The Mammalian Reflex

Blue Yard Hub Mammalian Reflex

Long before the technology had been developed to dive with air compressed into tanks man was already busy exploring the depths of the seas. It is also here that Greece has left its mark in diving history. Perhaps the most inspiring and well-known evidence of this feat are the sponge divers of Kalymnos. Sponge diving dates back many centuries but tangible evidence of freediving for sponges exists since the early 1800s. Sponge diving is the oldest known form of the original art of underwater diving. The only equipment that was used in the early days of sponge diving was the ‘skandalopetra’, a heavy rounded stone tied on a rope to the boat that enabled the diver to reach greater depths faster. Using only a knife the diver cut the sponge loose from the bottom and collected them in special nets. In 1869 the ‘skafandro’ was introduced, a rubber suit with surface air being pumped to the diver through a long hose from the boat. This suit enabled the diver to descend deeper and longer. Subsequently the volume of sponges retrieved increased. This innovation in diving, however, did not come without consequence. Statistics suggest that between 1886 and 1910 over 10.000 men from Kalymnos died either whilst diving or as a result of diving. The people at the time simply knew too little about the effects of (prolonged) diving on the human body. That knowledge followed, but only gradually, in the decades that followed. The dive of one man, Raimondo Bucher, initiated a large step in exploring and understanding the relationship between diving and human physiology.

The year is 1949. Raimondo Bucher, lieutenant in the Italian air force, has just accepted a potentially life threatening bet. He shall sail to the center of Lake Capri, take a deep breath and on that one breath descend to a depth of 100 feet (approximately 33 meters). There he will be awaited by a scuba diver who will be the recipient of a small package to be delivered by Bucher; the evidence of a successful freedive. Bucher will then return to the surface. If Bucher succeeds he will be 50.000 lire richer.

Scientists warn Bucher that according to Boyle’s Law the dive can only have a fatal outcome. Boyle’s Law describes how the volume of a gas reduces as pressure increases and vice versa. This is very important with diving as the huge mass of water ‘above’ the diver exerts an increasing pressure on the body as the diver descends deeper. This pressure is especially noticeable in parts of the body that contain air, like our lungs, ears and sinuses. The ‘open’ spaces in our body shrink as we dive deeper. According to Boyle’s Law the pressure at 33 meters deep would cause the lungs to shrink to the point of collapsing. Bucher ignores the warnings, dives, hands over the package to the scuba diver en returns to the surface with a big smile on his face.

Bucher not only won the bet but with this dive also proved all scientists wrong. Boyle’s Law, which had stood for over three centuries, appeared to fall apart underwater. How was this possible? Scientists were baffled and a whole new field of research emerged in search of an explanation. That explanation was found in what we now refer to as the mammalian reflex (also known as the diving reflex).

The mammalian reflex occurs in all mammals and is truly a miracle of Mother Nature. In its natural form (without an exposure suit or weights) the human body has the perfect buoyancy for freediving, also known as breath-hold diving or skin diving. At the surface we float but with minimal effort we can descend to great depths. Many of us, however, are not aware of the physiological changes that occur within our body as we submerge our face in water. The first response is the immediate closure of the airway. We hold our breath. At the same time our body reduces the heart rate. In humans this reduction can be anywhere from 10 to 25%. This slowing of the heart rate reduces our body’s consumption of oxygen. The body also redirects the blood flow by a process called peripheral vasoconstriction. Blood vessels in non-vital parts of our body (such as our fingers, toes, arms and legs) are narrowed allowing less oxygen-rich blood to reach those parts. Instead that same blood is directed to the more vital parts of our body, such as the heart, lungs and brain … all of which require significantly larger amounts of oxygen than other organs. This redirection allows for the vital organs to keep receiving oxygen for prolonged periods of submersion. The redirection also serves a second purpose. The extra blood deviated to the vital organs also strengthens organ walls as it is ‘squeezed’ into the tissues. At greater depths this blood even penetrates organ walls to counteract the external pressure being exerted on the body. This explains why Bucher was able to descend to 33 meters without his lungs collapsing.

The mammalian reflex increases in effect as you dive deeper. The current world record freediving is held by Austrian freediver Herbert Nitsch. On one breath he was able to descend to a depth of 253 meters in 2012. A record, by the way, that he set at the Greek island Santorini. He also holds the world record in the traditional Greek discipline of Skandalopetra at 107 meters.

[Photo credit: Brian Skerry]