Protecting cetacean brains from high blood pressure

In all mammals, blood pressure in the arteries exceeds that in the veins. The difference drives blood flow through the body, but it can also spike to dangerous levels. The rhythmic movements of galloping horses, for instance, produce abdominal pressure swings of 10–16 kPa on each stride. If those pulses are strong enough from particularly vigorous activity, they can damage the brain. But whereas horses can relieve the pressure just by breathing in and out, other large mammals, such as whales and dolphins—whose up and down tail movements likewise put a lot of stress on their bodies—must hold their breath while swimming and diving. Those mammals must have some other way to protect their brains.

Zoologist Margo Lillie and her colleagues from the University of British Columbia recently reasoned that a network of blood vessels known as the retia mirabilia (Latin for “wonderful nets”) could provide a mechanism for the job. The retia snake through the animals’ thorax, up their vertebral column, and into their brain case. Among cetaceans—whales, dolphins, and porpoises—they theorized that the retia could transfer pressure pulses among constituent vessels, thereby largely removing any artery-to-vein blood-pressure differences that could generate pulsing flows above the average levels produced by the heart.

To test that hypothesis, the researchers developed a computational model of the blood’s dynamics, based on the vascular morphology from 11 cetacean species. They found that the pulse-transfer mechanism could protect the cerebral vasculature from up to 97% of the pressure pulses that might occur from the tail’s downstroke. The retia essentially transfer each pulse in arterial blood entering the brain to the venous blood that exits it, keeping the pulse amplitude within the animals’ normal, physiological range. (M. A. Lillie et al., Science 377, 1452, 2022 .)