Study demonstrates how whales protect their brains while swimming

WASHINGTON: According to a new study, unique blood arteries in whale brains may protect whales from dangerous blood pulses caused by swimming.

There are various theories concerning the precise role of the retia mirabilia, or "beautiful net," networks of blood vessels that cradle a whale's brain and spine, but UBC zoologists believe they've broken the code, with computer modelling confirming their predictions.

The findings of the study were published in the journal Science. When horses gallop, they experience 'pulses' in their blood, where blood pressures inside the body rise and fall with each step.

Dr Margo Lillie and her colleagues propose for the first time in a recent study that the same phenomena happen in marine mammals that swim with dorso-ventral motions, i.e. whales. And they may have discovered why whales avoid long-term brain damage for this.

In all animals, average blood pressure is higher in arteries, which carry blood out from the heart, than in veins. According to Dr Lillie, a research associate emerita in the UBC department of zoology, this differential in pressure drives blood flow throughout the body, including via the brain.

Locomotion, on the other hand, may aggressively transport blood, creating pressure spikes or 'pulses' to the brain. The pressure differential between blood entering and exiting the brain during these pulses might cause harm.

According to Dr Lillie, the long-term damage of this type can lead to dementia in humans. However, whereas horses deal with pulses by breathing in and out, whales hold their breath when diving or swimming. "So, if cetaceans can't use their respiratory system to reduce pressure pulses, they must have discovered another solution," Dr Lillie explains.

Dr Lillie and colleagues hypothesised that the retia employ a 'pulse-transfer' mechanism to ensure that there is no change in blood pressure in the cetacean's brain during movement, in addition to the average difference.

Essentially, rather than damping the blood pulses, the retia shift the pulse from the arterial blood entering the brain to the venous blood exiting, maintaining the same 'amplitude' or intensity of pulse and so avoiding any variation in pressure in the brain itself.

The researchers gathered biomechanic information from 11 cetacean species, such as fluking frequency, and fed them into a computer model.

"Our hypothesis that swimming generates internal pressure pulses is novel, and our model supports our prediction that locomotion-generated pressure pulses can be synchronised by a pulse transfer mechanism that reduces the pulsatility of the resulting flow by up to 97%," says senior author Dr Robert Shadwick, emeritus professor of zoology at UBC.

The model could potentially be used to ask questions about other animals and what's happening with their blood pressure pulses when they move, including humans, says Dr Shadwick. And while the researchers say the hypothesis still needs to be tested directly by measuring blood pressures and flow in the brain of swimming cetaceans, this is currently not ethically and technically possible, as it would involve putting a probe in a live whale.

"As interesting as they are, they're essentially inaccessible," he says. "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."

"Understanding how the thorax responds to deep water pressures and how the lungs impact vascular pressures would be an essential next step," says co-author Dr Wayne Vogl, professor of cellular and physiological sciences at UBC. "Of course, direct measures of blood pressure and flow in the brain would be beneficial, but they are not yet technically achievable."