Diving is not for the faint-hearted; the deeper you go the more crushing the pressure bearing down on you becomes. However, intrepid fin whales will regularly dive down 100–200 m in search of their next meal, and consequently face pressures of 1000–2000 kPa. But these immense pressures present very real physiological challenges; for a start, how do they maintain their transmural blood pressure? ‘Transmural blood pressure is the pressure inside the artery minus the pressure outside and is produced by the heart when it contracts’, explains Margo Lillie, from the University of British Columbia, Canada. ‘Whatever the pressure is inside the thorax, where the heart is, the heart just bumps it up – it creates an overpressure. So, [in a terrestrial mammal] if the pressure in the thorax is 100 kPa the heart will bump it up to about 113 kPa.’ On land, where pressure outside the arteries is also at equilibrium with atmospheric pressure2548).
Lillie started by inflating different types of arteries with pressurized liquid. ‘With a terrestrial mammal's artery, as you increase the pressure the artery increases in diameter up to a point and then it mostly stops stretching – that point is around the physiological pressure [13 kPa]’, explains Lillie. However, most of the tested arteries stopped increasing in diameter almost immediately, reaching their maximum diameters at very low pressures of just 1–2 kPa. ‘This was entirely unexpected’, recalls Lillie. When she looked at a cross-section of the arteries, Lillie found that they had an unusually thick layer of collagen, the material responsible for stiffening arteries.
Intrigued, Lillie wondered what would happen if she exposed the arteries to negative transmural pressures, and found that they were unusually resistant, withstanding negative pressures of up to −50 kPa. Again, collagen was responsible and Lillie calculated that the collagen would prevent the artery from ever fully collapsing.
Lillie was perplexed. All her data suggested that whales' arteries, with their collagen-induced stiffness, were built for withstanding a range of pressures including negative pressure. Lillie wondered whether her previous assumptions had been wrong. Maybe, during descents, the pressure inside the thorax couldn't equilibrate quickly enough to ambient pressure. As the internal blood pressure is set inside the thorax, Lillie realised that lower-than-ambient pressures in the thorax could create low or even negative pressure in parts of the body equilibrated to the higher, ambient pressure. Lillie explains that while most organs readily pressurize, the air-filled lungs have a tougher time pressurizing. The air needs to be compressed to equilibrate, but with stiff ribs that resist the pressure, something else needs to push down on the air in the lungs. In human divers, blood will move from equilibrated areas into thoracic vessels, filling up space and compressing the air. Many scientists assumed that the same would happen in whales; however, when Lillie and her colleagues modelled it, she found this was not the case: ‘I started to realise, thinking about the volume of blood that has to shift, that what is important is the rate at which a whale is going down and how fast the tissues [blood] can respond, and I decided that they can't respond fast enough.’ So depending on an artery's location inside or outside the thorax, it may experience different transmural pressures. For humans this would be disastrous, but with their extra thick collagen coating, it's no problem for a fin whale's artery!