Seals are fascinating marine mammals that spend much of their lives in the water. Despite their aquatic lifestyle, they possess lungs and must surface to breathe air, similar to land mammals. This fundamental requirement drives many of their unique adaptations, allowing them to thrive in both marine and terrestrial environments.
The Basics of Seal Respiration
Seals breathe by inhaling air at the water’s surface, drawing oxygen into their lungs, and then exhaling carbon dioxide. They cannot extract oxygen directly from water like fish. After taking a breath, seals hold it as they dive underwater, relying on the oxygen stored within their bodies.
When a seal is about to dive, it exhales a significant portion of the air in its lungs. This exhalation helps them reduce buoyancy, making it easier to descend. Unlike humans who typically hold a full breath before diving, seals prioritize efficient oxygen management in their blood and muscles rather than in their lungs. Their respiratory system is also highly efficient, allowing them to exchange a large percentage of air with each breath, up to 90%, compared to humans who exchange about 20%.
Unique Lung Adaptations for Aquatic Life
Seals have developed unique lung adaptations for diving. One key adaptation is their ability to intentionally collapse their lungs during deep dives. This collapse prevents nitrogen from being absorbed into their bloodstream under high pressure, reducing the risk of decompression sickness, commonly known as “the bends.”
Lung collapse typically occurs at depths around 25 to 70 meters. By collapsing their lungs, seals isolate the air in their upper airways, preventing gas exchange at extreme depths. This mechanism, combined with the elasticity and robustness of their lung tissue, allows their lungs to withstand the pressure of deep water and then re-expand upon ascent.
Whole-Body Diving Adaptations
Beyond their specialized lungs, seals have whole-body physiological adaptations for diving. One significant adaptation is bradycardia, a slowing of their heart rate during a dive. A seal’s heart rate can decrease from a surface rate of 75-120 beats per minute to 4-6 beats per minute, conserving oxygen. This heart rate reduction is part of a diving reflex.
Seals also redistribute blood flow, shunting it away from tissues that tolerate lower oxygen levels and directing it to organs like the brain, heart, and lungs. This ensures tissues receive a continuous supply of oxygen during long dives. Their bodies contain a higher volume of blood than land mammals of similar size, and a greater concentration of hemoglobin (the oxygen-carrying protein in red blood cells), allowing them to store more oxygen. Their muscles are rich in myoglobin, an oxygen-storing protein that can hold up to ten times more oxygen than human muscles. These adaptations allow seals to tolerate the buildup of lactic acid, a byproduct of energy production without oxygen, further extending their dive times.