Simanaitis Says
On cars, old, new and future; science & technology; vintage airplanes, computer flight simulation of them; Sherlockiana; our English language; travel; and other stuff
THE WHALE’S WONDERFUL NET
IMAGINE THE CHALLENGES CONFRONTING terrestrial ancestors of dolphins and whales: Exponential increases in drag forces when moving through water, higher hydrostatic pressures at depth, and extreme breath-holding requirements to function at these depths. “Yet,” as Terrie M. Williams notes in “Recasting the Whale’s Wonderful Net,” Science, September 23, 2022, “the transitions did occur, resulting in 47 extant cetacean (dolphin and whale) family lineages that radiated throughout the global oceans. How this evolutionary leap was accomplished has been the subject of much speculation. On page 1452 of this issue, Lillie et al. continue this multimillennial investigation on aquatic adaptations in cetaceans, detailing how specialized vascular networks provide protection for their brains during submergence.”
Here are tidbits gleaned from these two Science articles.
What Do Whales Have in Common with Dogs and Giraffes? They’re all mammals, of course, but more specifically each has evolved specialized control of cerebral vasculature.
“Like running terrestrial mammals,” Williams observes, “the locomotor movements of swimming dolphins and whales may act to generate abdominal and thoracic pressures as the abdominal organs (viscera) push against the diaphragm. These pressure pulses are subsequently translated into the vasculature. Running mammals alleviate large pressure fluctuations by exhaling. However, such a strategy is unavailable for breath-holding cetaceans.”
Giraffes exhibit a similar change in cerebral blood pressure when they bend over to drink. They too have evolved specialized vasculature.
The Whale’s Wonderful Net. Williams writes, “A dense plexus of intertwined arteries and veins, the rete mirabile (translated “extraordinary, wonderful net”), is interposed between the aorta exiting the heart and the cerebral circulation in cetaceans. Rather than the relatively simple vascular pathways of many terrestrial mammals, arterial blood of dolphins and whales is dispersed through a series of sieve-like retia found in thoracic, intravertebral, and cranial regions before reaching the brain.”
Williams continues, “Cranial retia associated with the whale’s brain may also act as a filter that prevents harmful nitrogen bubbles from instigating decompression syndromes that lead to stranding. Furthermore, the complex net structure and large surface area characteristic of the rete mirabile may also assist in modulation of cerebral blood pressure, a function proposed previously for resting dolphins, but not fully understood for active divers.”
Methodology. M.A. Lillie and colleagues “modeled the extensive array of blood vessels, or retia mirabilia, found in cetacean brains across 11 species and concluded that this array minimizes blood pressure differentials, thus protecting the brain without reducing the pressure pulses and facilitating fluking locomotion.”
The researchers’ model is an electrical analog of cetacean blood dynamics. They describe, “The model contains four pressure sources implemented with a controlled pressure source. The heart generates a mean pressure (‘DC’), but no cardiac pulse was implemented. Sinusoidally oscillating arterial and venous pressures were introduced independently to represent intravascular fluking-generated pulses.”
Results. The researchers summarize, “We tested this hypothesis using a computational model based on morphology from 11 species and found that the large arterial capacitance in the retia, coupled with the small extravascular capacitance in the cranium and vertebral canal, could protect the cerebral vasculature from 97% of systemic pulsatility.”
Cetaceans Versus Other Aquatic Vertebrates. What’s more, the researchers note, “All animals deal with vascular pulsatility, and cardiorespiratory processes are generally coupled to locomotion to manage internal pressures. But in cetaceans, the combination of locomotion with dorsoventral fluke oscillation while breath-holding has created an extraordinary set of hemodynamic demands that appear to be met by the evolution of two morphologically distinct retia. Our results link the evolution of cetacean retia mirabilia to dorsoventral fluking during breath-hold diving and explain why retia are not present in other aquatic vertebrates that have different modes of locomotion.”
Yes, I always thought whales weren’t just ordinary fish. ds
© Dennis Simanaitis, SimanaitisSays.com, 2022
Simanaitis Says 