Sharks do not breathe air. As aquatic animals, they extract oxygen directly from the water. This fundamental difference sets them apart from air-breathing creatures and is central to their survival in marine environments. Their specialized biological systems are adapted for underwater life, meaning they cannot survive out of water.
How Sharks Breathe: The Role of Gills
Sharks breathe using gills, which are efficient organs for gas exchange in water. They typically have five to seven gill slits on each side of their head. Water enters the shark’s mouth or, in some species, through small openings called spiracles behind their eyes. This water then flows over the gill structures.
Each gill is supported by a gill arch, from which numerous gill filaments extend. These filaments are covered with microscopic folds called lamellae, which significantly increase the surface area for oxygen absorption. As water passes over these lamellae, oxygen diffuses from the water into the shark’s bloodstream, while carbon dioxide moves from the blood into the water. The efficiency of this process is amplified by a mechanism known as countercurrent exchange.
Countercurrent exchange involves blood flowing through gill capillaries opposite to the water flow over the gills. This opposing flow maintains a steep concentration gradient along the lamellae, ensuring oxygen moves from water to blood. This enables sharks to extract a high percentage of dissolved oxygen from the water, often up to 80% or more.
Sharks employ two primary methods to ensure continuous water flow over their gills. Many active, fast-swimming sharks, such as great whites and mako sharks, use ram ventilation, swimming continuously with open mouths to force water over their gills. If these sharks stop moving, water flow ceases, risking suffocation. In contrast, some sedentary sharks, like nurse sharks, use buccal pumping, actively pumping water with mouth and pharynx muscles to breathe while stationary. Some species can switch between both methods depending on their activity level.
Why Sharks Don’t Breathe Air: Distinguishing Features
Sharks are obligate aquatic breathers, meaning their respiratory system is adapted for extracting oxygen from water. Their gills are not functional in air. When a shark is removed from water, its delicate gill filaments collapse due to surface tension. This collapse drastically reduces the surface area for gas exchange, making it impossible for the shark to absorb sufficient oxygen from the air.
Unlike marine mammals such as whales and dolphins, sharks do not possess lungs. Mammals use lungs to breathe air, requiring them to periodically surface to inhale. Sharks lack these pulmonary structures and the circulatory system necessary to process atmospheric oxygen.
The physiological differences extend to their overall body structure. Sharks have a cartilaginous skeleton and large, oil-filled livers that aid in buoyancy, unlike bony fish with swim bladders. If a shark is stranded out of water, gill damage and the inability to extract oxygen lead to suffocation. Their biology is entirely geared towards a life submerged in water, making prolonged exposure to air unsustainable.