In this blog post, we’ll start by exploring the question of whether humans can dive into the deep sea like whales, and then examine the human body’s diving reflex mechanism and its scientific feasibility.
When you first learned that whales—which swim for long periods in the deep sea—are not fish but mammals like humans, were you surprised? It is astonishing that these creatures, which we used to consider fish, are actually warm-blooded mammals like humans that give birth to live young and nurse them with milk. It is even more remarkable that, despite these characteristics, they can dive to depths of hundreds of meters and remain underwater for hours without breathing. Through long-term evolution, aquatic mammals like whales have developed the ability to dive for up to two hours. This is not simply a result of their fast swimming speed or large size. Their bodies have adapted to life underwater with a level of sophistication that is hard for humans to imagine, and these adaptations include the ability to regulate heart rate and store oxygen in their muscles.
Whales possess myoglobin and hemoglobin—which transport and store oxygen—in their muscles and red blood cells, respectively, in quantities incomparable to those found in humans. These substances help whales efficiently utilize oxygen within their bodies even at great depths. Unlike humans, whales can replace up to 80–90% of the air in their lungs with fresh air in a single breath. In contrast, humans can only replace about 15%, resulting in a significant difference in their ability to hold their breath. This characteristic of whales is the envy of humans who aspire to dive for long periods, particularly divers for whom staying underwater for extended periods is crucial. Through research into the physical structure of whales, divers are conducting extensive training and studies to reduce oxygen consumption during dives and enhance their ability to survive underwater.
So, do humans truly lack the same capabilities as whales? While not to the same extent as whales, humans have also evolved physiological mechanisms to cope with the extreme underwater environment of oxygen deprivation and water pressure—namely, the diving reflex.
In biology, a reflex refers to a phenomenon that occurs automatically within an organism in response to a stimulus, independent of conscious will. The diving reflex is a circulatory reflex observed in mammals, birds, and amphibians when they enter cold water, helping the organism maintain life in an aquatic environment. For mammals in particular, this reflex is a vital life-sustaining system. Although the human body’s diving reflex is not typically evident in daily life, this automatic reflex plays a major role in emergency situations when a person falls into water. The diving reflex occurs automatically through the interaction of various mechanisms in the human body, thereby enabling survival underwater.
For example, when a person submerges their head underwater, the nose and mouth—which supply oxygen—are cut off from the air. Since every cell in our body requires oxygen to produce energy, the oxygen-deprived environment of water poses a threat to human survival. When this situation is detected, the body automatically redistributes blood flow to supply oxygen to the most vital organs. Since blood plays the most fundamental and crucial role in supplying oxygen and nutrients, where and how blood is distributed in such situations determines survival. This is why the diving reflex evolved within the circulatory system, which encompasses the heart and blood vessels.
The process by which the diving reflex occurs is extremely rapid and sophisticated. First, the brain recognizes that the head has been submerged in cold water. This happens when the respiratory center detects that blood oxygen levels have dropped due to being cut off from external air, and the vagus nerve senses the temperature difference between the air inside the body—specifically the air in the sinuses (the hollow spaces within the bones around the nose)—and the external environment, transmitting a signal regarding this temperature difference to the brain. The brain combines these signals regarding apnea and the temperature difference to recognize the situation as “diving” and triggers the diving reflex, a survival mechanism. When the diving reflex occurs, the heart rate decreases by about 10–50%, causing the pulse to slow down. As the heart rate decreases, the amount of oxygen consumed per hour decreases, allowing the diver to remain underwater for a longer period. Additionally, stimulation of the sympathetic nervous system causes peripheral arteries to constrict, directing blood away from the arms, legs, and fingertips toward major internal organs such as the lungs, brain, and heart. While every cell in the body requires oxygen and nutrients carried by the blood to perform its specific functions, in the oxygen-deprived environment of diving, these resources are prioritized for organs that play a critical role in sustaining life. This is because, while one can survive without limbs, the blood supply to the heart and lungs—the centers of the circulatory and respiratory systems—and to the brain, which serves as the central hub for all organs, is crucial. This is especially true for the blood supply to the diencephalon and medulla oblongata, which are involved in maintaining life.
In 2002, a research team from Lund University and Mid Sweden University in Sweden uncovered a significant finding regarding the relationship between the human diving reflex and water temperature. It was already known that the diving reflex occurs even without apnea, but that it is triggered more strongly when combined with cold water. However, the research team discovered that the body perceives the “cold water” condition not as the absolute temperature of the water, but as a significant difference between the water temperature and the pre-dive temperature—that is, the air temperature. Furthermore, since the areas responsible for sensing this temperature difference are concentrated in the eyes, forehead, and the area above the nose, they determined that what is crucial for triggering the diving reflex is not how much of the body is submerged, but whether the face is submerged. Considering that the nose is the primary pathway for oxygen exchange, the fact that the perception of temperature differences also occurs in the area above the nose implies that the two signals the brain receives when recognizing the diving reflex originate from nearly the same location. Divers utilize this fact to extend their dive time and maintain a more comfortable state underwater by inducing the diving reflex by splashing cold water on their faces before entering the water.
Previously, it was believed that as one descends below the surface, the increasing water pressure would cause the thoracic cavity—the space where the ribs surround the lungs—to be damaged by the pressure, leading to death if a person went deeper than 60 meters. However, as it became known that divers could dive to depths of over 100 meters, scientific doubts arose; this phenomenon can also be explained by the diving reflex. When the diving reflex occurs, blood that has pooled in the lungs settles between the alveoli and inside the thoracic cavity. At this point, all blood vessels and organs within the thoracic cavity allow plasma to pass through their tissues. In other words, plasma—the liquid component of blood excluding cells—flows into the spaces between the cells. Because plasma is a liquid, its volume does not decrease even when subjected to physical pressure. This allows it to transmit the hydrostatic pressure applied to the thoracic cavity, preventing the ribcage from collapsing and enabling humans to survive at great depths. Thus, the diving reflex is a crucial physiological mechanism that allows humans to survive under high water pressure. Mammals specialized for aquatic life, such as whales, seals, sea otters, and dolphins, utilize this diving reflex more powerfully and effectively, allowing them to remain active underwater for extended periods. Through the diving reflex, these animals drastically reduce their heart rate, concentrate blood flow to their most vital organs, and minimize oxygen consumption underwater. Thanks to this ability, they can dive to depths of hundreds of meters to hunt for food, evade predators, and live freely.
In contrast, humans, who spend most of their time on land, do not possess the same diving capabilities as these aquatic mammals, which are perfectly adapted to the underwater environment. However, when a situation arises where a person falls into the water or must remain underwater for an extended period, the human diving reflex becomes a vital life-sustaining mechanism. When our face is submerged, oxygen is cut off, and we sense the cold temperature, our brain automatically triggers the diving reflex to direct blood flow to vital organs and regulate heart rate. As a result, even when submerged, our body autonomously prepares for survival. This response occurs without our conscious awareness and functions as an instinctive physiological reaction to protect our lives.
The diving reflex reminds us once again of the mystery of life shared by humans and animals. This sophisticated physiological mechanism, which has evolved to allow our bodies to sustain life underwater, remains a significant subject of research for scientists. Future research on the diving reflex is expected not only to contribute to finding ways for divers to stay underwater longer and more safely but also, in the long term, to open up possibilities for humans to operate at deeper depths.
