The great whales are the largest animals ever to exist on Earth, and their internal anatomy reflects a complete biological commitment to an aquatic existence. To support a massive body mass, nearly every system has undergone profound changes from their land-mammal ancestors. Whether they are filter-feeding baleen whales (Mysticeti) or predatory toothed whales (Odontoceti), the scale of their organs and the complexity of their adaptations allow them to thrive in the deepest parts of the ocean. These internal modifications enable them to manage immense pressures, store vast amounts of oxygen, and process thousands of pounds of food while navigating a dark, three-dimensional world.
The Musculoskeletal Framework
The internal framework of a whale provides support and leverage for movement in a buoyant, fluid environment. The backbone, or vertebral column, is a central, highly flexible structure that lacks the fused sacrum seen in terrestrial mammals, which anchors the pelvis and hind limbs. The vertebrae are numerous and concentrated toward the tail to allow for the powerful vertical undulation that drives aquatic motion. The ribs are not rigidly connected to the sternum, allowing the chest cavity to compress easily under the extreme pressure of deep dives without fracturing.
The pectoral flippers, used primarily for steering and stopping, contain the same skeletal elements found in a human arm and hand, demonstrating shared mammalian ancestry. These forelimb bones are shortened and flattened, with an immobile elbow joint that forms a rigid, paddle-shaped structure. The tail flukes, the primary source of propulsion, contain no bone; they are composed entirely of dense, fibrous connective tissue. This powerful surface is driven by massive muscle systems housed in the peduncle, which can account for up to a third of the whale’s total body weight.
Cardiovascular and Respiratory Adaptations
The circulatory and respiratory systems of a whale are extensively modified to manage the physiological demands of prolonged, deep diving. The heart of a large whale, such as the blue whale, is the largest on the planet, weighing approximately 400 pounds and capable of pumping around 60 gallons of blood with a single beat. The aorta, the main artery leaving the heart, is highly elastic and functions as a hydraulic reservoir. This elasticity allows the vessel wall to maintain pressure and smooth blood flow even as the heart rate slows dramatically. This mechanism allows the heart rate to drop from a surface pace of around 25 to 37 beats per minute to as low as 2 to 10 beats per minute during a deep dive.
Respiratory Adaptations
Respiratory efficiency is maximized at the surface, where whales can exchange up to 90% of the air in their lungs with each breath, compared to only 10 to 20% in humans. To cope with increasing pressure at depth, the lungs are relatively small and designed to collapse partially or completely. This collapse forces residual air out of the alveoli and into reinforced airways. This controlled mechanism prevents nitrogen from being forced into the bloodstream under high pressure, which causes decompression sickness in human divers.
The Retia Mirabilia
A complex network of intertwined blood vessels called the retia mirabilia, or “wonderful nets,” is distributed throughout the thorax, spinal column, and base of the brain. This vascular network manages blood flow and pressure fluctuations, acting as a buffer against pressure pulses generated by powerful tail movements and the hydrostatic pressure of the deep ocean. The retia mirabilia also serve as a reservoir for oxygenated blood, helping to shunt the supply toward the heart and brain, which are the most oxygen-sensitive organs, while restricting blood flow to peripheral tissues.
Fueling the Body The Unique Digestive System
The internal mouth structure is the first major point of difference between the two whale suborders, reflecting their distinct feeding strategies. Baleen whales possess hundreds of plates of baleen, made of the protein keratin, which hang from the upper jaw. These plates act as a sieve to filter small organisms like krill and copepods from the water. In contrast, toothed whales have specialized teeth, which vary widely in number and size, used for grasping and securing prey like fish and squid before swallowing them whole. The sperm whale, for example, typically only has teeth on its lower jaw, used to capture deep-sea prey.
The massive food intake required is processed by a digestive system that features a complex, multi-chambered stomach, similar in structure to that of ruminants. The first chamber is the non-glandular forestomach, which is lined with a tough, protective layer and functions primarily as a holding chamber for unchewed food. Here, mechanical grinding occurs, and microbial activity begins breaking down the chitin found in the exoskeletons of crustaceans.
From the forestomach, food passes into the main or fundic stomach, which is the site of most chemical digestion, secreting acids and enzymes. The final chamber, the pyloric stomach, features a lower pH environment optimal for the final stages of gastric digestion before the material moves into the small intestine. In some species, the small intestine can reach extraordinary lengths; the sperm whale has the longest intestinal system in the world, which can exceed 300 meters, providing the surface area necessary for nutrient absorption.
Specialized Sensory Systems
The sense of hearing is paramount for a whale’s survival, as sound travels efficiently through water, making it the primary means of navigation and communication. Toothed whales have evolved sophisticated internal mechanisms for receiving and processing the high-frequency clicks used in echolocation. Sound waves are funneled through the fatty tissues of the lower jaw, which acts like an acoustic window, directing vibrations through an oil-filled channel to the specialized inner ear bones.
For sound production, toothed whales use structures in their nasal passages called phonic lips, which vibrate as air is forced past them to create clicks. The melon, a fatty organ located in the forehead, functions to shape and focus these clicks into a concentrated beam projected forward. Baleen whales, which communicate over long distances using low-frequency sounds, utilize bone conduction, where the skull itself vibrates to transmit sound waves directly to the ear complex. While whales have functional eyes, they are small relative to body size and possess adaptations for low-light conditions, such as numerous rods and a reflective membrane.