Freediving, in its simplest form, is being in and under water while holding one’s breath, without the use of a breathing apparatus. It relies on the diver’s ability to hold their breath, diving as deep as they can until resurfacing again.
An average human can hold their breath for 30 to 45 seconds on land. However, freedivers have the capacity to go up to 9 minutes while diving and up to 24 minutes when lying completely still under water.
Between the 1970s and 1980s, scientists explored the theory that humans spent a large portion of our evolutionary phase in a semi-aquatic existence known as the aquatic ape theory.
Some characteristics of the human body functioning that support this theory are that we have a layer of subcutaneous fat to keep us warm, our relative hairlessness compared to similar land mammals, and the way our fingertips wrinkle due to prolonged exposure to water which enables us to grip things easier.
There is one other promising factor of the aquatic ape theory that has changed the freediving world forever, the mammalian diving reflex. It is seen in aquatic animals such as dolphins and otters, and in humans to a lesser extent.
When you freedive, here are 5 changes that occur in your body as a result of the mammalian diving reflex.
1. Heart Rate Slows Down
Bradycardia refers to a slower than normal heart rate. When your body first hits the water, your heart automatically lowers its beating rate by 8% to 10% to conserve oxygen.
Many professional freedivers have taught themselves to summon this phenomenon and have trained themselves to lower their heart rate by 40% to 50%. This comes with a tremendous amount of discipline and control.
2. Capillaries Constrict
As you dive deeper into the ocean, the water becomes progressively colder. Peripheral vasoconstriction occurs to allow the body to retain heat for longer as protection against hypothermia. It causes the capillaries in your limbs to constrict, thus reducing blood flow by muscle contraction in the blood vessels’ walls. It also allows more oxygen to be delivered to important and oxygen-sensitive organs like your heart and brain.
Your spleen, which acts as a blood filter that controls the amount of red blood cells and blood storage in the body, also contracts and squeezes supplementary oxygenated blood cells into the circulatory system. This buys you almost 30 seconds more under water without breathing.
3. Air Spaces Shrink
Another way your body adapts to the pressure of the ocean is that your lungs shrink, sometimes up to half their size because their spongy texture is unable to cope with the surrounding pressure. Due to the mammalian diving reflex, the vessels in your lungs become engorged with almost 1.8 litres of blood to counteract the pressure.
However, as much as the mammalian diving reflex aids us, there can be unpleasant ramifications if the boundaries are pushed too far.
4. Temporary Colour Blindness
Some freedivers have reported that they experience temporary colour blindness when their bodies are running out of oxygen.
Professional freediver Martin Stepanek can hold his breath for over 8 minutes and was the first person to dive over 121 metres on a single breath. “The parts of your eyes that monitor colours are very demanding on oxygen. When the level of oxygen drops, even a little bit, you’re going to lose the colour in your vision,” Stepanek said.
Apart from losing colour vision, freedivers also tend to experience the loss of peripheral vision.
“It’s almost like tunnel vision, and that’s kind of when I would be pushing my luck,” Stepanek said. “Usually, that is a very bad sign. That would be followed by the tunnel closing completely and that would be unconsciousness,” he added.
When his peripheral vision starts closing in, it is a cue for him to stop going further.
5. A Drunken State
The deeper a diver descends, the higher the concentration of nitrogen and oxygen in the lungs. This can result in nitrogen narcosis. Narcosis has been referred to the “rapture of the deep” and many divers compare it to a feeling of pleasant drunkenness. Divers sometimes use the “Martini Rule” to estimate the effects of narcosis during a dive. In theory, it has the ability to affect all divers that descend beyond 20 metres.
Nitrogen narcosis could result in either pleasant or unpleasant thoughts and emotions. It could bring out euphoria where everything feels sublime and amusing, or it could cause paranoia and negative thoughts. Either way, it prevents you from thinking clearly, hinders your multi-tasking ability and causes short-term memory loss.
However, experienced freedivers can eventually learn to cope with its effects.
Experts in freedive physiology agree that a buddy system is essential to freediving safely. When possible, it is advisable to dive under the direct supervision of a dive partner. It might be the best way to prevent a fatal blackout.
Freediving can be one of the safest recreational activities if you take proper training courses, follow rules and stay within your limits.