When a whale pumps its tail up and down to swim, a wave of increased blood pressure moves from the tail to the head – but a network of vessels redirects the animal’s blood to protect the brain
An intricate network of blood vessels in whales’ brains may protect them from damage caused by powerful pulses of blood pressure generated during swimming.
Whales get around by moving their tail up and down in the water which, in combination with breath-holding, sends a wave of pressure from the tail to the head. This would typically cause injury to the brain, but whales manage to evade such damage.
“The squeezing actions create pressure pulses which can travel in the blood through veins or arteries,” says Robert Shadwick at the University of British Columbia in Canada. “Unlike a running mammal, [whales] cannot alleviate the locomotion-induced pulses by exhaling air.”
Researchers first discovered the webs of blood vessels known as retia mirabilia – Latin for “wonderful nets” – in deep-diving whales in the 1600s, but until now, their function was poorly understood. To investigate, Shadwick’s team created a computer model that simulates pressure changes in a whale’s body while swimming. They based their model on the physical characteristics of 11 cetaceans, from bottlenose dolphins to baleen whales.
Their analysis revealed that retia mirabilia help maintain steady blood pressure in the brain without dampening the intensity of the pulses or the power of the tail movement.
The web of blood vessels reroutes the rush of pressure from arteries entering the brain to veins leaving the brain. This protects the brains of cetaceans from pressure swings without changing how blood moves in the rest of the body.
“The simulations showed that the retia [mirabilia] could eliminate over 90 per cent of the harmful effect of locomotion-induced pulses by this transfer mechanism,” says Shadwick. “The outcome of the simulations was certainly surprising.”
The work also helps explain why other marine mammals such as seals and sea lions lack retia mirabilia. Because these animals swim via side-to-side undulations, they avoid sending a hazardous pressure pulse to the brain, reducing the need for retia mirabilia.
Journal reference: Science, DOI: 10.1126/science.abn3315
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