I had a professor, or maybe more than one, who argued that breath-holding animals don’t get decompression sickness because they don’t accumulate enough nitrogen in their tissues. However, despite their name being associated with research suggesting otherwise, this statement is false.

While there is some truth to the idea that different tissues absorb and release nitrogen at different rates (fast tissue versus slow tissue), leading to varying nitrogen accumulation, studies and incidents of beached marine mammals have demonstrated the presence of bubbles in their tissues, which is considered evidence of decompression sickness or “the bends.”

Marine mammals possess various anatomical, physiological, and behavioral adaptations that help minimize the risk of decompression sickness. A significant preventive measure is the reduction of nitrogen loading in tissues. In this aspect, there is a grain of truth regarding breath-holding divers. With a single breath, there is a limit to the amount of nitrogen that can be absorbed, unlike human SCUBA divers who continuously breathe gas under pressure, significantly increasing the risk of decompression sickness. However, even in breath-holding animals, any air retained during a dive is subject to pressure, presenting some risk.

Pinnipeds (seals, sea lions, walruses) expel air from their lungs before diving, primarily seen in phocid seals. They have reduced air-trap spaces, such as nasal sinuses, facilitating the removal of air. Additionally, about half of their rib length consists of flexible cartilage, allowing the ribcage to collapse under less pressure and pushing any remaining air into rigid, poorly blood-perfused areas where gas exchange is limited. Pinnipeds typically experience chest collapse at relatively shallow depths of 25-50 meters, aided by the flattened shape of their hearts, which enables heart function during collapsed chest periods.

Cetaceans (whales, dolphins, porpoises), however, tend to dive with full lungs. Unlike other mammals, cetaceans have a few ribs with two heads (bicapitate ribs) at the front end, while the rest of their ribs have a single head (unicapitate ribs). Bicapitate ribs possess a firm connection with the vertebrae, allowing limited rotation, while unicapitate ribs enable torsion at the connection point, allowing the ribcage to “rotate” and collapse. This collapse occurs at greater depths for cetaceans compared to pinnipeds.

Both cetaceans and pinnipeds, as well as marine otters, have reinforced cartilage in their airway passages, making them rigid. Under sufficient pressure, the alveoli (air sacs) collapse, pushing the air into rigid passages and minimizing gas exchange. Odontocetes (toothed whales) have sphincter muscles at the terminal ends of these passages, although their exact function remains unclear.

The collapsed alveoli severely restrict gas exchange, potentially leading to thickening of alveolar tissue, further inhibiting gas exchange and nitrogen uptake. At this stage, there are two distinct pulmonary regions: air-filled regions that impede gas exchange and collapsed regions where gas exchange would typically occur. It is believed that the majority of blood perfusion occurs in the collapsed regions, resulting in a ventilation-perfusion mismatch. Any remaining gases in the alveolar regions are minimal but can be readily absorbed, leading to further collapse. Most of the nitrogen-laden air becomes trapped in the rigid airways, where no gas exchange takes place. The presence of sphincter muscles in odontocetes may help close the passageway terminals before external pressures collapse the lungs, leading to alveolar collapse even earlier.

During diving, both pinnipeds and cetaceans demonstrate voluntary control over heart rate, allowing them to reduce blood flow to the lungs when unnecessary, further minimizing gas exchange. They can maximize effort to reach the point of collapse sooner and regulate the speed of ascent to prevent bubble formation in tissues.

Previous research has primarily focused on how marine mammals prevent nitrogen loading, partially due to their breath-holding capabilities. However, we now know that they can naturally accumulate significant amounts of nitrogen, which can pose hazards and even be fatal under certain circumstances. Some researchers propose shifting the emphasis towards understanding how marine mammals manage their nitrogen load. As technology advances, it will be intriguing to see the progress in this field, addressing various issues that were mere dreams half a century ago.