The question from class was, “what is the mammalian dive reflex and can we do it in lab?”

The latter portion of the question was easy. Of course we can do it and did, as you can see from the data above that we collected in the lab.  As for what the dive reflex is, it is something present in about all mammals, which yes includes humans.  The dive reflex (a.k.a. diving response) is a physiological response to diving into cold water (not to be confused with the Seinfeld episode dealing with cold swimming pool water and male physiology).  The reflex generally consists of a set of physiological responses that are activated when the face more specifically is cooled (such as by the water during a scuba dive, hence the name).  It is hypothesized that the diving response is a reflexive mechanism enabling the body to tolerate low levels of oxygen.

What the diving reflex  typically looks like is as follows; 1) bradycardia, which is a slowing of the heart rate.  In marine mammals, the diving reflex can reduce their heart rate by 80-85%.  In humans, this reduction is closer to 40% at its best;  2) peripheral vasoconstriction, which is the narrowing of blood vessels to reduce blood flow by muscle contraction in the blood vessel’s wall.  This causes reduced blood flow to the limbs ensuring that oxygen sensitive organs like the brain and heart continue to receive needed oxygen;  3) a blood shift, which occurs during deep scuba dives allowing blood plasma and water to pass through organs and circulatory walls to the chest cavity to protect the organs from the increase in external pressure.  The lungs gradually fill up with blood plasma, which is then reabsorbed when pressure drops.

Typically, the colder the water, the faster the dive response will occur.  Temperatures above 21°C (70°F) will not normally elicit a response.  The slowing of the heart rate occurs fairly quickly upon facial contact with cold water (see figure above; the heart rate of this subject went from 89 beats per minute to 39 bpm within 10 seconds).  This decrease in heart rate would help to lower oxygen consumption, since the heart muscle is working at a lower intensity.  This response appears to be mediated by the trigeminal nerves (the 5th cranial nerve) that transmit information to the brain, which then innervates the vagus nerve (10th cranial nerve) causing bradycardia and peripheral vasoconstriction.

The reduced blood flow to the limbs and blood shift tend to occur a bit more gradually.  These responses are thought to be preventive in nature, because it begins before the level of oxygen becomes critically low.  In addition, the large amount of blood that accumulates in the vessels of the lungs acts as a protective measure, because fluids - compared to tissue and bones - cannot be compressed.  This blood accumulation prevents the lungs from collapsing under the high pressure of a deep-water environment.  Vasoconstriction shunts blood away from the arms and legs, and the amount of blood we have available is concentrated in a "miniature" circulatory system including the lungs, heart and brain (in the subject above their blood pressure when from 111/59 to 145/96 as the immediate vasoconstriction response begins).

There are other actions associated with the diving reflex.  In infants, the windpipe by the vocal chords spontaneously closes to prevent water from entering the lungs.  This reflex is initiated as soon as there is contact with water (this reflex disappears at about 6 months of age; see the Nirvana ‘Nevermind’ album cover).  In adults, following a number of dives, the spleen contracts and releases a large quantity of red blood cells to the circulatory system.  The release of more red blood cells allows more oxygen to be stored in the blood.