free diving

What Happens to Your Body 300 Feet Beneath the Ocean?

Words: Steve Root
Photography: Creative Commons

We dive into the effects on humans under extreme conditions.

Next to floating in your birthday suit in outer space, freediving to 200, 300 … even 900 feet in cold, open ocean water is one of the harshest environments to which you could subject your body. But those depths are reached regularly by free divers, whether swimming entirely under their own power, with the use of fins or assisted by a weighted sled device. Regardless of the method, each is a thrill ride that entails zero breathable oxygen, organ-crushing pressure, bone-chilling cold and, if not done correctly, a good chance you’ll emerge from the experience unconscious — if not plain old dead.

So what actually happens to your body down there?

To find out, we went to Kirk Krack, founder of free diving training agency Performance Freediving International, coach of dozens of world record holders and hundreds of national record holders.

“It’s all about becoming water,” says Krack. And he’s not just spewing mystical mumbo-jumbo. Krack is referring to a highly specialized form of training that draws upon something called the Mammalian Dive Reflex (MDR), a strange physiological phenomenon we inherited when our ancestors first ambled out of the ocean millions of years ago.

Sort of an autonomic channeling of our inner deep-diving elephant seal, MDR allows the body to manage and withstand low levels of oxygen. It kicks in the moment you hit cold water and it induces some mind-boggling transformations:

© YouTube / World of Adventure

Cardiovascular System

One of the first effects of MDR is bradycardia, a slowing of the heart rate by 10 to 30 percent, and that’s just the automatic response. Trained divers can reduce that to 50 percent, with some reports as slow as 14 beats per minute, as compared to 60 to 100 BPM on average for a person at rest. At that rate, it shouldn’t even support consciousness, but somehow … it does. 

Then there’s peripheral vasoconstriction, a high-fallutin’ term meaning a tightening of the blood vessels in the outer extremities. It’s a neat little trick the body performs on itself, essentially saying, “hey fingers and toes (hands and feet, you’re next, followed by arms and legs, if necessary), you guys don’t require as much oxygen to survive as the heart and brain, so just chill for a bit until we get through this.” Voila! Oxygen-rich blood is diverted to the more critical organs. 

Finally, if one dives deep enough, something called “blood shift” occurs. Here, blood plasma and water pass through cellular walls, essentially filling the chest cavity with blood to offset compression by water pressure. Since the process reverses as pressure decreases on ascent, divers sometimes resurface with blood streaming from their nose or mouth as a result, but it sure beats having a torso flattened like a soda can under a car tire.

Respiratory System

Swim down just 33 feet, and you’ve already doubled the pressure on your body. At 100 feet, pressure triples, and at 300 feet, lungs are squeezed to the size of softballs. In 1996, studies showed that diver Francisco Ferreras-Rodriquez experienced chest compression from 50 inches on the surface down to 20 inches at 436 feet deep. 

But pressure is not just the enemy. When it comes to buoyancy, it can be an ally. A lungful of air wants to make a body float, but once you reach a depth of about 20 feet, the air has been compressed to a point that the body attains neutral buoyancy: It “floats” at that level. Continue deeper and a body enters the range of negative buoyancy and starts to sink, which is why divers at that point can simply place their arms at their sides like a skydiver and start to effortlessly descend. 

The Brain

The brain is an oxygen hog, so peripheral vasoconstriction kicks in to shunt oxygen from the extremities to the all-important grey matter. However, sometimes it’s not enough, and a loss of motor control can occur. And that’s a bad thing. In fact, on competitive dives, upon resurfacing, judges are on the lookout for any sign of motor-control loss.

The brain also plays a major role in terms of the psychology of a diver. Obviously, there’s the fear factor to contend with. Freediving is inherently dangerous, and one has to be able to face those fears and keep their wits about them.

The brain must be able to discern between a true need to breath and just an urge, or something known as a “pressure contraction” in which the diaphragm sends a message to the brain saying, hey, you’re not breathing! “You know it’s coming,” says Krack, “so you tell yourself to give it five more seconds, wait for it to subside. But it’s a big battle. That’s where a lot of people turn, they’re not psychologically prepared.”

Skull, Sinuses and Ears

Whereas soft tissue wilts under pressure, harder substances such as bone hold up pretty well, and those sinus and ear cavities in your head can come in handy as air storage tanks. On descent, a properly trained diver can transfer some 85 ounces or more of compressed and oxygen-rich air from the lungs to the mouth and into the sinuses and ears where it will remain until the process is reversed on ascent. 


Where your dive will determine where light disappears on a dive. For example, in the Cayman Islands (where Krack runs his annual Deja Blue event), 250-foot visibility is common. However, “as you descend into the water column,” says Krack, “the light spectrum starts to disappear. The color spectrum ROYGBV disappears in that order, with red turning flat brownish/green at about 15 feet and the blue and violet part of the color spectrum remaining at depth, giving it a dull effect.” In other regions, such as the Pacific Northwest, it will be pitch black by around 100 to 130 feet due to plankton. Of course, darkness can amplify your ‘fear of the unknown’ of what’s down there.

So, what’s the maximum depth we can withstand without catastrophic failure? That’s still an unknown: The current men’s world-record free dive is 253 meters (831 feet) by Herbert Nitsch.

“If we were hitting any state of physiological limits, like sprinting,” says Krack, “we’d be measuring records in tenths or hundredths of meters, but we still measure in whole meters. Sixty years ago, it was said to be impossible to go more than 50 meters. Then we broke 60 and then 70. When we broke 100, they said it was impossible to go further. Today, they don’t even bother saying what is possible and impossible anymore.”

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