turns their face into a
cold draft, the diving response could slow the heart dangerously, says
Schagatay.

But it鈥檚 not just abnormal heart rhythms at fault. Not everyone could survive
the pressure at the depths they鈥檙e now reaching in competitions. Most
researchers are reluctant to put a figure on how much pressure the human body
can survive鈥攑artly because they keep being proved wrong. Just 40 years
ago, the medical world thought the lungs would never cope with the pressure 50
metres down, yet that鈥檚 just a warm-up by today鈥檚 standards. But Lundgren is
willing to stick his neck out. 鈥淚 think we are now at the limit,鈥� he says.
Schagatay agrees. 鈥淲e were not meant to be deep divers,鈥� she says.

Indeed, something strange seems to be happening to the blood during dives
deeper than 30 metres, according to experiments done at the Wirral Hyperbaric
Centre at the Murrayfield Hospital on Merseyside (see 鈥淒ive, dive, dive鈥�). The
researchers took arterial blood samples from two divers immediately after a
simulated deep dive, and within 30 seconds of taking the sample, the blood had
coagulated. 鈥淭he blood was definitely sticky,鈥� says Dave Alcock, Director of
Operations at the unit. The red blood cells also looked abnormal under the
microscope. They are planning more tests to find out what鈥檚 really going on.

Even if this effect proves to be harmless, as the pressure increases further
and more and more blood is sucked into the chest space, the tiny blood vessels
become ever more distended. Their strength and the point at which they would
rupture is a very individual thing. Already, some divers cough blood after a
dive鈥攁nd that can only mean they have sustained some sort of damage. 鈥淚f
they really knew what they were putting their bodies though, I don鈥檛 think they
would dive,鈥� says Lundgren.

The quest for greater depths is sure to continue鈥擣rancisco 鈥淧ipin鈥�
Ferreras has already claimed an unofficial record of 162 metres. But each body
and each day is different. If someone has a greater tendency to bleed, or if the
water is just that bit warmer, the depth could be too great. 鈥淏reath-hold diving
is a wonderful thing,鈥� says Lundgren. 鈥淏ut no researcher would say it鈥檚 safe to
go so far or so deep. It鈥檚 a very, very fine line they are treading at those
诲别辫迟丑蝉.鈥�

THE hatch clangs shut like the airlock of a submarine, sealing champion
freedivers Tanya Streeter and Frederic Buyle into the pressure chamber housed in
the Wirral Hyperbaric Centre at the Murrayfield Hospital on Merseyside. The
chamber is usually pressurised with air for the benefit of divers with
decompression illness. But today it鈥檚 been flooded with around 20,000 litres of
tepid water, quite a test for their newly installed heating system. The plan is
to simulate exactly the pressure changes that would be experienced during a real
dive.

Throughout the 鈥渄ive鈥�, a team led by Dave Alcock, Director of Operations at
the unit, will monitor the divers鈥� heart rates and blood pressures and take
blood samples as the pair surface. The divers have arterial lines in their
wrists, dangling uncomfortably outside their wetsuits. In return for the probing
and discomfort, they hope to learn a bit more about what their bodies are going
through鈥攁nd perhaps pick up some tips about breathing exercises and
warm-ups that could help improve their personal bests.

As technicians take their stations around the chamber, the pair don their
masks and nose clips. Resting their feet on the bottom, head and shoulders above
the surface, they do their warm-up deep-breathing exercises, sucking in or
blowing out air through tight lips for around half a minute at a time.

Buyle gives the ready signal and all goes quiet鈥攂riefly. As the divers
sink beneath the surface, Alcock twists a handle on the control panel to release
compressed air into the chamber. He controls the pressure increase to simulate a
descent of one metre every second. As they pass the one minute mark, Buyle is
still signalling OK, so they keep pressurising to the equivalent of 80 metres.
As they hit the target depth, the warning lights blink on鈥攖he signal for
the crew to put on ear protectors鈥攁nd the technicians open all the release
valves. Compressed air whooshes out for around a minute, returning the divers to
the surface. To them it felt just like the real thing.