It’s been awhile since my last dolphin trivia blog entry, so I was getting all geared up to start off the month with a new batch of assorted dolphin facts.   But as I was culling through some notes I’ve been squirreling away over the past few weeks, a singular topic kinda emerged from the random bits and it just drew me in...

How Do Dolphins Stay Underwater So Long?

Dolphins are, of course, not fish - they have to surface in order to breath just like we do.   But where a dolphin’s lungs are comparable in size to those of other mammals, their respiratory system is anatomically unique.   Their lungs contain a lot more alveoli (air cells) and are comprised of two layers of capillaries instead of one.   Also, the membrane around the lungs & inner chest walls of dolphins is thick & elastic.   These anatomical differences provide for a dramatically more efficient exchange of gas — with each breath, a dolphin renews 80-90% of its lung capacity, whereas in humans, this number is normally 20% or less.

Given that dolphins can dive for as much as 15 minutes at a time and at depths of as much as 200m (about 600 feet), they have to make each breath last much longer than we can.   But even with hyper-efficient lungs, dolphins can’t take in enough oxygen to sustain long periods underwater.   Yet we know that they don’t run out of air and drown, so, how do they do it?

When diving, dolphins employ a kind of selective circulation where bloodflow to the skin, digestive system, some organs, & extremities slows (or even ceases), leaving only the heart, brain, & tail muscles working.   And due to the atmospheric pressure exerted on the dolphin during a deep dive, its lungs & rib cage collapse at around 100m down.

This collapse forces air out of the lungs into nasal passages & air sacs and squeezes even more blood out of its heart.   This kind of abrupt surge of blood rushing through a human’s cartoid artery would likely burst all the blood vessels in the brain and result in instant death.   But in dolphins, the blood from the aorta is forced into a sponge-like mesh of capillaries that slows the blood and reduces the pressure before it reaches the brain (imagine a sponge over the end of your garden hose for an idea of how this works).

Dolphins don’t suffer "bends" or other decompression sickness that human divers do even though they dive deeper and more frequent.   In fact, they suffer no ill effects of breathing immediately after diving a considerable depth.   Primarily, this is because, while humans breathe highly compressed air as they descend, dolphins hold their breath during dives.

Credit Where It’s Due
Bits & pieces of this info were gleaned from an array of websites but I really hit the jackpot with two sites in particular:   Michael Harwood’s The Amazing Bodies of Dolphins essay and The [Dolphin] Respiratory System chapter from the Thinkquest Team’s "Dolphins: The Oracles of the Sea".   Check these sites out for more interesting info!
 

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