DIVING PHYSIOLOGY

Tavishi

When I was younger, I really enjoyed swimming. In my head, I'd always imagine myself as a seal. Of course, there were significant differences. For one, I did not have nearly enough skill to compare myself to a pinniped. I also would NOT do well diving in the Arctic- I'm a born and bred Arizonan, water below 80 degrees is far too cold for me. I also, was, uh, a human. So yeah, naturally, my body wasn't made to swim. But there are certain advantages physiologically that marine mammals have allowing them to dive. If we go back far enough, mammals first evolved before marine mammals, when this guy, Pakicetus, decided to go back to his roots. (Of course, it was a lot more complex than this, this is quite simplified.)

Different marine mammals evolved from different ancestors- but Pakicetus was the pioneer of the cetaceans, or whales and dolphins. (Which, by the way, are all whales. Odonticets are toothed whales, and mysticets are baleen whales.) Pinnipeds likely evolved from the musteloids, a group of weasely little guys (quite literally so. Weasels, otters, etc.) But because of this, you may ask how do pinnipeds differ from these land mammals in being able to swim so proficiently? For one, to account for pressure, pinnipeds have insane amounts of hemoglobin and myoglobin, allowing them to store oxygen in their bodies much longer, thus allowing for long, long, dives. The average hemoglobin in humans is around 12-18 mg/dL, whereas in elephant seals it is around 20-27 mg/dL, which is an insane amount. Pinnipeds also have a large blood volume and an excessively large spleen. The spleen stores oxygen rich blood during dives, releasing some of it when necessary, acting as somewhat of a storage organ. All three of these characteristics are common among diving mammals.

Pinnipeds also experience physiological changes once they enter water. These diving responses, including bradycardia and peripheral vasoconstriction. Bradycardia is a decrease in heartbeat. Peripheral vasoconstriction (or just vasoconstriction of all non-heart blood vessels) results in reduced blood flow, and thus, reducing oxygen consumption. Peripheral vasoconstriction itself first necessitates bradycardia, which, in tandem, allow pinnipeds to dive much deeper and to go without oxygen. In order to allow for such fast and strong vasoconstriction, the blood vessels are quite heavily innervated. Interestingly enough, pinniped diving responses can be triggered merely by facial stimulation. The placing of a seal's face in water is all that is necessary to initiate bradycardia; for example, the wetting of a seal's nose, or the stimulation of trigeminal/glossopharyngeal nerves can trigger physiological changes occurring during dives.