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
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.