Unraveling the Mammalian Dive Reflex (Part II)

Peripheral Vasoconstriction and Blood Shift

Diving deep initiates a range of cardiovascular responses in the freediver, a consequence of these athletes having trained their bodies to budget their oxygen supply when responding to increasing pressure.

One of these responses is peripheral vasoconstriction, a key adaptation in the mammalian diving reflex (MDR) that functions as an oxygen-conserving reflex whilst diving. As a freediver or marine mammal descends deeper and deeper the effects of this reflex becomes more pronounced, the blood vessels in the extremities slowly constricting and shutting off blood supply to the hands, feet, and eventually the arms and legs. At the deepest part of a very deep dive the only parts of the anatomy that have blood flowing through them are the brain and vital organs. It is at this point that bradycardia reaches a peak, the heart rate of some elite freedivers measuring only eight or nine beats per minute. Associated with the blood shift is a dramatic increase in arterial blood pressure, in many cases exceeding 280/200mmHg with occasional systolic peaks higher than 300mmHg (normal blood pressure: 120/80mmHg), this hypertensive response reflecting the overall extreme peripheral vasoconstriction.

With training, the most important adaptation associated with the Mammalian Diving Reflex is the blood shift outlined above. The efficiency of the shift determines how effectively the freediver tolerates elevated concentrations of metabolic products such as carbon dioxide and cerebral and cardiac resistance to hypoxemia. These two factors are ultimately dependent upon enhanced oxygen storage (conservation) and a large blood volume with associated high levels of hemoglobin and myoglobin. One way to increase blood volume and increase hemoglobin concentration terrestrially is by performing endurance exercise, but this is only part of the story, since, during a dive, there is a contraction of the spleen and concomitant increase in hematocrit, a mechanism that synchronously increases hemoglobin content.

To attain this point, much of the blood is progressively rerouted to those areas that cannot sustain their metabolism without oxygen, namely the brain, central nervous system and the heart. Restricting the circulation of blood to a tissue or an organ results in a steady decline of the oxygen available for support of oxidative metabolic processes and subsequent dependence upon whatever anaerobic resources are available. Eventually, a depression of metabolism takes place. This lowering of the rate at which the many complex chemical processes can occur, thus conserving metabolic energy, is, in fact, a central feature of the adaptation to asphyxia and is exactly the situation that the freediver strives to achieve. Another mechanism that occurs during descent is a blood shift away from the skeletal muscles, which temporarily can provide the energy needed for continued exercise by anaerobic metabolism. Elite freedivers have trained their bodies to increase the efficiency of this oxygen-conserving mechanism by regularly exposing themselves to high carbon dioxide levels by employing training techniques such as apnea walking. Although the reflex controls that are responsible for this shift of blood to the brain and the heart are only partially understood, it is a mechanism that can be changed by correct training and is one the keys to maximizing human aquatic potential.

Overall, the intense cardiovascular response to freediving in elite athletes resembles response patterns of diving mammals such as Emperor Penguins and Weddell seals, although these aquatic-based divers do not experience the same circulatory disruption as their terrestrial based counterparts.

In terms of mammalian competition, the Emperor penguin is definitely one of the elite, since it has the ability to attain depths greater than 500m and stay submerged for as long as twelve minutes. In ‘shallower’ dives it may remain submerged for periods in excess of twenty minutes. However, even these performances are overshadowed by the Weddell Seal which can descend to 700m and has recorded submergence times of eight-two minutes.

If you are wondering why Homo Sapiens cannot match these aquatic athletes it is probably due to the fact that the occurrence of arrhythmias and large increases in blood pressure observed in terrestrial-based freedivers reflect species differences and less than perfect adaptations by humans. However, that is not to say that we are closing in on our species’ freediving potential, and there is no plausible physiological reason to suggest that Homo Sapiens can’t become more like Homo Aquaticus in terms of performance. While we may not reach the depths of the Weddell Seals, a 200m No-limits dive is probably less than five years away. How long it will take to reach 300m is open to debate, but it is a depth that is definitely within the realms of physiological possibility.??



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