If you look at parts of the circulatory system of whales and dolphins, you might think you’re looking at a Jackson Pollock painting, not blood vessels. These cetaceans have particularly dense, complex networks of blood vessels associated primarily with the brain and spine, but scientists didn’t know why. A new analysis shows that the nets protect cetacean brains from the pulses of blood pressure the animals endure while diving deep in the ocean, researchers report Sept. 23. Science.
Whales and dolphins “have gone through these really amazing vascular adaptations to support their brains,” says Ashley Blawas, a marine scientist at the Duke University Marine Laboratory in Beaufort, NC, who was not involved in the research.
Networks of blood vessels called retia mirabilia, meaning “wonderful nets,” exist in some animals other than cetaceans, including giraffes and horses. But webs are not found in other aquatic vertebrates that move differently from whales, such as seals. Thus, scientists had suspected that cetacean retia mirabilia played a role in controlling increases in blood pressure.
When whales and dolphins dive, they move their tails up and down in a wave-like manner, which creates a surge in blood pressure. Land animals that experience similar surges, such as galloping horses, are able to release some of this pressure by exhaling. But some cetaceans hold their breath to dive for long periods of time (SN: 9/23/20). Without a way to relieve this pressure, these bursts could tear blood vessels and damage other organs, including the brain.
In the new study, biomechanics researcher Margo Lillie of the University of British Columbia in Vancouver and her colleagues used data on the morphology of 11 cetacean species to create a computational model that can simulate the animals’ retia mirabilia. He revealed that the arteries and veins in this tangle of blood vessels are very close together and may even sometimes come together. As a result, the retia mirabilia could equalize differences in blood pressure created by diving, perhaps by redistributing blood pulses from arteries to veins and vice versa. In this way, the networks are relieved of, or at least attenuate, the massive blood pressure surges that might otherwise reach and damage the brain.
Networks “equalize the [blood flow] in a way that you never lose that blood that’s in the vein and doesn’t collapse on its own, and you don’t have that arterial blood going too quickly to the brain,” says marine biologist Tiffany Keenan of the University of North Carolina Wilmington who was not involved in the study. “It’s really nice to know what we’ve always wondered but no one has been able to show.”
But studying cetaceans is difficult because of their protected status and limited access to specimens, which usually come from stranded animals, the researchers say. For this reason, a limitation of the new study is that the researchers had to input data from different species to build 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 like a quilt,” says Andreas Fahlman, a marine scientist at the Oceanogràfic Foundation in Valencia. Spain, who was not involved in the study.
As a result, the model may be missing important aspects that may be specific to other species, which have unique anatomies and even move differently, with some staying closer to the surface or others diving deeper. Taking a closer look at the circulatory system of whales and dolphins, perhaps using non-invasive techniques such as sensors that can measure blood flow and pressure, can confirm that the computational model reflects real-life dynamics.