Upon submersion, mammalian divers invariably exhibit a characteristic ‘dive reflex’: the heart slows and the peripheral vessels, supplying blood to muscles and non-vital organs, constrict, thereby prioritising blood flow to the brain. This stereotyped response, dubbed ‘the master switch of life’ by Per Scholander following his pioneering work, imposes unusual demands on the major arteries. The slowed heart rate results in greater cardiac filling between beats, meaning the volume pumped per contraction (stroke volume) increases. However, this large volume of blood is driven against a high resistance generated by the peripheral vasoconstriction, thus putting strain on the central vessels.
Seals are particularly adept divers, and as revealed in Scholander's classic work, exhibit a pronounced dive reflex. In a recent paper, Arnoldus Blix and his colleagues explored how the outflow of the seal heart is adapted to their diving lifestyle. In a post-mortem investigation, the Norwegian team investigated both the mechanical properties and the detailed anatomy of the hooded seal aorta.
It had been previously established that the seal aorta exhibits an unusual ballooning, referred to as the ‘aortic bulb’. Blix and his colleagues isolated the bulb and measured pressure as they loaded it with saline. The vessel was extremely elastic; it could comfortably accommodate the estimated diving stroke volume with minimal changes in pressure. During dives, the aorta thereby functions as an elastic reservoir, or ‘windkessel’, that cushions the circulation from dangerous pressure fluctuations.
Given its vital role, Blix and colleagues next investigated the microanatomy of the aortic bulb. Many large arteries are nourished by blood vessels (vasa vasorum), but they are traditionally reputed to remain superficial. However, in the thick aortic bulb of seals, a rich arborisation of blood vessels clearly penetrated the entire wall. These vessels provide the oxygen necessary to maintain the integrity of the elastic wall. It is possible that the surprising extent of this vasculature is unique to the peculiar bulb in seal arteries, but the authors speculate that similar vasa vasorum may have simply been overlooked in the vessels of other animals, including humans.
In an independent article, published in the same journal, Hirofumi Tanaka and colleagues studied arterial stiffness in another group of expert divers: the Ama people of Japan. Ama women free dive in pursuit of pearls and may dive over 100 times per day. To investigate whether this is reflected in arterial adaptations, the team assessed arterial stiffness in pearl divers in comparison to both physically active and inactive non-divers. This involved a suite of techniques, including measuring arterial blood pressure and taking ultrasound images of the carotid artery in resting individuals.
Tanaka and colleagues demonstrated that both divers and physically active non-divers generally had less-stiff arteries than sedentary individuals. However, in several key parameters, for example the profile of the pressure waves in the arteries, the divers clearly stood out. Much like those of seals, the divers’ arteries were better adapted to buffer the pressure changes between heartbeats.
These two studies demonstrate that in marine mammals and humans, the distinct cardiovascular challenges imposed by diving are alleviated by similar mechanisms. The elastic arteries of seals have been sculpted by millions of years of natural selection, whilst in the Japanese divers this adaptation is a direct result of a lifetime of pearl collecting.
