Dolphins regulate every heartbeat while diving

Almost a hundred years ago, the Norwegian biologist Per Scholander described this challenge “Diving response” changes that are activated in the body of marine mammals when they are under water. The existence of this physiological reaction to diving – nicknamed Scholander “the main switch of life” – is considered to be a clue to the secrets of dolphins, seals and whales. they support their long snorkeling and are taken almost as a dogma. However, researchers from the Oceanogràfic Foundation in Valencia question its validity and offer an alternative.

In the new work with dolphins, which is now published in the journal Scientific reports Researchers deny that at the beginning of the dive a specific body reaction to the dive occurs, a physiological response as such.

Dolphins have an unusual ability to control heart rate at will.
On the contrary, they claim that Dolphins have an unusual ability to control heart rate at will just as people control breathing or blinking. This may be a major evolutionary adaptation in favor of diving.

“Our data shows that dolphins voluntarily change the heart rate and the amount of blood pumped at each stroke,” says Andreas Falleman, lead author of a study conducted in collaboration with the Dolphin Quest organization in Hawaii and the Health Research Institute. Faith in Valencia.

“This mechanism will allow them to regulate blood flow during the dive, what they plan to do from the very beginning, and make changes on the fly if necessary. Scalable adaptation that helps them balance the need for oxygen with their availability, ”said Fallman.

There is no automatic response to diving
The “dive response” described by Scholander consists of a series of physiological changes to make better use of available oxygen . One such change is a decrease in heart rate, bradycardia, during a dive.

Researchers at the Oceanographic Foundation confirm that this change occurs, but do not consider it as a “characteristic that manifests itself in evolution specifically for diving,” says Falman, but an extreme manifestation of a phenomenon called sinus respiratory arrhythmia that occurs in all vertebrates and seeks to synchronize heartbeat with breathing If the body receives more blood into the lungs, only when they are full of oxygen, will the exchange of gaseous respiration be more effective. ”

Thus, the arrhythmia of the respiratory sinuses makes the heart beat more often when it is inspired, and slower between inspirations . In humans and other mammals, this also happens, but in dolphins, arrhythmia is much more pronounced in their breathing: they breathe once or twice a minute – we do it a dozen times – and the time between breaths, much longer, can be very variable.

“The response to immersion does not exist as such, but rather is an extension of the respiratory sinus arrhythmia,” says Falman.
The new work suggests that the slowing of the heart rate associated with the “dive response” is actually the slowest time between inspiration that can be expected from dolphins who associate the heartbeat with breathing. “The reaction to diving as such does not exist, but is an extension of the respiratory sinus arrhythmia, especially pronounced in dolphins,” said Fahlman.

Researchers at the Oceanographic Research Foundation came to this conclusion when, studying the “diving reaction” from dolphins, they tried to subtract the effect of arrhythmia from the respiratory sinuses. By removing from their measurements changes in cardiac function, which, in their opinion, may be associated with sinus arrhythmia, they found that there was almost no change to measure ,

“Our results show that the changes associated with immersion, after subtracting the effect of sinus arrhythmia, are much less pronounced than expected,” they write. Scientific reports , “When we look at sinus arrhythmia, dolphins do not observe a noticeable decrease in heart rate — it remains to be seen whether these results apply equally to other marine mammals.”

True evolutionary adaptation specific to diving
So, is this phenomenon, which occurs in all vertebrates – arrhythmias of the respiratory sinuses, enough for explaining mammals’ adaptation to aquatic life? The authors of this work believe that there is something more.

Your data suggests that, in addition, Dolphins can voluntarily regulate heart rate and the amount of blood that pumps the heart with each beat in order to very accurately control the amount of whole blood that reaches the lungs.

Irrigation of the lungs is key to controlling the behavior of gases in tissues, a process on which the life of divers depends. There is oxygen and nitrogen in the air. When immersed, oxygen must pass from the lungs into the blood, but not nitrogen, because when it rises to the surface, as the pressure changes, dangerous bubbles form. Nitrogen is formed in the tissues.

There is evidence that porpoises, also cetaceans,
The idea that marine mammals voluntarily control the amount of blood reaching the lungs and what time is not new. There is evidence that porpoises, also cetaceans, do.

“Our results show that dolphins further reduce heart rate at the beginning of a dive when it is longer,” say Oceanogràfic researchers. “This is a result similar to that obtained for porpoises. They confirm the hypothesis that cardiovascular changes during diving are a complex physiological response, including many factors, such as voluntary preliminary correction, 39, exercise, stress and fear, that is, it would be a very plastic physiological characteristic instead automatic response or reflex. ”

Controversial proposal
The authors know that their proposal is controversial but this is the one that best fits — they did it — with new information about how the heart of the dolphin works, which now contributes to its study.

The study of the cardiovascular system of dolphins is not easy and not only in the natural environment. Even in aquariums it is necessary to train animals so that they themselves decide to cooperate voluntarily with cardiologists. Taking data by force would make them worse, which would invalidate the measure.

For the work that is published, ultrasound of the chest 11 dolphins to examine your heart before, during and after voluntary apnea on the surface and at rest. As the researchers write in scientific reports, “these are the first voluntary and non-invasive measurements” of parameters related to the function of the heart in dolphins.

In addition, we had data provided by Stefan Midler , a veterinary cardiologist who was able to measure heart rate and blood volume sent to the heart at each pulse, using echocardiography. “Stefan is the only person in the world who can do this with dolphins, because it’s very difficult,” said Falman.

In order for animals to learn to quietly hold their breath, when their guardian speaks about this, the specialist in underwater ultrasound has spent several months of training, but the wait was worth it to get information too. an attitude.

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