'Wonderful nets' of blood vessels protect the brains of dolphins and whales during dives.

 These networks, known as retia mirabilia in cetaceans, have long been a mystery.

Sperm whales (shown) dive deep underwater in search of food. The animals' brains are not damaged because of their dense, intricate networks of blood vessels.  WILDESTANIMAL/MOMENT/GETTY IMAGES AND MORE
Sperm whales (shown) dive deep underwater in search of food. The animals' brains are not damaged because of their dense, intricate networks of blood vessels.  WILDESTANIMAL/MOMENT/GETTY IMAGES AND MORE

If you look at parts of a whale or dolphin's circulatory system, you might think you're looking at a Jackson Pollock painting rather than blood vessels. Scientists had no idea why these cetaceans had such dense, complex networks of blood vessels primarily associated with the brain and spine. Researchers report in the September 23 issue of Science that the networks protect the brains of cetaceans from the blood pressure pulses that the animals experience while diving deep in the ocean.

Whales and dolphins "have gone through these really amazing vascular adaptations to support their brain," says Ashley Blawas, a marine scientist at Duke University Marine Lab in Beaufort, North Carolina, who was not involved in the study.

The blood vessel networks, known as retia mirabilia, or "wonderful nets," are found in animals other than cetaceans, such as giraffes and horses. However, the networks are not found in other aquatic vertebrates, such as seals, which move differently than whales. Scientists had suspected that the retia mirabilia of cetaceans plays a role in controlling blood pressure surges.

When whales and dolphins dive, their tails move up and down in an undulating motion, causing blood pressure to rise and fall. Exhaling allows land animals that experience similar surges, such as galloping horses, to relieve some of the pressure. 

However, some cetaceans can dive for extended periods of time by holding their breath (SN: 9/23/20). These blasts could tear blood vessels and harm other organs, including the brain, if there is no way to relieve the pressure.

Margo Lillie of the University of British Columbia in Vancouver and colleagues used data on the morphology of 11 cetacean species to create a computational model that can simulate the animals' retia mirabilia in the new study. 

It revealed that the arteries and veins in this tangle of blood vessels are extremely close to one another and may even be joined at times. As a result, the retia mirabilia may be able to compensate for differences in blood pressure caused by diving, possibly by redistributing blood pulses from arteries to veins and vice versa. The networks thus eliminate or at least weaken, massive blood pressure surges that would otherwise reach and devastate the brain.

The networks "equalize the [blood flow] in such a way that you never lose that blood that's in the vein, it doesn't collapse down on itself, and you don't have that shooting arterial blood going really fast into the brain," says Tiffany Keenan of the University of North Carolina Wilmington, who was not involved in the study. "It's really cool to know what we've always wondered but no one could show us."

Despite this, researchers say that studying cetaceans is difficult due to their protected status and limited access to samples, which are usually from stranded animals. As a result, one limitation of the new study is that the researchers had to use data from multiple species to create their model.

"They take a little bit from here and a little bit from there, mixing a dolphin with a beluga whale with a beaked whale — it's kind of like a quilt," Andreas Fahlman, a marine scientist at the Oceanogràfic Foundation in Valencia, Spain, who was not involved in the study, says.

As a result, the model may be missing important details that are unique to other species, which have different anatomies and even move differently, with some staying closer to the surface and others diving deeper. A closer examination of the circulatory systems of whales and dolphins, possibly using nonintrusive techniques such as sensors that can measure blood flow and pressure, may help confirm that the computational model accurately reflects real-life dynamics.


M.A. Lillie and colleagues Retia mirabilis: Protecting the cetacean brain from blood pressure pulses caused by locomotion. Science, Vol. 377, p. 1452, September 23, 2022. doi: 10.1126/science.abn3315


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