How can a human hold their breath underwater for over 11 minutes and dive to depths of over 200m unaided?
Free diving is classified as the sport of individuals descending underwater while holding their breath. There are various competitive and non-competitive classifications of this sport, and national competitions are held across the world every year. The current record for static apnea (individuals just holding their breath while submerged, normally in a pool) is 11 minutes 35 seconds for a man (Stephane Mifsud) and 8 minutes 23 seconds for a woman (Natalia Molchanova). But how do they do this?
The technical term for holding your breath is apnea; the body will actually make physiological adaptations to the lack of oxygen while underwater. These are caused by the mammalian diving reflex and involve lowering the heart rate and restricting blood flow to areas of the body, among other adaptations. Individuals can also actually develop their ability to hold their breath for longer periods of time through various forms of training.
One form of training is to perform an ‘apnea walk’. This is where an individual holds their breath at rest for a minute, and then walks as far as they can without needing to take a breath. This activity can help to prepare the body for the anaerobic conditions it will experience in free diving. Also, prior to free diving, individuals will sometimes hyperventilate to lower carbon dioxide levels in their blood, which will ultimately slightly delay signals travelling to the brain that warn of asphyxiation.
First aid trained buddy – Due to the complications that can occur with free diving, every sensible free diver has a first aid trained buddy ready at the surface. They can help them if they get into trouble.
A snorkel is often used by the diver to aid release of air. The mask is crucial so the diver can see which way they are swimming and the surface when they are ascending.
Weights help the diver descend, and in various free diving disciplines (such as variable weight apnea), weights are used to speed up the process.
Flippers (fins) Sometimes used, and sometimes not. They are used in the disciplines of free diving when distances are key, such as the dynamic apnea with fins discipline.
A good wetsuit will help the individual retain heat in the colder water of the deep, and also smoothes the body surface of the diver to reduce drag during descent.
Mammalian diving reflex
There are four primary adaptations that the human body makes during free diving. The first is a drop in heart rate, which ultimately slows body functions and causes the body to use less oxygen. Alongside this, vasoconstriction occurs and the spleen releases extra red blood cells – the latter ensures oxygen levels rise slightly and the former ensures oxygen is delivered to the vital organs as opposed to the limbs. Large muscles constrict to aid this process. Also, dependent on the depth, blood plasma will fill blood vessels in the lungs to reduce volume, which will stop damage occurring when the body is exposed to pressure created by diving to depths greater than 30m. Dolphins and seals display these same traits and archaeologists have seen evidence of individuals using free diving to source food and resources from as early as the 5th Century BCE.