How "wonderful net" of cerebral vasculature protects cetacean brain from swimming-generated pulses

An extensive network of blood vessels known as the retia mirabilia (translated as "wonderful nets") helps to protect the brains of deep-diving whales and dolphins from the pulses of blood pressure generated as they swim beneath the waves, according to a new study. The findings reveal a previously unknown function for the retia mirabilia in cetaceans and explain why they are not present in other aquatic vertebrates that have different modes of locomotion. More than 50 million years ago, the terrestrial ancestors of modern cetaceans renounced their land-locked lives and returned to the oceans. This revolutionary transition required drastic changes to the terrestrial mammalian form and physiology to survive the unique challenges of living underwater. One of the most challenging aspects of this environment is enduring the extreme pressure animals experience at depth, both externally and internally, while providing a constant supply of oxygenated blood to the brain. While holding their breath at depth, the powerful tail movements that allow these creatures to propel their large bodies through the water can disrupt this supply by causing blood pressure pulses (pulsatility) in arterial and venous vessels that rise and fall with each stroke. However, how cetaceans have adapted to protect their brains from potential damage due to increased pulsatility generated by fluking locomotion is poorly understood. Margo Lillie and colleagues explored whether the extensive array of blood vessels, or retia mirabilia, play a role in this ability. Unlike the relatively simple vasculature of many terrestrial mammals, cetaceans have massive vascular located in the thoracic, intravertebral, and cranial regions, whose function is not yet known. Lillie et al. developed hemodynamic models of the retia mirabilia based on morphology from 11 cetacean species and found that the large arterial capacity of the retia, combined with the small extravascular capacity in the cranium and vertebral canal, could protect the delicate cerebral vasculature from blood pressure differentials. This "pulse-transfer" mechanism ensures that blood pressure remains steady in the brain without dampening the pressure pulses themselves. In a related Perspective, Terrie Williams discusses the study in greater detail.