Some whale species can dive hundreds or thousands of meters deep. Yet they suffer no damage to the head: thanks to a special network of blood vessels.
Sperm whales can dive up to a thousand meters deep, gray whales migrate thousands of kilometers through the ocean - just two of the feats that large whales are capable of. The animals are driven by the powerful blow of their tail flukes, which sends strong pressure waves through the whale's body every time. At the same time, the mammals have to hold their breath underwater, which increases the pressure in their bodies even more. Yet their blood vessels and brains suffer no damage, as would be expected in humans. Robert Shadwick of the University of British Columbia and his team describe in "Science" why this does not happen.
Whales have a special network of blood vessels in their skulls called the rete mirabile: a network of fine arteries that branches off from an artery and later rejoins to form an artery. It was discovered as early as the 17th century and is also found in other mammals, for example, on the kidneys. In large whales, however, its significance was unknown for a long time.
Shadwick and co therefore developed a computer model that enabled the research group to simulate the pressure fluctuations in the whale body and its consequences for the vascular system. The model is based on the physiology of eleven different species, ranging from bottlenose dolphins to great whales.
According to the model, the rete mirabile plays a crucial role in equalizing pressure: it keeps blood pressure in the brain constant without the whales having to weaken their tail beats. The network of blood vessels transmits the increase in pressure from the arteries, which carry blood into the vessel, through the mesh until the arteries merge into the veins, which leave the brain. This distributes pressure over a large area, which is why the rete mirabile acts like a protective helmet: it protects the whale brain from pressure fluctuations without changing blood flow in the rest of the body. According to the simulation, the vascular network "swallows" about 90 percent of the pressure increase triggered by the tail beats: enough to dive deep or swim powerfully without causing a brain stroke.
Seals or harbor seals do not have this vascular network in their heads, which is related to their swimming style. They glide with lateral wave movements and thus ensure that new pressure pulses do not keep coursing through the body.