An extensive network of blood vessels known as rete mirabile (marvellous net) helps protect the brains of whales and dolphins, the stars of our #Scienceofthebeast, when they swim beneath the waves. In this way, they are safeguarded from the pulses of blood pressure generated when diving at great depths.

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When submerging in water at certain depths, humans must be very aware of the differences in pressure so as not to damage our bodies. Other animal species, such as cetaceans, can withstand this extreme pressure when swimming. In addition, because of their locomotion, whales and dolphins make powerful movements with their tails, which change the blood pressure as they move up and down.

When the terrestrial ancestors of modern cetaceans gave up their terrestrial life and returned to the oceans more than 50 million years ago, they underwent a transition with drastic changes. It involved changing the shape and physiology of ancient land mammals to survive the unique challenges of living underwater.

One of the most challenging aspects of this environment is withstanding the extreme pressure at great depths, both externally and internally, while providing a constant supply of oxygenated blood to the brain.

A study published in Science reveals how they do this: there is a hitherto unknown function in the so-called rete mirabile (marvellous network, retia mirabilia in plural) of cetaceans.

We have modelled the circulatory system of cetaceans to predict how retia mirabilia would affect haemodynamics in the brain of a swimming cetacean. Margo Lillie

“Our computer model predicts blood flow and pressures in a swimming cetacean. The behaviour of blood flow in the circulatory system has a lot in common with the flow of current in electronic circuits, and for years scientists have used our understanding of the latter to make computer models of the circulatory system,” explains Margo Lillie, a researcher in the department of zoology at the University of British Columbia in Canada who leads the work.

Unlike the relatively simple set of blood vessels in many terrestrial mammals, cetaceans have a massive ‘vasculature’ located in the thoracic, intravertebral and cranial regions, the function of which was unknown.

“We modelled the cetacean circulatory system to predict how retia mirabilia would affect haemodynamics in the brain of a swimming cetacean. To do this we needed data on this network: how much ‘retial vasculature’ there is in each species, the morphology of the blood vessels and the mechanical properties of their retial arteries,” adds Lillie.

The authors developed these haemodynamic models of the retia mirabilia based on the morphology of 11 cetacean species and found that the large arterial capacity of this type of network, combined with the small extravascular capacity in the skull and vertebral canal, could protect the delicate cerebral vasculature from the differences in blood pressure experienced by these aquatic mammals.

“The brain needs a lot of blood and this makes it particularly susceptible to damage from pulsatile flow. By maintaining a high blood flow, any pulsatility entering the brain can reach deep into the smallest vessels, which are easily damaged,” the scientist continues.

A system found only in these aquatic vertebrates
The cetacean’s swimming movement was incorporated as a pressure pulse in the abdomen, which could be transmitted through the arteries and veins to the brain. The frequency of this pulse covers the range in which cetaceans swim.

In this way, they hold their breath at depth, while powerful tail movements allow them to propel themselves. In addition, they interrupt the supply by triggering pulses of blood pressure (pulsatility) in the arterial and venous vessels that rise and fall with each advance.

This ‘pulse transfer’ mechanism ensures that blood pressure remains stable in the brain without dampening the pressure pulses themselves. The scientists also explain that they are not present in other aquatic vertebrates that have different modes of locomotion.

The life of diving mammals is different from that of mammals living on land. From this study we can observe how they have adapted to infer the problems they have had to face during evolution.

On the other hand, it also helps to understand physiology, how different species are constructed and how we function in the aquatic environment.

“From both a technical and ethical point of view, it is extremely difficult to study physiology in any cetacean species. This includes both the smallest and the largest cetaceans. We need to measure pressure and blood flow in the brains of swimming cetaceans, but that is not technically possible now. In the future it may be,” Lillie concludes.