Paul J. Ponganis and Gerald L. Kooyman of the Center for Marine Biotechnology and Biomedicine at Scripps Institution of Oceanography provide the following answer.
 Image: COURTESY OF SCOTT HILL/ NOAA/NMML A SPERM WHALE can dive down more than 2,000 meters and can stay submerged for up to an hour. |
Some sea creatures exploit great depths. The biggest physiological challenges in adapting to pressure are probably faced by those animals that must routinely travel from the surface to great depth. Two such animals are the sperm whale and the bottlenose whale. From the days of whaling, these animals have been recognized as exceptional divers, with reports of dives lasting as long as two hours after they were harpooned. Today, with the use of sonar tracking and attached time-depth recorders, dives as deep as 6,000 feet (more than a mile below the surface of the ocean) have been measured. Routine dive depths are usually in the 1,500- to 3,000-foot range, and dives can last between 20 minutes and an hour.
Diving to depth can result in mechanical distortion and tissue compression, especially in gas-filled spaces in the body. Such spaces include the middle ear cavity, air sinuses in the head, and the lungs. Development of even small pressure differentials between an air cavity and its surrounding tissue can result in tissue distortion and disruption¿a condition in human divers known as "the squeeze." In some species of cetaceans, the middle ear cavity is lined with an extensive venous plexus, which is postulated to become engorged at depth and thus reduce or obliterate the air space and prevent development of the squeeze. Cetaceans also have large Eustachian tubes communicating with the tympanic cavity of the ear and the large pterygoid sinuses of the head. These air sinuses of the head have an extensive vasculature, which is thought to function in a manner similar to that of the middle ear and facilitate equilibration of air pressure within these spaces. Lastly, most marine mammals lack frontal cranial sinuses like those present in terrestrial mammals.
Another organ susceptible to compression damage is the lung. In deep-diving whales and seals, the peripheral airways are reinforced, and it is postulated that this allows the lungs to collapse during travel to depth. Such collapse has been observed radiographically and confirmed with blood nitrogen analyses in the deep-diving Weddell seal.
Collapse of the lungs forces air away from the alveoli, where gas exchange between the lungs and blood occurs. This blunting of gas exchange is important in the deep diver because it prevents the absorption of nitrogen into the blood and the subsequent development of high blood nitrogen levels. High blood nitrogen pressures can exert a narcotic effect (so-called nitrogen narcosis) on the diver. It may also lead to nitrogen bubble formation during ascent¿a phenomenon known as decompression sickness or "the bends." Collapse of the lungs in the deep diver avoids these two problems.
Loss of gas exchange at depth has another important implication: the lungs of the deep diver cannot serve as a source of oxygen during the dive. Instead deep-diving whales and seals rely on large oxygen stores in their blood and muscle. Several adaptations enable this. First, these animals have mass specific blood volumes that are three to four times those found in terrestrial mammals (i.e., 200 to 250 milliliters of blood per kilogram body mass, in contrast to a human value of 70 milliliters blood per kilogram). Second, the concentration of hemoglobin (the oxygen-transport protein in blood) is also elevated to a level about twice that found in humans. Third, the concentration of myoglobin, the oxygen storage protein in muscle, is extremely elevated in these animals, measuring about 10 times that in human muscle.
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Add CommentI understand that free divers (those not breathing compressed air from scuba tanks) don't get the bends. Apparently that's because no new air is added to the lungs during the dive, preventing super-saturation of the blood. I guess the air that was pressed out of their lungs into their blood just goes back into their lungs as they surface (rather than into nerve tissue/joints, etc). So, it seems like seals wouldn't get the bends either, even if they did not collapse the lungs. What am I missing? Thanks. (Could lung collapse also be a way to reduce bouyancy for the dive?)
Reply | Report Abuse | Link to thisThe answer to the centuries-old mystery of why whales beach themselves can stated in only one word:
Reply | Report Abuse | Link to thisBAROSINUSITIS
Barosinusitis in diving sea mammals is a pressure-related injury in the sinuses and air sacs located inside their heads.
It is well-known that rapid and excessive changes in the surrounding water pressure can cause physical trauma in all diving mammals, including man.
Severe oscillations in pressure (seaquakes) are common above the epicenter of certain shallow-focused undersea earthquakes, especially those located in the rift valley of mid-ocean ridges.
Navy sonar, oil industry airguns, and underwater explosives cause the same barosinusitis injury as caused by undersea earthquakes.
In toothed whales, the sinuses and air sacs serve as acoustic mirrors reflecting sound inside their heads in such a fashion to enable their echo-navigation system to function properly. An injury in this critical part of their biosonar system naturally disrupts echo-navigation, causing the animals to lose their normally excellent sense of direction. It also prevents them from diving and feeding themselves.
Lost at sea, the flow of the surface currents directs the injured whales/dolphins downstream from the point of injury. Control over the swim path of the injured sea mammals happens because water is 700 times denser than air. The increased density induces a powerful drag (resistance) to swimming in any direction except downstream with the flow. Thus, surface currents quickly point lost whales and dolphins headfirst into the path of least resistance or least drag.
If the pod does not recover, surface currents are likely to deposit them on a sandy beach simply because current just happens to be the same energy that carries each grain of sand to build the beach in the first place. In general, whales/dolphins are directed to beaches that are building sand; not to beaches that are eroding.
Unable to navigate or dive and terrified by the pack of starving predators trailing them, the wounded whales/dolphins huddle together in a tight group for protection against sharks and killer whales. They swim downstream with the flow of the surface currents.
Land masses that extend out to sea opposing the flow of the surface currents, serve to trap sand, flotsam, seaweed, and lost sea mammals swimming with the flow. Cape Cod is the best example of such a natural trap in the United States. Cape Sorrel in Tasmania and Golden Bay in New Zealand are also natural traps for non-navigating whales/dolphins.
Capt. David Williams
Deafwhale Society, Inc.