As kids, many of us had competitions to see how long we could hold our breath under water. It may have seemed like a simple and fun way to compete with our friends, but our bodies were actually doing amazing things to help us stay under water for longer periods of time. Humans and other mammals have an evolutionary mechanism called the mammalian dive response that allows us to stay submerged under water for long stretches.

Our dive response uses both the sympathetic and parasympathetic nervous systems at the same time. The sympathetic nervous system activates the “fight or flight” reaction when we’re stressed, which includes an elevated heart rate. The parasympathetic nervous system regulates normal bodily functions, such as lowering heart rate and blood pressure.

Typically, these systems’ actions oppose each other. But in this case, they work together to increase the amount of time we can spend without taking a breath. When our face is submerged in water, nerve cells sense the cold. This, in addition to holding our breath, causes our heart rate to slow and our peripheral blood vessels to constrict, which makes our blood pressure increase. In addition, studies have shown that the spleen contracts to release extra blood cells into circulation. All of these factors combine to extend the period of time we are able to spend under water without replenishing our oxygen.

We can see striking examples of a strong dive reflex in seals and whales. When seals dive, blood flow to their skin and peripheral organs decreases significantly and their heart rates slow down. This helps them preserve oxygen to prolong the length of their dives. Some seals can even hold their breath for two hours! Whales typically have a diving heart rate of four to eight beats per minute, but scientists have seen some whales go as low as two beats per minute.

In humans, the best example of dive response is the Haenyeo and Ama divers in Korea and Japan. These women have trained their dive response since birth and dive multiple times a day every day. Due to their diving frequency, Haenyeo and Ama divers are able to store oxygen in blood and muscle more efficiently and have better oxygen exchange capability. Their circulating red blood cell count can increase by 10% when they dive, which gives them better oxygenation of the blood, a stronger dive response and a much longer dive time.

Next time you’re swimming under water, think about all the things your body is doing to help you win the breath-holding competition at the pool.