What is the rete mirabile and what does it do in cetaceans?

The term is Latin and grandiloquent: rete mirabile, the “wonderful net”. It was a 17^th-century Italian anatomist, Marcello Malpighi, who is said to have popularised the expression, applying it to a lacework of tiny interwoven blood vessels. At the time its function was not understood. The rete mirabile still fascinates today: it is one of the most ingenious devices evolution has produced, and in diving cetaceans it reaches an extreme refinement.

A vascular lacework

Imagine a watering hose in which, instead of a single main tube, you had a comb of hundreds of small tubes, all parallel and in close contact with one another. That is the architecture of the rete mirabile: an artery divides into a multitude of fine arterioles, interwoven with a multitude of venules arising from a main vein. The structure is so dense that under the microscope the vessels are literally seen touching, separated by a thin wall. This proximity allows direct exchange, of heat, of gas, of pressure, between the two flows, which run in opposite directions.

In cetaceans, particularly well developed retia mirabilia are found at the base of the brain, in the rib cage, and along the vertebral column. The fins (flippers, dorsal fin and tail flukes) contain them as well, where they serve mainly for thermoregulation.

The elegance of thermal countercurrent

The first role of the rete mirabile is thermal. The fins of cetaceans pose a problem: barely protected by insulating blubber, they would otherwise be sieves for heat. In cold water a cetacean would lose hundreds of watts an hour through them. Evolution solved the problem with a mechanism known as countercurrent exchange.

Here is how it works. Warm blood from the heart travels down into the fin through the artery, at a temperature of about 36 °C. As it goes, it passes the cold blood returning to the heart through the vein, at a temperature close to that of the surrounding water, say 5 °C. Thanks to the proximity of the two flows within the rete mirabile, heat passes by conduction from the warm arterial blood to the cold venous blood. Arterial blood therefore reaches the tip of the fin already cooled, and loses little heat to the water. Venous blood reaches the trunk already rewarmed, and does not chill the body core.

The energy balance is surprisingly favourable: less than five per cent of the heat produced by core metabolism is lost through the fins. It is exactly the same principle as in the legs of emperor penguins or the feet of herons: these animals can stand on ice for hours without exhausting themselves.

The radiator also works in reverse

In tropical waters the thermal problem is reversed. A sperm whale or a common bottlenose dolphin hunting at full speed produces a great deal of metabolic heat. How is it to be shed? The rete mirabile can then work in radiator mode: the peripheral vessels dilate, which “short-circuits” the countercurrent exchange and lets warm blood go straight to the skin to cool. The fins then become efficient heat sinks. This is why, in warm water, cetaceans are sometimes seen slapping their flippers at the surface: they are using their fins as cooling vanes.

Protecting the brain during a dive

The second role of the rete mirabile is haemodynamic. When a sperm whale dives to a thousand metres in under ten minutes, its body undergoes considerable changes in pressure. The rib cage is compressed, the organs deform, blood is massively redistributed. Without protection these fluctuations would be transmitted straight to the brain, an organ particularly sensitive to pressure spikes.

The cervical rete mirabile acts as a hydraulic damper. Its multitude of small vessels, branched and interwoven, works like a system of interconnected dampers: it smooths the fluctuations, blunts the peaks, steadies the flow. The blood that finally reaches the brain is therefore at a remarkably constant pressure, despite the vascular storm shaking the rest of the body. It is the biological equivalent of a capacitor in an electrical circuit: it filters out the transients.

A reservoir of oxygenated blood

A third role, sometimes less well known: the thoracic rete mirabile stores a significant volume of blood. During a dive this blood can be actively redistributed towards the vital organs by contraction of the vessel walls. It is one component of the diving response, which also includes bradycardia and peripheral vasoconstriction. The sperm whale thus carries, in the form of vascular reservoirs, a substantial part of the oxygen it will consume during the sixty to ninety minutes its dive can last.

An ancient evolutionary innovation

The rete mirabile is not an invention peculiar to cetaceans. It is found, in various forms, in an extremely wide range of animals: tunas and mackerel sharks (where it keeps their lateral muscles at a temperature above that of the surrounding water, which is what makes these fish “warm-blooded”), giraffes (a cerebral rete that damps the pressure changes caused by head movement), sheep, cats, some diving seabirds. But nowhere is the system as developed as in deep-diving cetaceans. Natural selection has clearly staked a great deal on this device.

The expansion of the rete mirabile in the ancestors of cetaceans was probably one of the key innovations that allowed them to exploit the deep-sea ecological niches left vacant by the extinction of the large marine reptiles at the end of the Cretaceous. See our page on the evolution of cetaceans.

Sources

• Vogl, A.W. & Fisher, H.D. (1981). “The internal carotid artery does not directly supply the brain in the Monodontidae”. Journal of Morphology, 170(3).
• Pabst, D.A., Rommel, S.A. & McLellan, W.A. (1999). “The functional morphology of marine mammals”. In Reynolds & Rommel (eds.), Biology of Marine Mammals.
• Costidis, A. & Rommel, S.A. (2012). “Vascularization of air sinuses and fat bodies in the head of the bottlenose dolphin”. Journal of Morphology, 273(11).
• Ponganis, P.J. (2015). Diving Physiology of Marine Mammals and Seabirds. Cambridge University Press.
• Carey, F.G. (1973). “Fishes with warm bodies”. Scientific American, 228.

---

Further reading