This reflex optimizes mammals' respiration to stay underwater for a long time. It's exhibited strongly in aquatic mammals (seals, otters, dolphins, etc.), but exists in a weaker version in other mammals, humans included. Diving birds, such as penguins, have a similar diving reflex. Every animal's diving reflex is triggered specifically by cold water contacting the face -- water that's warmer than 70°F won't cause the reflex, and neither will submersion of body parts other than the face. Also, the reflex is always exhibited more dramatically, and thus can grant longer survival, in young people and animals.

Upon initiation of the reflex, three changes happen to the body, in this order:

  1. Bradycardia is the first response to submersion. Immediately upon facial contact with cold water, the human heart slows down ten to twenty-five percent. In the seal the changes are even more dramatic, going from about 125 beats per minute to as low as 10 on an extended dive. Slowing the heart rate lessens the its need for bloodstream oxygen, leaving more to be used by other organs.

  2. Next, peripheral vasoconstriction sets in. When under high pressure induced by deep diving, capillaries in the extremities start closing off, stopping blood circulation to those areas. Note that vasoconstriction usually applies to arterioles, but in this case is completely an effect of the capillaries. Toes and fingers close off first, then hands and feet, and ultimately arms and legs stop allowing blood circulation, leaving more blood for use by the heart and brain. Human musculature accounts for only 12% of the body's total oxygen storage, and our muscles tend to cramp up during this phase. Aquatic mammals have as much as 25 to 30% of their oxygen storage in muscle, and thus they can keep working long after capillary blood supply is stopped.

  3. Finally, and most interesting, is the blood shift that occurs only during very deep dives. When this happens, organ and circulatory walls allow plasma/water to pass freely throughout the thoractic cavity, so its pressure stays constant and the organs aren't crushed. In this stage, the lungs' alveoli fill up with blood plasma, which is reabsorbed when the animal leaves the pressurized environment. This stage of the diving reflex does not occur in humans1.

You and a friend can take your own diving reflexes for a test drive easily, with a largeish pan, some ice water, and a stopwatch. First, have the friend get your pulse rate by holding his hand on your wrist for fifteen seconds, counting the number of beats, and multiplying by four. Then, fill the pan with ice water, and submerge your face in it for fifteen or twenty seconds. Afterwards, have your friend take your pulse again, and note how much slower it is than the initial reading -- almost counterintuitive, since you'd expect your heart rate to go up from the shock of cold water.

I'm somewhat doubtful about even posting this last bit, because I don't have any medical documentation to back it up. Maybe there have been studies done, but I couldn't find any, only a few anecdotal case studies. If you take this advice, you do so at your own risk! At any rate, the idea is to slow the damage done during a heart attack by triggering the victim's diving reflex. Wet towels, ice, cold packs, and so forth are placed on the victim's face until an ambulance arrives, hopefully slowing down the victim's oxygen need for that time. This would seem to be a correct notion, as the diving reflex is very powerful, and can completely knock out the high heart rate (tachycardia) caused by heavy exercise.


1. Or maybe it does. Hexter tells me "According to some of the best freedivers stage three can happen in humans if they have trained a lot, and go deep enough."
Props out to j3z_ for a couple of factual corrections to this writeup!

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