Whales are mammals like humans, and their long evolution has given them the ability to hold their breath underwater for up to two hours. These aquatic mammals have great oxygen storage capacity in their lungs and muscles, and they use the dive reflex to reduce oxygen consumption. Humans also have a diving reflex mechanism that automatically directs blood flow to vital organs and lowers heart rate when submerged, allowing us to survive in extreme conditions.
Who wasn’t surprised when we first learned that whales, which swim for long periods of time in the deep ocean, are not fish but mammals like humans? It’s amazing that what we think of as fish are actually mammals that have warm blood and give birth to and nurse their young, just like humans. It’s even more amazing that despite these characteristics, they can dive to depths of hundreds of meters and stay underwater for hours without breathing. Aquatic mammals like whales have evolved over time to have the ability to dive for up to two hours. Their bodies have adapted to life underwater in ways that are hard for humans to imagine, including the ability to regulate their heartbeat and retain oxygen in their muscles.
Whales have unparalleled amounts of myoglobin and hemoglobin, which transport and store oxygen, in their muscles and red blood cells, respectively. These help whales utilize oxygen efficiently, even at great depths. Unlike humans, whales can exchange up to 80-90% of the air in their lungs for fresh air in a single breath. Humans, on the other hand, are only able to exchange about 15%, which is a significant difference in their ability to hold their breath. This characteristic of whales is envied by humans who aspire to the ability to dive for long periods of time, especially divers for whom staying underwater for long periods of time is important. Divers train and study whale anatomy to reduce their oxygen consumption during dives and improve their ability to survive underwater.
But do we really lack the same abilities as whales? While not as advanced as whales, we do have an evolutionary and physiological mechanism that allows us to cope with the extreme conditions of anoxia and hydrostatic pressure below the surface: the diving reflex.
In biology, a reflex is a phenomenon that occurs automatically in living organisms in response to a stimulus, regardless of willpower. The dive reflex is a circulatory system reflex that occurs in mammals, birds, and amphibians when they enter cold water and helps them stay alive in the aquatic environment. For mammals in particular, this reflex is a critical life-support system. While the human body’s ability to dive is often unnoticed in everyday life, this automatic reflex can play a huge role in a drowning emergency. The dive reflex is an automatic response that is triggered by the interaction of several mechanisms in the body that allow us to stay alive underwater.
For example, when a person submerges their head underwater, the nose and mouth, where oxygen comes in, are closed off from air. Since every cell in the body needs oxygen to generate energy, the anoxic environment of being underwater is a life-threatening situation for the human body. When the body senses this, it automatically redistributes blood flow to oxygenate the most vital organs. Since blood is the most basic and important source of oxygen and nutrients, where and how it is delivered in these situations is critical to survival. This is why the dive reflex evolved in the circulatory system, which includes the heart and blood vessels.
The process of developing the dive reflex is very quick and sophisticated. First, the brain recognizes that the head is submerged in cold water, which means that the respiratory center recognizes that the blood oxygen level has decreased due to being cut off from the outside air, and the vagus nerve senses the temperature difference between the air in the sinuses, the hollow spaces in the bones around the nose, and the air outside the body, and sends a signal to the brain about the temperature difference. The brain combines the apnea and temperature difference signals to recognize that you are diving and triggers the dive reflex, a survival mechanism. When the dive reflex kicks in, your heart rate drops by 10-50% and your pulse starts to slow down. With a lower heart rate, you use less oxygen per hour, which allows you to stay underwater for longer periods of time. The sympathetic nervous system also causes the peripheral arteries to constrict, allowing blood to flow from the arms, legs, and fingertips to the major organs in the body, such as the lungs, brain, and heart. All cells in the body equally need the oxygen and nutrients carried by the blood to do their jobs, but in the oxygen-deprived environment of diving, the organs that play a critical role in keeping an individual alive are prioritized. This is because even though we can survive without limbs, the heart and lungs, which are the center of the circulatory and respiratory systems, and the brain, which is the backbone of all organs, especially the brainstem and soft brain, are critical for life support.
In 2002, a team of researchers from Lund University and Mead University in Sweden made a significant discovery about the relationship between the human dive reflex and water temperature. It has always been known that apnea alone triggers the dive reflex, but that the reflex is stronger when cold water is added to the mix. However, the researchers found that it is not the absolute temperature of the water, but the body’s perception of a large difference between the pre-dive temperature and the air temperature that triggers the dive reflex. They also found that it is not how much of the body is submerged, but how much of the face is submerged that is important for the dive reflex to occur, as the areas that perceive this temperature difference are concentrated in the eyes, forehead, and upper nose. Given that oxygen enters and exits the body primarily through the nose, it makes sense that the perception of temperature differences also occurs in the upper nose, meaning that the two signals the brain receives when it recognizes the dive reflex come from roughly the same place. Divers take advantage of this fact by inducing the dive reflex by splashing cold water on their face before entering the water to extend their dive time and make them more comfortable underwater.
Previously, it was believed that because of the increasing pressure of the water as you descend below the surface, humans would die if they descended to depths of more than 60 meters, as the pressure would damage the pleural cavity, the space where the ribs surround the lungs. However, scientific questions arose when it became known that divers could dive to depths of more than 100 meters, which could also be explained by the dive reflex. The dive reflex causes the blood to pool in the lungs, between the alveoli and inside the chest cavity. At this point, all the blood vessels and organs in the chest cavity allow plasma to pass through the tissues, which means that plasma, the liquid part of the blood that excludes cells, flows between the cells. Because plasma is a liquid, it doesn’t shrink in volume under physical pressure, which is what allows it to conduct the water pressure in the chest cavity, preventing the rib cage from collapsing and allowing humans to survive at great depths. The diving reflex is an important physiological mechanism that enables humans to survive the high pressures underwater. Mammals that are specialized for life underwater, such as whales, seals, sea otters, and dolphins, utilize the dive reflex more powerfully and effectively, allowing them to stay underwater for extended periods of time. These animals use the diving reflex to dramatically slow their heart rate, direct blood to their most vital organs, and minimize oxygen consumption underwater. This ability allows them to dive to depths of hundreds of meters, hunt for food, avoid predators, and live freely.
Humans, on the other hand, who spend most of their time on land, don’t have the same ability to dive as these aquatic mammals, who are perfectly adapted to the underwater environment. However, in the event of a drowning or extended period of time underwater, the human dive reflex becomes an important life-support mechanism. When our face is submerged in water, cutting off oxygen and sensing the cold temperature, our brain automatically triggers the dive reflex to pump blood to our vital organs and regulate our heart rate. This allows your body to autonomously prepare for survival in a drowning situation. This happens without us even realizing it, and it works as an instinctive physiological response to protect our lives.
The dive reflex reminds us of the mystery of human and animal life. This sophisticated physiological mechanism that evolved to help our bodies stay alive underwater is still an important topic of research for scientists. In the future, research into the dive reflex will not only contribute to finding ways to help divers stay underwater longer and safer, but it will also open up the possibility of the potential for humans to operate at deeper depths in the long term.
