Unlike mammals, sharks do not have lungs. Their specialized biology is suited for extracting oxygen directly from water, allowing them to thrive in marine environments. Instead of lungs, sharks utilize gills for gas exchange, a defining characteristic of their underwater existence.
How Sharks Breathe
Sharks breathe using gills, specialized organs that extract dissolved oxygen from water. Water enters the shark’s mouth, flows over these gills before exiting through several gill slits on the sides of their heads. Water movement over the gills occurs through two main methods: buccal pumping and ram ventilation.
Buccal pumping involves the shark actively drawing water into its mouth using muscles in its cheeks and throat, then pushing it over the gills. This method allows some sharks, particularly bottom-dwelling species like nurse sharks and tiger sharks, to breathe while remaining stationary. These sharks can rest on the seafloor and still maintain a flow of oxygenated water over their gills.
Ram ventilation requires the shark to swim continuously with its mouth open, forcing water to flow over its gills as it moves forward. Fast-swimming species, such as great white sharks, mako sharks, and whale sharks, often rely on this method. If these sharks stop swimming, the water flow over their gills ceases, which can lead to suffocation. Some shark species, like the tiger shark, can switch between buccal pumping and ram ventilation depending on their activity level.
Gill Respiration Efficiency
Shark gills are efficient structures designed to extract oxygen from water, which contains significantly less oxygen than air. Most shark species have five to seven gill slits on each side. Inside these slits, the gills are supported by gill arches, bearing numerous gill filaments.
Each gill filament is covered with tiny, folded structures called lamellae, which increase the surface area available for gas exchange. This extensive surface area is crucial because water holds only about 1% oxygen compared to air’s 21%. As water flows over the lamellae, dissolved oxygen diffuses into the shark’s bloodstream, while carbon dioxide diffuses from the blood into the water.
The efficiency of shark respiration is due to countercurrent exchange. Within the gill filaments, blood flows in the opposite direction to the water passing over them. This opposing flow maintains a constant concentration gradient, meaning that even as the blood gains oxygen, it continuously encounters water with a higher oxygen concentration. This allows sharks to absorb up to 80% of the available oxygen from the water, a much higher percentage than humans extract from air.