Scientists have long known that land mammals, such as horses, experience “pulses” in their blood while galloping, as blood pressure inside the body fluctuates on every stride. Now, a team of researchers from the University of British Columbia (UBC) has found that the same phenomenon occurs also in marine mammals that swim with dorso-ventral movements, such as whales, but that the mechanism that protects them from brain damage during these pulses is different from that employed by terrestrial animals.
In all mammals, blood pressure is higher in the arteries exiting the heart than in veins. This difference in pressure drives the blood flow through the body and the brain. However, locomotion often causes spikes in pressure, or “pulses” to the brain, which can cause significant damage. In order to avoid such problems, animals evolved different ways of compensating for such differences in pressure between the blood entering and exiting the brain. Horses, for instance, deal with these pulses by breathing in and out. But how do whales manage to avoid brain damage?
“If cetaceans can’t use their respiratory system to moderate pressure pulses, they must have found another way to deal with the problem,” said study lead author Margo Lillie, a research associate emerita in Zoology at UBC.
By collecting biochemical parameters from 11 whale species, and using a computer model to analyze the data, the scientists found that special blood vessels in whale brains and spines – known as “retia mirabilia,” or “wonderful net”- may protect them from the damage caused by pulses occurring while they swim.
According to the researchers, the retia uses a “pulse-transfer” mechanism to ensure there is no difference in blood pressure in the whales’ brain during movement. Thus, rather than dampening the pulses occurring in the blood, the retia transfers them from the arterial blood entering the brain to the venous blood exiting it, while keeping the same amplitude or strength of the pulse in order to avoid differences in pressure in the brain itself.
“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,” explained study senior author Robert Shadwick, a professor emeritus of Zoology at UBC.
“Understanding how the thorax responds to water pressures at depth and how lungs influence vascular pressures would be an important next step,” added study co-author Dr. Wayne Vogl, a professor of Cellular and Physiological Sciences at UBC. “Of course, direct measurements of blood pressure and flow in the brain would be invaluable, but not technically possible at this time.”
The study is published in the journal Science.