If the Olympics allowed marine mammals to compete, they would dominate free diving competitions in terms of breath holding duration and extreme physiology. Marine mammals also have adaptations that help prevent tissue damage from the lack of oxygen during such dives. For example, Weddell and northern elephant seals store large amounts of oxygen in their blood and muscles, thanks to abundant hemoglobin, which permits them to hold their breath for extended periods of time while still ensuring tissues have enough oxygen. In addition, simply dunking their head underwater or holding their breath, triggers the autonomic nervous system to reduce heart rate and causes blood vessels to constrict. This response to diving related apnea, called the “mammalian dive reflex”, helps animals balance oxygen demand in the body with the ability to supply oxygen to tissues. Interestingly, elite human free divers share this ability to mount a diving reflex in response to apnea.

A new study published this month in the American Journal of Physiology – Regulatory, Integrative and Comparative Physiology, compared the responses to apnea in elite human free divers and northern elephant seals. Even outside water under normal circumstances, elephant seals may pause breathing after exhaling. Researchers collected blood samples following rest-associated apneas in northern elephant seals and during voluntary apneas in elite human free divers. As to be expected, both humans and seals had elevated carbon dioxide levels in their blood after holding their breath for about 6.5 minutes. Despite this similarity in carbon dioxide responses, pH was only reduced in humans. In humans, the drop in pH that happens in response to increases in carbon dioxide typically results in a strong desire to breathe. The ability to prevent changes in pH, along with enhanced buffering, allows seals to continue holding their breath as it avoids activating the desire to breathe. Seals may also be able to prevent oxidative stress by producing high amounts of carbon monoxide from the turnover of hemoglobin. Although this new study was conducted in humans and seals after similar apnea lengths, seals are capable of holding their breath for much longer during dives (up to 90 minutes).   

Interestingly, another study published several years ago in Philosophical Transactions of the Royal Society equipped elite human free divers with a wearable near-infrared spectroscopy device. This allowed researchers to collect continuous physiological measurements during dives of 21-107 meters in depth and 49-249 seconds in duration. When comparing free diving grey seals and elite diving humans, the researchers measured similar patterns of reduced heart rate (i.e. the mammalian dive reflex). In fact, heart rate declined to only 27 beats per minute for one of the divers, which is not as low as that achieved by elephant seals (as low as ~5 beats per minute), but still quite impressive. With respect to brain oxygen levels, the researchers found a striking contrast between humans and seals wherein elite free diving humans had levels low enough to knock someone out. In fact, first author Dr. Chris McKnight remarked in a University of St. Andrews press release, “We measured heart rates as low as 11 beats per minute and blood oxygenation levels, which are normally 98 per cent oxygenated, drop to 25 per cent, which is far beyond the point at 50 per cent at which we expect people to lose consciousness and equivalent to some of the lowest values measured at the top of Mount Everest.”

Seals for the win in breath holding duration and heart rate reduction; humans for the win in tolerating low brain oxygen concentrations!

Sources:

CV Brown, JC McKnight, AR Bain, JC Tremblay, A Patrician, BI McDonald, CL Williams, AG Hindle, LJ Pallin, DP Costa, Z Dujic, DB Macleod, TM Williams, PJ Ponganis, PN Ainslie. Selected and shared hematological responses to apnea in elite human free divers and northern elephant seals (Mirounga angustirostris). American Journal of Physiology – Regulatory, Integrative and Comparative Physiology. 327(1): R46-R53, 2024.

McKnight JC, Mulder E, Ruesch A, Kainerstorfer JM, Wu J, Hakimi N, Balfour S, Bronkhorst M, Horschig JM, Pernett F, Sato K, Hastie GD, Tyack P, Schagatay E. When the human brain goes diving: using near-infrared spectroscopy to measure cerebral and systemic cardiovascular responses to deep, breath-hold diving in elite freedivers. Philosophical Transactions of the Royal Society B. 376: 20200349, 2021.

https://news.st-andrews.ac.uk/archive/divers-beat-seals-during-deep-dives/

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Categories: Comparative Physiology, Extreme Animals, Hibernation and Hypoxia, Nature's Solutions, Ocean Life

Tags: American Journal of Physiology, American Physiological Society, diving, freedive, freediving, scuba, seal, travel