We have all been asked the question “if you could have any superpower, what would it be?” For me, I've always wanted to have the ability to breath underwater. It turns out that humans already have a sort of underwater “superpower”, but it only kicks in when we hold our breath beneath the surface. For decades, scientists have been conducting research on what is called the 'human dive response'. The human dive response is an evolutionary adaptation that allows humans and many other mammals to hold their breath underwater for far longer than what would be expected based on how our bodies function on land. This unique ability is only experienced when you are both holding your breath and completely submerged underwater. Once these criteria are met, your body automatically responds by undergoing several interesting physiological changes that function to slow the depletion of the precious oxygen taken in your last breath. Why does this only happen underwater though? What allows our bodies to subconsciously recognize the appropriate time to initiate the human dive response? As we'll see, splashing cold water on your face does more than just waking you up in the morning.
What causes the diving response?
If you have ever jumped into a freezing cold pool or lake, it is very difficult to breath at first and often requires a conscious effort to begin breathing again. Apnea is the term used to describe the arrest of breathing, and acts as a sort of “master switch” to neurologically induce the dive response. This can be accomplished by either voluntarily or involuntarily holding your breath. We can consciously take a deep breath and hold it, which is obviously the method used by professional free divers before making a decent up to several hundred feet . However, things become more interesting when our body responds to signals by causing us to involuntarily hold our breath. Reflex apnea is the situation where someone begins to hold their breath after being suddenly or unexpectedly submerged in water. This reflex has also been observed without submerging your face in water, however the response is much greater when the face is immersed. One study found that apnea with face immersion resulted in a 25% reduction in heart rate while apnea without face immersion resulted in only a 10% reduction in heart rate. This condition of reduced heart rate, or bradycardia, is one of the characteristic physiological changes that our bodies experience during the human dive response. The reduced heart rate slows the amount of oxygenated blood that is being distributed throughout your body, making the oxygen that was taken in during the last inhalation last longer. The average heart rate of a heathy adult on land is approximately 60-80 beats per minute (BPM) and if you're an athlete that number might be closer to 40. However one study found that elite free divers experienced a reduced heart rate as low as 20-24 beats per minute.
Researchers found that another key signal to initiate the dive response is the stimulation of cold receptors on the face. The most important regions on the face were found to be the forehead, eyes, and nose, which elicit a greater degree of bradycardia and reduction in blood flow. There are various hypothesis on the roll of cold water in the human dive response, however it is widely accepted that these regions of the face are neurologically responsible for augmenting stronger physiological changes associated with the dive response. Next time you're stressed or anxious, perhaps repeat that morning routine of splashing cold water on your face to slow your heart rate to calm down a bit.
How does our body interpret and respond to these signals?
The autonomic nervous system (ANS) is responsible for controlling the involuntary functions of the human body such as heart rate, hormone secretion, and digestion. The ANS is made up of the parasympathetic nervous system, controlling various functions when the body is at rest, and sympathetic nervous system, which prepares the body for more intense physical activity like responding in a fight-or-flight situation. When the human dive response is initiated, the parasympathetic system is allows for the reduction in heart rate. The sympathetic system comes into play by causing the blood vessels in our extremities to narrow and allow for less blood to pass through, an effect called peripheral vasoconstriction. This physiological effect is often observed when the body becomes too cold and needs to preserve the warm blood for the more critical organs our core and heat. While this may also be a function in the dive response, the constricting of blood vessels in the extremities is also thought to preserve the oxygenated blood for those more vital organs.
This all may sound contradictory at first. If you can’t breath, wouldn’t you expect the sympathetic system to take over and evoke a fight-or-flight response? In which case, one would experience an adrenaline rush that would increase heart rate and increase blood flow to all areas of the body, including extremities. Instead you have the two seemingly opposing nervous systems working together to produce the most efficient physiological response to conserve oxygen for the regions of the body that depend on it the most.
Not every dive response is the same
Emotional factors also come into play when experiencing the dive response. One study found that when subjects were distracted or harassed, diving bradycardia didn’t occur despite having the face submerged underwater. Interestingly though, it has also been observed that fear is able to produce a significant dive response, for example when a victim suddenly or unexpectedly fell into water. This effect, along with rigorous training, could explain how big wave surfers are able to survive wipeouts that hold them under water in less-than-relaxing conditions for minutes at a time. Unfortunately, this hypothesis of a fear induced diving response would be difficult to formally test due to medical ethics.
Experienced free divers and breath holders experience more significant physiological changes than those not accustom to holding their breath under water. It was even observed that people who participated in just 2 weeks of breath hold training experienced an increase in both the speed of onset and magnitude of the responses that characterize the human dive response such as bradycardia and vasoconstriction. You don't need to train for weeks to test out your human dive response though. Next time you're in a pool or ocean, see how long to can old your breath underwater verses above water. Try to feel for changes in things like your heart rate. Share what you experienced in the comment section below!