How do whales and humans dive differently, and how can humans survive underwater?

Aquatic mammals like whales have developed excellent diving abilities through long evolution, and humans can also survive underwater thanks to a physiological mechanism called the dive reflex. In this article, we’ll explain how the dive reflex works and how divers use it to survive.

 

When we first learned that whales swim in the deep ocean for long periods of time, we were surprised to learn that they are not fish, but mammals like humans. Aquatic mammals like whales have evolved over time to be able to dive for up to two hours. As members of the same mammalian family, their ability to dive far exceeds ours, and we’ve all felt a sense of wonder and mystery at one time or another. Whales have an unparalleled amount of myoglobin and hemoglobin, which transport and store oxygen, in their muscles and red blood cells, respectively. Whereas humans can only exchange about 15% of the air in their lungs in a single breath, whales have the ability to exchange 80-90% of their lungs for fresh air. This physical trait is envied by humans who crave the ability to stay underwater for long periods of time, especially divers for whom staying underwater for long periods of time is important. But even if our anatomy doesn’t allow us to match these abilities, humans, like aquatic mammals, have physiological mechanisms that allow us to survive extreme conditions below the surface, such as anoxia and pressure: the diving reflex.
In biology, a reflex is a phenomenon that occurs automatically in living organisms in response to a stimulus, regardless of the will. The dive reflex is a circulatory system reflex that occurs in mammals, birds, and amphibians when they enter cold water, helping them to stay alive in the aquatic environment. When a person submerges their head underwater, the nose and mouth, the holes through which oxygen comes in, are closed off from air. Every cell in the body basically needs oxygen to work, so the anoxic situation underwater becomes an urgent challenge that the body must solve to stay alive. Since blood is the most basic and important source of oxygen and nutrients, where and how it is delivered in this situation is critical to survival. This is why the dive reflex has evolved in the circulatory system, which includes the heart and blood vessels.
Here’s how the dive reflex works First, the brain recognizes that the head is submerged in cold water, which means that the respiratory center recognizes that the blood oxygen concentration has decreased due to being cut off from the outside air, and the vagus nerve detects the difference in temperature 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 rather the body’s perception of a large difference between the pre-dive temperature and the air temperature that triggers the reflex. They also found that it is not how much of the body is submerged, but rather how much of the face is submerged that triggers the reflex, 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 water as you descend below the surface, humans would die if they descended to a depth 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 under physical pressure, which is why it’s able to conduct the water pressure in the chest cavity, preventing the rib cage from collapsing and allowing humans to survive at great depths.
For aquatic mammals that spend every waking moment underwater, diving time and physiological activity while submerged is critical not only for survival, but also for hunting prey and avoiding predators. As such, their bodies adapt in many ways, including the diving reflex, to stay in top shape during this time. However, other mammals that spend most of their time on land also have the dive reflex as an automatic mechanism to prepare for a drowning situation. When your brain realizes that your face is submerged, cut off from oxygen, and you’re feeling cold, it pumps blood to your vital organs and regulates your heart rate, all of which happens without you even realizing it, your body is protecting you from drowning. When we think about this diving reflex, we are reminded of the mystery of how persistent and intricate life is.