Whales, which spend their entire lives in the ocean, do not possess gills. These massive marine animals are classified as mammals, characterized by the presence of lungs for extracting oxygen from the air, not water. Since their ancestors were land-dwelling creatures, whales retained the pulmonary system of air-breathing vertebrates, adapting it for an aquatic existence. Consequently, they must regularly surface to exchange gases with the atmosphere, a behavior fundamentally different from that of fish.
Whales are Mammals
Whales belong to the order Cetacea, and their classification as mammals dictates their respiratory anatomy. Like all mammals, they are warm-blooded, give birth to live young, and nurse their offspring with milk, distinguishing them from fish. The presence of lungs means whales cannot extract dissolved oxygen from the surrounding water.
Their evolutionary journey saw ancient land mammals return to the sea approximately 50 million years ago. While they developed fins and flukes, they retained lungs, meaning their physiology is built around the efficient use of atmospheric air, not the low concentration of dissolved oxygen in water.
The Mechanics of Whale Respiration
The physical act of a whale breathing is centered on the blowhole, a specialized, highly modified nostril. This opening is situated on the top of the head, allowing the whale to take a breath without lifting its entire body out of the water. Baleen whales possess two blowholes, while toothed whales have a single blowhole.
Breathing is a conscious, voluntary action for a whale, unlike the involuntary reflex of human breathing. The whale must actively contract muscles to open the blowhole, which is sealed by a fibrous plug to prevent water entry. This separation of the respiratory and digestive tracts ensures the whale does not inhale water while feeding.
When the whale surfaces, it executes a rapid, forceful exhalation that clears the lungs of stale air, visible as the “blow.” Whales then take a quick, deep inhalation before diving again. This process is efficient, allowing them to exchange up to 90% of the air in their lungs with each breath, compared to 10% to 15% for humans. This high rate of gas exchange enables them to hold their breath for extended periods.
Gills Versus Lungs: A Biological Comparison
The fundamental difference between gills and lungs lies in the medium from which they extract oxygen. Gills are the respiratory organs of aquatic animals, designed to take in dissolved oxygen from water using specialized filaments that maximize surface area for gas exchange.
Lungs, conversely, are internal structures adapted to extract a much higher concentration of oxygen from the air. The lung design involves internal branching tubes and air sacs, like the alveoli in mammals, where gas exchange occurs. The necessity for whales to surface highlights a limitation: lungs cannot function underwater because they are not designed to process the low density of dissolved oxygen.
The two systems are not interchangeable. The high metabolic rate of a whale requires a substantial and rapid intake of oxygen that only air-breathing lungs can provide. If a whale possessed gills, the extracted dissolved oxygen would be insufficient to sustain their large body size and energy demands.