Our bodies are ideally suited for standing, walking, and running. Some scientists even argue that running is what set us apart from our ancestors and gave us the competitive advantage over other hominids. Even so, we have such a profound response to water that it changes our body from our blood flow to our skin.

The diving reflex is triggered when we hold our breath (apnea) and cold water comes in contact with sensory receptors on our forehead, eyes, and in our nose. The blood vessels around our non-vital organs, our limbs, and our skin constrict, concentrating our blood – and most importantly, its oxygen – around our brain, heart, and lungs. It also acts to strengthen the torso against the pressures of deeper dives and preserves core body temperatures.

Interestingly, capillary restriction also seems to be the cause behind the wrinkling of the skin on our hands and feet when exposed to water for prolonged periods. This response is believed to occur to give us better traction on wet surfaces and better help us grip wet objects.

This vasoconstriction causes a spike in our blood pressure that is offset by the slowing of our heart. The average resting heart rate is around 60 bpm, but when our diving reflex is triggered it drops to around 40 bpm. The heart rate of professional freedivers slows to 20-30 bpm! To further help us survive longer underwater, our spleen contracts and releases red blood cells into our body, prolonging the amount of time we can hold our breath.

The purpose of the diving reflex is the same for us as it is for aquatic mammals such as seals or dolphins: to preserve oxygen. In aquatic mammals the response is more pronounced and they have more adaptations to help them cope such as the ability to carry more oxygen in their blood and the ability to better regulate their blood pressure and heart rate.

Next time you go to the pool, try an experiment. See how long you can hold your breath before you get in, then see how long you can hold your breath with your face underwater.