In this blog post, we will look at the diving reflex mechanism of the human body and its scientific possibilities, starting with the question of whether humans can dive as deep as whales.
When people first learned that whales that swim in the deep sea for a long time are not fish but mammals like humans, who was not surprised? It is surprising that they, whom we thought were fish, are actually mammals with warm blood like humans, and they give birth to offspring and breastfeed them. It is even more amazing that, despite these characteristics, they can dive to depths of hundreds of meters and stay underwater for hours without breathing. Over millions of years, aquatic mammals such as whales have evolved to be able to dive for up to two hours. This is not possible simply because they are fast swimmers or large in size. Their bodies have adapted to life in the water in ways that are difficult for humans to imagine, including the ability to regulate heart rate and the ability to store oxygen in their muscles.
Within the body of a whale, there is a much greater distribution of myoglobin and hemoglobin, which carry and store oxygen, in the muscles and red blood cells than in humans. They help whales efficiently use oxygen in their bodies even in deep waters. Unlike humans, whales can exchange up to 80-90% of the air in their lungs with fresh air in one breath. Humans, on the other hand, can only exchange about 15%, showing a significant difference in their ability to hold their breath for a long time. This trait of whales is the envy of humans who long for the ability to dive for long periods of time, especially divers who need to stay underwater for long periods of time. Divers are conducting a lot of training and research to reduce oxygen consumption during diving and improve their ability to survive underwater by studying the body structure of whales.
So, do humans really lack the same abilities as whales? Although not quite as good as whales, humans also have an evolutionary physiological mechanism to cope with the extreme underwater environment of anoxia and water pressure, which is the diving reflex.
In biology, reflexes are automatic responses to stimuli in the body, regardless of the will. Diving reflexes are circulatory reflexes that appear when mammals, birds, amphibians, etc. enter cold water, helping the creature to maintain life in the water environment. In mammals, in particular, these underwater reflexes are a very important life-sustaining system. The human body’s ability to reflexively dive is not often demonstrated in everyday life, but this automatic reflex plays a major role when we fall into the water in an emergency. The diving reflex is automatically generated by the interaction of various mechanisms in our body, which enables us to sustain life underwater.
For example, when a person immerses his or her head in water, the nose and mouth, which are the entry points for oxygen, are blocked from the air. Since all cells in our body need oxygen to generate energy, the oxygen-free environment of water is a threatening environment for the human body. When this situation is detected, the body automatically redistributes blood flow to supply oxygen to the most important organs. Blood plays the most basic and important role of supplying oxygen and nutrients, so where and how blood is supplied in such situations determines survival. This is why the diving reflex has evolved in the circulatory system that encompasses the heart and blood vessels.
The process of diving reflex is very quick and precise. First, the brain recognizes that the head is submerged in cold water, which is then recognized by the respiratory center as the blood oxygen level has dropped due to the lack of external air. The vagus nerve then detects the difference in temperature between the air in the body and the air outside the body, which is the air in the sinuses, the empty spaces in the bones around the nose, and sends a signal to the brain about the temperature difference. The brain combines the signals from the apnea and the temperature difference to recognize the situation as diving and triggers the diving reflex, a survival mechanism. When the diving reflex is triggered, the heart rate slows down by 10-50%, and the pulse rate begins to slow down. When the heart rate slows down, the amount of oxygen used per hour decreases, allowing you to stay underwater for longer. In addition, the peripheral arteries contract due to the excitement of the sympathetic nerve, causing blood to collect from the arms, legs, fingertips, etc. to the major organs in the body, such as the lungs, brain, and heart. All cells in the body need oxygen and nutrients carried by blood to perform their respective roles, but in a situation of oxygen deficiency, such as diving, organs that play an important role in maintaining the life of the individual are supplied first. Even if we don’t have limbs, we can still live, but the blood supply to the heart and lungs, which are the center of the circulatory and respiratory systems, and the brain, which acts as the central nervous system for all organs, is important, especially for the limbic and subcortical regions involved in maintaining life.
In 2002, a research team from Lund University and Mid Sweden University in Sweden discovered a significant fact in the relationship between the human diving reflex and water temperature. It was already known that the diving reflex occurs even when there is no apnea, but the diving reflex occurs more strongly when cold water is added. However, the research team found that the situation of cold water is not the absolute temperature of the water, but the difference in temperature before diving, that is, the difference in temperature with the air, which the body senses. In addition, they found that the parts that recognize this temperature difference are concentrated in the eyes, forehead, and the area above the nose, so the important thing for the diving reflex to occur is not how much the body is submerged in water, but whether the face is submerged in water. Considering that the nose is the main point where oxygen enters and leaves the body, the fact that the perception of temperature differences also occurs in the area above the nose means that the two signals received when the brain recognizes the diving reflex come from almost the same place. Divers sometimes use this fact to increase their diving 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 humans would die if they descended below a depth of 60 meters because the water pressure would increase as they descended. However, when it became known that divers could dive to depths of 100 meters or more, scientific questions arose, which can also be explained by the diving reflex. The blood that has gathered in the lungs due to the diving reflex is located between the alveoli and inside the chest cavity. At this time, all the blood vessels and organs in the chest cavity allow the tissue to pass through the blood plasma. In other words, plasma, the liquid part of blood that excludes cells, flows into the cells. Since plasma is a liquid, it does not shrink in volume even when physically pressured, which prevents the collapse of the rib cage by transmitting the water pressure applied to the chest cavity and enables humans to survive even at deep depths. The diving reflex is an important physiological mechanism that enables humans to survive even under high water pressure in the water. Mammals that specialize in aquatic life, such as whales, seals, sea otters, and dolphins, can use this diving reflex more powerfully and effectively to stay active underwater for long periods of time. These animals use the diving reflex to dramatically lower their heart rate, concentrate blood flow to their most important organs, and minimize oxygen consumption underwater. This ability allows them to dive to depths of hundreds of meters to hunt prey, escape predators, and live freely.
On the other hand, humans, who spend most of their time on land, do not have the diving ability of these aquatic mammals, which are perfectly adapted to the underwater environment. However, the human diving reflex is an important life-sustaining mechanism when we fall into the water or need to stay underwater for a long time. When our face is submerged in water, oxygen is cut off and we sense the cold temperature, our brain automatically triggers a diving reflex to transport blood to the vital organs and regulate our heart rate. As a result, our body autonomously prepares for survival even when we are drowning. These reactions occur without our awareness and function as instinctive physiological responses to protect our lives.
The diving reflex makes us feel the mystery of life that humans and animals possess. This sophisticated physiological mechanism, which has evolved to enable our bodies to sustain life underwater, is still an important research topic for scientists. In the future, research on the reflex will not only help divers find ways to stay underwater longer and safer, but also open up the possibility of humans’ potential to operate at greater depths in the long term.
