How Seals Breathe
Seals, like all mammals, breathe air using their lungs and do not possess gills. They must regularly come to the water’s surface to inhale. Their respiratory system is adapted for efficient air exchange, allowing them to take in a large amount of oxygen with each breath. Seals use their nostrils for breathing, which automatically close tightly when they submerge to prevent water from entering their lungs. This mechanism allows them to move seamlessly between aquatic and terrestrial environments.
When a seal prepares for a dive, it often exhales most of the air from its lungs, which might seem counterintuitive to humans. This exhalation helps them reduce buoyancy, making it easier to dive deeper into the water. While underwater, seals control their oxygen levels efficiently, relying on oxygen stored within their bodies rather than in their lungs.
Life Underwater
Seals possess remarkable physiological adaptations that enable them to spend significant periods underwater. One such adaptation is bradycardia, where their heart rate slows dramatically during a dive. This slowed heart rate conserves oxygen by reducing the metabolic demand of the body. They also redirect blood flow, sending oxygen-rich blood primarily to essential organs like the brain, heart, and lungs, while restricting it from tissues that are more tolerant of low oxygen levels, such as muscles and extremities.
Their bodies are also highly efficient at storing oxygen. Seals have a greater blood volume compared to land mammals of similar size, and their blood is rich in hemoglobin, the protein that carries oxygen in red blood cells. Additionally, their muscles contain high concentrations of myoglobin, a protein similar to hemoglobin that stores oxygen directly within the muscle tissue, providing an immediate oxygen reserve for movement during dives. Some species, like elephant seals, can remain submerged for up to two hours, while smaller species might hold their breath for 15 to 40 minutes.
Seals can tolerate higher levels of carbon dioxide in their blood than land mammals and have a reduced urge to breathe, which helps them extend their dive times. Their lungs also undergo a controlled collapse at depth, which helps prevent nitrogen from dissolving into the bloodstream and causing decompression sickness, often referred to as “the bends.” This lung collapse also preserves a small oxygen reservoir in the upper airways, which can be utilized during ascent.
Seals as Mammals
Seals are classified as mammals, a biological group characterized by specific traits that distinguish them from other animal classes. As mammals, seals are warm-blooded, give birth to live young, and nurse their offspring with milk. They also possess hair or fur, which helps with insulation.
Fish, in contrast, are aquatic vertebrates that extract dissolved oxygen directly from water using specialized respiratory organs called gills. Gills are feathery structures with numerous blood vessels that facilitate gas exchange underwater. The presence of gills is a defining feature of fish, allowing them to respire without surfacing.