What Happens to Our Body When We Enter Cold Water?

Despite spring being on its way, outdoor water temperatures will still be pretty cold in the UK for a few months. Some of you may be considering trying cold water immersion, some of you may have been swimming or dipping throughout the winter, and some of you may be planning on venturing back into the open water soon, but do you actually know what happens to your body when you enter cold water? Having this awareness, especially for new open water swimmers/cold water dippers, should make it a safer and more relaxed experience.

Firstly, what is considered cold water? Anything under 21 degrees (69F), which is often the temperature of open water in the UK in the summer if we're lucky! However, 15 degrees (59F) or below is officially considered to give you more therapeutic benefits. 

The human body is constantly trying to maintain homeostasis or balance. As we enter cold water, the cold shock response is activated and a number of physiological reactions occur in an effort to deal with this cold threat and to keep us alive.

We have temperature receptors all over our skin, and in cold water these send an alarm message to the hypothalamus (our temperature centre) in the brain, indicating that our body is in danger from this cold threat.  

The Sympathetic branch (fight to flight mode) of our Autonomic Nervous System is activated, and the brain sends a signal to the peripheral blood vessels in the skin and extremities to constrict (vasoconstriction). The blood is redirected to the vital organs (heart, lungs, brain, etc.) to keep them warm and oxygenated so they can continue to function and keep us alive in this "dangerous" cold environment. This fight or flight response also releases adrenalin, noradrenalin and cortisol as hormones in the blood and neurotransmitters in the brain.   This Sympathetic activation usually causes a sudden inspiratory gasp reflex in new cold water dippers which can lead to hyperventilation (increased breathing rate) and temporary increases in heart rate and blood pressure.

Once the body is immersed up to the neck or the face is immersed (not necessarily the whole head), the human dive response quickly triggers the Parasympathetic branch (rest, digest, or relaxation mode) of the Autonomic Nervous System and this slows the respiratory rate and lowers the heart rate and blood pressure, to reduce the increased workload on the heart. In response to the release of stress hormones, the body then secretes endorphins, dopamine and serotonin, which are more relaxing hormones to boost mental balance and ease pain.

The body also produces heat by increasing your metabolism. Brown fat is initially activated to generate heat by taking sugar and white fat from the blood. Research suggests that the more cold dipping we do, the more brown fat our body stores. As the body temperature drops, the shivering response in the muscles is triggered in an effort to prevent hypothermia. Ideally, swimmers/cold dippers exit the water before shivering starts or as soon as they experience it.  

This cold shock response all happens within 30 seconds to a minute after entering cold water. Research has shown that after just 4-6 cold water immersions, our bodies become much more acclimatised. The water always feels cold, but it is a great way to build mental and physical resilience and the release of all of those hormones makes you feel great for the rest of the day! We’ll look at all the benefits of cold water immersion another time. 

If you want more in-depth information on this subject, I strongly recommend reading: "Winter Swimming: The Nordic Way Towards a Healthier and Happier Life" based on research by Dr Susanna Soeberg.