The threat from decompression sickness, or the bends, is very real for scuba divers but dolphins and whales have evolved to avoid this problem
It’s a pressure thing
When under water for extended periods of time, carbon dioxide and nitrogen accumulate in the blood. The deeper an animal dives, the more pressure its lungs are under. The problems only begin to occur when it’s time to ascend.
Returning to the surface reduces the pressure on the lungs. If this pressure is relieved too quickly, nitrogen forms bubbles in the bloodstream. These bubbles can cause blood vessel blockage or collapse and trigger dangerous chemical reactions in the blood.
Whales and dolphins escape this painful fate by collapsing their lungs during ascent. The alveoli, air pockets in the lungs, squeeze shut to prevent drastic pressure changes. This is the same for deep diving whales, like Cuvier’s beaked whale that can reach 2,992 metres (9,800 feet) in a single dive.
The deeper a dolphin dives, the smaller the volume of gas in the lungs becomes. This is not the case for humans, as we can’t control our alveoli at will. SCUBA divers must perform safety stops during an ascent to prevent pressure being reduced too quickly. Dolphins avoid this by controlling the pressure in their lungs at depth.
Dolphins also replace the air in their lungs much more quickly than humans. A human breathing heavily can replace around 65 per-cent of the air in its lungs in a single breath. Dolphins can exchange 95 per-cent of their breath.
A human can exhale 15 litres (4 gallons) of air per breath. A dolphin can pump out 130 litres (34 gallons) per every exhaled breath. This helps maximise the expulsion of carbon dioxide that would otherwise build up in the body at depth.
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Image from flickr.com/bmiphone