Why don鈥檛 marine mammals like whales get 鈥渢he bends鈥�?
Stephen Johnson
Eugene, Oregon, US
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Diving mammals don鈥檛 get the bends, or decompression sickness, because they don鈥檛 breathe compressed gases 鈥� or any gases for that matter 鈥� when they dive.
Decompression sickness is caused by the absorption of soluble gases in body tissues during diving. Because the pressure under water is greater than at the surface, these tissues strive to reach equilibrium with the outside water pressure. Humans can breathe while underwater through the use of compressed gas tanks. This gives humans an enormous exposure to nitrogen and other soluble gases. When a diver returns to the surface, these gases can form dangerous bubbles as they leave the body. Because of this, human dive times and depths are severely limited, while diving mammals can reach depths of hundreds, sometimes thousands, of metres and stay there for long periods.
Diving mammals do all of their dives on a single breath of air. In fact, many exhale before diving. In addition, when sea mammals dive, their lungs are compressed, which drastically reduces the ability of the blood to pick up air. Instead, diving mammals store extra oxygen in their bloodstream and muscles. As a percentage of body weight, diving mammals have about three times as much blood as humans do. They also have a of the oxygen-transporting protein haemoglobin in their blood: a whale鈥檚 blood is around 60 per cent haemoglobin, versus around 30 per cent in humans. Their muscles can store significant amounts of oxygen too, as they have eight times as much myoglobin, a protein that does this, as human muscles.
The bends, or , was first discovered in the 19th century when hard rock mines reached depths that required the air supply to be significantly compressed. These early miners, or caissons, often developed symptoms that caused them to hunch over on their return to the surface, hence the term the bends. The illness is treated with oxygen at high pressure.
Mike Follows
Sutton Coldfield, West Midlands, UK
Actually, marine mammals can get decompression sickness, but their physiology largely protects them. To answer this question fully, it is perhaps best to start with why humans are so susceptible.
The pressure in a column of fluid increases as you move down through it. There is 1 atmosphere of pressure at sea level because of the weight of air pressing down from above. Water pressure increases by 1 atmosphere for every 10 metres of descent. For example, at a depth of 30 metres, the pressure is four times atmospheric pressure and scuba divers breathe air at this pressure. Even if air at atmospheric pressure could be delivered to divers, the surrounding water pressure would prevent their lungs expanding and they would suffocate.
Air dissolves in the bloodstream, which is how gases are moved around the body so that oxygen can be delivered to cells and carbon dioxide can be removed. Nitrogen in the air is also absorbed by the body tissues, as it is five times as soluble in fat as in water. As a scuba diver ascends, the pressure decreases and nitrogen comes out of solution. Bubbles of nitrogen gas can form if the ascent is too rapid. These can cause injury. Divers ascend slowly so their blood can surrender the nitrogen gas to the lungs, from where it can be safely exhaled.
When I started diving, it was assumed that decompression sickness was impossible for breath-hold divers, which includes freediving humans (who dive without scuba gear) and marine mammals that dive on a single breath taken at the surface. Except for a reduction in oxygen and an increase in carbon dioxide, it was thought that the exhaled air would be identical to what was inhaled. It was believed there would be no reason for bubble formation because air wasn鈥檛 being inhaled at higher pressure during the dive.
This changed in 2002 with the mass stranding of 14 whales in the Canary Islands. They all showed symptoms of decompression sickness. The stranding was close to a naval military exercise where acoustic sonar was being used. It is thought that the whales made a rapid ascent to escape the sound. Subsequent research has shown that deep-diving whales collapse half of their lungs to reduce the reservoir of nitrogen that can be absorbed into their tissues.
The absorption of nitrogen increases with pressure, dive duration and the supply of nitrogen. The Cuvier鈥檚 beaked whale (Ziphius cavirostris) can dive to almost 3 kilometres, exposing it to 300 times the pressure experienced at the surface. A diving whale experiences 100 times the pressure that scuba divers do when they dive to a depth of 30 metres. However, while whales take a single breath before they dive, scuba divers breathe compressed air in order to stay submerged for around an hour, which increases the volume of air passing through their lungs (and their exposure to nitrogen) by a factor of about 400. This makes humans more susceptible than whales to decompression sickness even before any evolutionary adaptations.
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