Whales Can Swim Without Suffering Brain Damage – This Is Why

By D.C. Demetre •  Updated: 09/23/22 •  5 min read

According to new research, whale brains have special blood vessels that may shield them from swimming-related blood pulses that could harm the brain.

The precise function of the “retia mirabilia,” or “wonderful net,” of blood vessels that surround the brain and spine of a whale is the subject of numerous theories. With the help of computer modelling, zoologists from the University of British Columbia now think they have the answer to the mystery.

When galloping, land mammals like horses experience “pulses” in their blood, where blood pressures inside the body rise and fall with each stride. In a new study, lead author Dr. Margo Lillie and her team make the first suggestion that whales, which swim with dorso-ventral movements, also experience the same phenomenon. And they may have discovered why whales avoid long-term brain damage.

Whale Blood Pressure Control

rete beluga whale arteries

A resin cast of a beluga whale’s spinal canal displaying arteries of the rete.
Credit: Wayne Vogl

The average blood pressure in arteries, or the blood leaving the heart, is higher than in veins in all mammals. According to Dr. Lillie, a research associate emerita in the UBC department of zoology, this difference in pressure controls blood flow throughout the body, including through the brain.

However, moving around can jar the blood, sending ‘pulses’ of pressure to the brain. Damage may result from the pressure differential between blood entering and leaving the brain during these pulses.

According to Dr. Lillie, long-term damage of this nature can cause dementia in people. Whales hold their breath when diving and swimming, in contrast to horses, who regulate their breathing by inhaling and exhaling.

“So if cetaceans can’t use their respiratory system to moderate pressure pulses, they must have found another way to deal with the problem,”

Dr. Lillie said.

Blood Pulse Transfer

In addition to the average difference in blood pressure, Dr. Lillie and colleagues hypothesised that the retia use a “pulse-transfer” mechanism to ensure that there is no difference in blood pressure in the cetacean’s brain during movement. The retia basically transfer the blood’s natural pulses from the arterial blood entering the brain to the venous blood exiting, keeping the pulse’s ‘amplitude’ or strength constant and preventing any variations in brain pressure.

Fluking frequency was one of the biomechanic parameters that the researchers collected from 11 different species of cetaceans and entered into a computer model.

“Our hypothesis that swimming generates internal pressure pulses is new, and our model supports our prediction that locomotion-generated pressure pulses can be synchronized by a pulse transfer mechanism that reduces the pulsatility of resulting flow by up to 97 percent,”

senior author Dr. Robert Shadwick said.

Rete Mirabile

The rete mirabile utilizes countercurrent blood flow within the net (blood flowing in opposite directions) to act as a countercurrent exchanger. It exchanges heat, ions, or gases between vessel walls so that the two bloodstreams within the rete maintain a gradient with respect to temperature, or concentration of gases or solutes.

Exchanges can be incredibly effective with the retia. For instance, in bluefin tuna, the metabolic heat in the venous blood is almost entirely transferred to the arterial blood, maintaining muscle temperature; this heat exchange is close to 99 percent efficient.

Retia mirabilia in the legs and feet of birds with webbed feet transfer heat from the departing (hot) blood in the arteries to the incoming (cold) blood in the veins. As a result of this biological heat exchanger, heat loss is decreased because the internal temperature of the feet is much closer to the outside temperature. The flippers and nasal passages of penguins also contain retia.

The retia mirabile got its name from the Greek physician, surgeon and philosopher Galen of Pergamon (circa 129 – 216 AD), who is considered one of the most accomplished of all medical researchers in antiquity. However, he mistakenly thought that humans also have a rete mirabile in the neck, apparently based on the dissection of sheep and misidentifying the results with the human carotid sinus, and ascribed important properties to it

Essentially Inaccessible

According to Dr. Shadwick, the model may be used to answer questions about how other animals, including humans, move and what happens to their blood pressure pulses. Although the researchers claim that the hypothesis still needs to be directly tested by examining the blood flow and pressure in the brains of swimming cetaceans, doing so would be both unethical and technically impossible at this time because it would require putting a probe inside a live whale.

“As interesting as they are, they’re essentially inaccessible. They are the biggest animals on the planet, possibly ever, and understanding how they manage to survive and live and do what they do is a fascinating piece of basic biology,”

he explained.

“Understanding how the thorax responds to water pressures at depth and how lungs influence vascular pressures would be an important next step. Of course, direct measurements of blood pressure and flow in the brain would be invaluable, but not technically possible at this time,”

said co-author Dr. Wayne Vogl.

References:

  1. M A Lillie, A W Vogl et al. Retia mirabilia: Protecting the cetacean brain from locomotion-generated blood pressure pulses. Science 2022 Sep 23;377(6613):1452-1456.
    doi: 10.1126/science.abn3315
  2. Grant, Mark (2000). Galen on Food and Diet. Routledge
  3. P. J. Ponganis, Diving Physiology of Marine Mammals and Seabirds (Cambridge Univ. Press, 2015)

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