Simulated diving and heart rate

When air-breathing animals dive, they must rely for oxygen entirely on the stores they have “on-board” at the beginning of the dive, loading up with as much oxygen as possible before submerging and making it last as long as possible.
Champion divers such as the Weddell seal may dive for as long as 80 minutes, though most dives are considerably shorter than that.
To perform such feats of diving, diving animals rely on anatomical features and physiological responses that increase oxygen storage while reducing nonessential use of oxygen during the dive.
The main features of this “dive response”, first described by Irving and Scholander in the 1940’s, include:
1. Peripheral vasoconstriction: the constriction of blood vessels and thus blood flow to non-essential organs (like the gut and many muscles), which saves the oxygen in the blood for more important organs (like the brain)
2. Apnea: holding your breath (kind of an obvious one!), and
3. Bradycardia – a slowing of the heart rate. Because it is supplying fewer organs with blood, the heart can beat more slowly and still supply adequate blood pressure to the brain, the most metabolically sensitive organ. A slower heart rate has the additional benefit in that the heart itself then requires less oxygen.

While it is not surprising that excellent divers like seals have these responses, what about man?
After all, we hardly spend hours at a time under water (except with SCUBA tanks, of course).
In 2001, free diving champion Scott Campbell set a world record for static breath holding (not underwater) of 6 minutes 45 seconds. And he wasn’t even exercising! (After all, a diving seal has to swim at least part of the time).

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I tested for the diving effect by placing my face in cold water and measured the change in heart rate. The diving response is thought to be a response of the body to reduce oxygen use by the body and conserve oxygen for use by the nervous system while under water. We tested the hypothesis that humans would exhibit this response and verified a strong diving response is present. I also tested to see if the diving response could be used to reduce heart rate that had been elevated by vigorous exercise. Finally I cut off circulation to my arms to test whether a reduction in the total body area to be perfused would alter the diving response. In both of these experiments I found that the diving response was the same, significantly reducing the heart rate....

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