Most people picture diving with a tank on the back, a regulator in the mouth and a schedule of decompression stops on the way up. Freediving removes all of that. You take one breath at the surface, go down, and come back up on the same breath. Since you never breathe compressed gas, there is nothing to decompress from. The equipment is a mask, a pair of long fins and a wetsuit.
The simplicity is deceptive. A freediver's body goes through a sequence of physical and physiological changes that we understand reasonably well, and most of them are not specific to humans. Seals, sea lions, sperm whales and beaked whales rely on the same mechanisms, in a far more developed form. This post walks through what happens on a dive, in order, and then looks at what the marine mammals do differently.
Pressure
Underwater, pressure increases by one bar for every 10 meters of depth. At the surface the body is under one bar of atmospheric pressure; at 10 meters it is under two, at 30 meters four. By about 250 meters, almost no sunlight remains. At the bottom of the Mariana Trench, close to 11 kilometers down, the pressure is more than a thousand bar. James Cameron, who went there in a submersible in 2012, described the seafloor as "desolate, lunar-like".1
A freediver works with a tiny slice of that range, but the numbers are still surprising. The record in the "no limit" discipline, where the diver rides a weighted sled down and a lift bag back up, is 214 meters, set by Herbert Nitsch in 2007. That discipline has since been abandoned by most competitors as too dangerous. The disciplines that remain, where the diver goes down and comes back by muscle power alone, have records just under 140 meters. For a body that evolved on land, that raises an obvious question: what is actually limiting the depth?
The diving reflex
The first change begins before the dive, when the face goes into cold water. Thermoreceptors in the skin of the face respond to the sudden temperature change and trigger what physiologists call the diving reflex, a set of responses whose net effect is to reduce oxygen consumption.2 The heart rate drops, in trained divers often well below 50 beats per minute. At the same time, blood vessels in the limbs constrict, which pushes blood away from the muscles of the arms and legs and toward the brain, heart and lungs.
What makes this interesting is that it runs against the normal response to exercise. Kicking down against positive buoyancy is real work, and on land that work would raise the heart rate. In the water, the reflex wins, and the heart slows while the legs are working. None of this requires training or intent. It is inherited, and it is present in every human, which is why cold-water immersion is used as a test of it in the lab.
Boyle's law and the blood shift
As the diver descends, the air-filled spaces of the body feel the pressure first: the ears and sinuses, which have to be equalized, and the lungs. Boyle's law says that at constant temperature the product of pressure and volume is constant, so the volume of air in the lungs halves by 10 meters and is down to a quarter by 30 meters.
That arithmetic produced a prediction that held for decades. The lungs have a residual volume, the amount of air left after a full exhalation, below which the airways and alveoli should start to collapse. For a typical adult, the air in a full breath is compressed to the residual volume somewhere between 30 and 40 meters. In the 1960s, physiologists expected that going deeper would crush the chest.
Jacques Mayol and Enzo Maiorca, following the adage "they did not know it was impossible, so they did it", went deeper anyway, and the body turned out to have a way around the problem. As the lungs compress, the blood that peripheral vasoconstriction has already displaced from the limbs moves into the vessels of the chest and the pulmonary capillaries. Blood is incompressible, so it fills the volume the air has given up and keeps the chest wall and the lungs from collapsing. This is the blood shift. It is the reason the 40-meter limit was wrong, and it is one of the few cases where a sport overturned a textbook prediction in physiology.
Neutral buoyancy and the free fall
A full breath before the dive keeps the diver afloat at shallow depth (it is Archimedes' principle that brings you back to the surface, or, in the more learned form every high-school student once learned by heart: "Any body immersed in a fluid is subjected to a vertical upward force equal in magnitude to the weight of the fluid displaced"). As the air compresses, the lungs displace less water and the upward force shrinks. At some depth, usually between 15 and 30 meters depending on the diver's build, the wetsuit and the amount of lead on the belt, the diver becomes neutrally buoyant, and below that point negatively buoyant. From there the diver stops kicking and falls.
The free fall is the part of the dive most freedivers describe when asked why they do it. It is also the most efficient part. The muscles are at rest, so oxygen consumption is at its lowest, and the only thing left to manage is equalization and the mind.
Why relaxation is a safety matter
At depth, the usual instinct in an extreme sport, to push against the difficulty, is the wrong one. Muscular tension in the chest increases the pressure gradient across the pulmonary capillaries and can push plasma into the alveoli. The result is an immersion pulmonary edema, which shows up as coughing and blood-tinged sputum after the dive and is one of the recognized injuries of deep freediving. The training for depth is therefore largely training in staying relaxed while the chest is being compressed, and divers learn early that mental tension and muscular tension are hard to separate.
What marine mammals do differently
Everything above also applies to marine mammals, with the numbers scaled up. I am an occasional freediver, and Vancouver is a good place for it. You can dive from the cliffs along Howe Sound into water that looks empty from the surface and turns out to be full: thousands of sea stars on the rock, sea urchins, the occasional octopus, and a lot of small, strange life you would not expect to exist. And seals and sea lions, which can scare you. I took the video below near Lions Bay. The sea lion came straight at me and turned away at the last moment, which is apparently a normal way for them to inspect a stranger.
A sea lion checking me out, Lions Bay, British Columbia.
These animals routinely dive past 300 meters. Their heart rate, around 100 beats per minute at the surface, can fall below 10 during a dive. It is the same reflex we have, with a much larger range.
The deep records belong to the toothed whales. Sperm whales regularly dive beyond 1,000 meters. Cuvier's beaked whale (Ziphius cavirostris) holds the record for any mammal, with a tagged dive to 2,992 meters and another lasting 222 minutes.3
Their lungs are not the explanation. Relative to body mass, the lungs of deep-diving whales are if anything small, and they collapse early in the dive, which has the benefit of keeping nitrogen out of the blood. The oxygen store that matters is in the muscle, bound to myoglobin.
Myoglobin with a surface charge
Myoglobin is the protein that stores oxygen in muscle, the muscle counterpart of hemoglobin in blood. Deep-diving mammals carry up to ten times more of it per gram of muscle than we do, enough that their muscle is nearly black. That raises a biochemical problem. Proteins at very high concentration tend to aggregate, and aggregated myoglobin does not bind oxygen.
In 2013, Michael Berenbrink's group at the University of Liverpool compared myoglobin sequences from about a hundred mammal species, from the pig to the water shrew, the beaver, the seal and the sperm whale.4 The divers share a feature the others lack: a strongly positive net surface charge on the protein. Molecules that all carry the same charge repel each other, which allows myoglobin to be packed at concentrations that would otherwise cause it to clump. Because the charge can be read from the sequence, the authors could also place the trait on the mammalian tree and estimate when the ancestors of modern cetaceans acquired it, roughly 50 million years ago.
A high myoglobin store is combined with a large blood volume, a sharp reduction in metabolic rate during the dive and a high tolerance to carbon dioxide and lactic acid in the muscle. Each of these exists in humans in a weaker form.
The same physiology, at a different scale
That is the point I find most interesting about freediving. The diving reflex that slows a freediver's heart in cold water is the one that slows a seal's. The blood shift that protects a human chest at 40 meters is the mechanism that protects a sperm whale's at 2,000. The difference between us and them is not in the mechanisms but in how far evolution has pushed each one. Freediving is, in that sense, a way of finding out how much of that machinery we still have.
1. Infiniment profond, France Culture podcast, with freediver Guillaume Néry and deep-sea ecologist Sophie Arnaud-Haond (in French). James Cameron quote: National Geographic, March 2012.
2. Lindholm P, Lundgren CEG. The physiology and pathophysiology of human breath-hold diving. J Appl Physiol 106, 284–292 (2009). doi:10.1152/japplphysiol.90991.2008
3. Schorr GS, Falcone EA, Moretti DJ, Andrews RD. First long-term behavioral records from Cuvier's beaked whales (Ziphius cavirostris) reveal record-breaking dives. PLOS ONE 9, e92633 (2014). doi:10.1371/journal.pone.0092633. The 222-minute dive: Quick NJ et al. J Exp Biol 223, jeb222109 (2020). doi:10.1242/jeb.222109
4. Mirceta S, Signore AV, Burns JM, Cossins AR, Campbell KL, Berenbrink M. Evolution of mammalian diving capacity traced by myoglobin net surface charge. Science 340, 1234192 (2013). doi:10.1126/science.1234192