cold water swimming
cold is emotional
COLD WATER SWIMMING
The page is intended to pull together information from a range of sources, such as health websites to medical journals, to describe the complex physiological and emotional responses exhibited when you go for a swim in cold water. It summarise a complex subject so it is not exhaustive, though it covers many questions and topic raised about cold water swimming. It is intended as a reference to dip into, but makes more sense if you read through it all. We learned a lot researching it and hope it helps you too. All the sites used are listed in the references at the end, if you want to dig deeper yourself. If you spot and errors or have questions not covered, send a message via our contacts page.
- This articles is not intended to tell you where to swim, how to swim, when to swim, what to wear, which are up to you.
- You always swim at your own risk. If unsure – book an open water coaching session, see our contact page.
- Cold water swimming should be avoided by anyone with an underlying heart condition. If in doubt, consult your doctor.
WHY CHOOSE COLD WATER SWIMMING?
Some enjoy the feeling of cold water, even becoming addicted to it. Some avoid heated indoor pools. Some miss the exercise over winter so put up with a cold dip. Some do it for health benefits. Some for social and doing an activity with others. Whatever the reasons, cold water swimming will stress your body in ways no other activity will. There are plenty of studies that show swimming in cold water is beneficial if done carefully, however there is still much that is unknown and research is ongoing.
For most people it tends to be when the water is under 15C and often triggers shock, gasping and a desire to get out. These feelings are a natural reaction by your body; they are a survival stress response from our tropical evolution, so anything colder is considered a threat. As a comparison, the thermoneutral water temperature for an immobile undressed person is about 35°C where very gradually cooling occurs over a long period. Generally, the greater the temperature difference between your skin and the water, the more likely it will trigger a stress response, and the colder the water the greater the overall response.
There is a misconception that all stress is bad for you. Stress can be good for you if used in ways to benefit your health. Bad stress is known as distress, but good stress is known as eustress, which is a term invented by endocrinologist Hans Selye in 1975.
- Distress is where you feel unable to control whatever is making you anxious so it becomes intolerable and bad for your health. Distress has a terrible impact on productivity, creativity, and mental health.
- Eustress is controllable and tolerable, short term and exciting, pleasurable, motivating, challenging yet achievable and leads to a positive outcome.
A stress response is an automatic survival instinct evolved to preserve homeostasis. Homeostasis is the balanced state of internal, physical, and chemical conditions needed to preserve life: if homeostasis is successful, life continues; if it is unsuccessful, it results in a disaster or death.
At a cellular level a steady internal body temperature is needed to metabolism enzymes to repair, grow and live. Think of your body as a big chemical factory with a drive to survive. Cellular metabolism is supported blood flow delivering hormones, nutrients and oxygen from upstream digestive and respiratory systems. So internal sensors automatically monitor things like temperature, blood pressure, hydration, respiratory rate, glucose levels to achieve homeostasis and your general wellbeing.
Threats to homeostasis are detected through your senses ie sight, smell, hearing, touch etc. This sensory information is fed to your brain to decide an appropriate response. This happens in an instance and why you can react with lightning speed when needed. Your brain draws on past memories and their associated emotional value, so previous threats and responses are recalled quicker for survival.
In the right doses, you can use stress to benefit your wellbeing. Friedrich Nietzsche said: “That which does not kill us, makes us stronger.”
All life requires stress to function, grow, and survive. Without challenging your body or mind they will wither. At the right dose, stress induces changes in cells, hormones, neurons, organs, and nervous system function to produce increased skill, strength, health, immunity, or cognition. The ideal is to push yourself enough but without being overwhelmed. Setting challenging yet achievable goals. Finding the “sweet spot” where stress is beneficial: too little leaves you unchallenged; too much may lead to health problems. However, everyone is different with different thresholds, which will also vary day to day.
Hormesis is the use of good stress or eustress for beneficial effects. This is an approach whereby your body derives long-term benefits and strength from short, potent episodes of challenge or stress. To be beneficial, the stress must subside at some point. For example, over-exercise can lead to injuries whereas shorter bursts provide sufficient stress to avoid injury and get stronger. Methods of training your body to handle a controlled amount of stress so you are better able to handle other stresses is also known as cross-adaption.
Cold water swimming is one approach but there are plenty of others, it triggers your stress response to release a flood of hormones that cause other physiological changes. Short durations have a positive impact on your wellbeing. Staying in too long may negate any benefits and risk severe hypothermia.
Cold water swimming is not for everyone, especially anyone with a heart condition. You may have read articles saying how good it is for wellbeing and feel pressure to jump on the bandwagon. Ignore the pressure, you have options because many other activities provide the same main benefits, namely:  Exercise: regular, moderate-intensity exercise is one of the best stresses, stretching the body and mind  Social: isolation is not good for wellbeing so meet and chat with others  Challenge: provides a sense of achievement  Outside: being outside in a natural environment helps mood and wellbeing.
So, if you don’t like cold water swimming, you try other activities like running, cycling, walking. These can be done on your own but try to do some as part of a group to provide beneficial social interaction. Exercise can be indoors but being outside brings other benefits. Here are a few other suggested “stressors” you could use to your benefit: Heat exposure: sit in a sauna if you don’t like the cold; Diet: intermittent fasting is another stress. Eating chillies is another; Cognitive activities – challenge your mind, crossword puzzles or learn a language. So mix and match to suit you, however, if you are still interested in cold water as your main form of stressor, then read on.
Getting into cold water is possibly the biggest stressor the average person’s body will tackle. Nothing else will shock your body in the same way as ice cold water hitting every part of your skin. It is your biggest organ and covered with hundreds of thousands of nerve endings all sending alarm signals to your brain at the same time. There are ways to get the benefits while reducing this shock, let’s look at four factors:  Location of the cold water  Temperature of the water  Duration you stay in cold water  Exposure and amount of your body you wet. The following are a few suggestions you can try if you want to use cold water as a stressor but minimise the shock and risk.
First off, you get all the benefits in the first few minutes, starting when the cold water triggers the initial shock until the point when you calm yourself and get your breathing back under control. Beyond this and you are straying away from all the cold therapy benefits into hypothermic territory. Bear this in mind if you want exercise beyond the immediate cold therapy and the longer you swim for exercise the colder you will get.
- Take a shower: this offer the most control while done in a safe environment at home. You decide how long to stay in, the temperature of the water, the duration and how much of your body to spray. Start on your legs and work your way to a full shower
- Take a cold bath fill your bath with cold water, add ice cubes to lower the temperature further if you want. Have someone with you or nearby if you are new to it. Maybe wear a top to help regulate your body temperature.
- Place your face in cold water
- Lidos such as Portishead lido often reduce their water temperatures over winter, running their heating at a lower setting to reduce heating costs. It is outside, you swim in with others, there are lifeguards and changing rooms.
- Sea temperatures rarely get as cold as lakes and rivers in the winter. Seas are like big radiators and slowly cool to their coldest around March. Clevedon sea is an estuary and gets colder than seas in Devon and Cornwall.
- Lakes and rivers are affected by air temperatures so go up and down over a winter
- Wetsuits keep you warm, wear a full wetsuit or a shortie or just gloves and booties depending how you feel about the cold.
- Stand in the water up to your waist, duck under if you feel like it. You will not get as cold standing in the water as you will swimming in it because you warm a small layer of water around your body.
- Swim breaststroke or head up front crawl to keep you head out of the water
To feel the benefits from hormesis and eustress, you need to feel the stressor is manageable and you are in control; feeling helpless makes the stressor toxic. To get the best from a swim session, pay attention to the dose/response relationship: high doses are toxic; low doses need to be uncomfortable but achievable so they are beneficial and activate positive responses, like resilience, repair, cellular pathways etc.
Cold water is the ideal stressor: it is a controllable stressor and there is probably nothing else that will shock your body so suddenly and acutely. Control in terms of how long you stay in, whether you swim or stand still, up to your waist or neck, head in or out, wetsuit or skins. It is up to you.
So, you need to use cold water as the stressor in a controlled way that works for you. You know your body and capabilities and what will work day to day, dip to dip. There comes a point where you get diminishing returns for effort expended, when you stay in too long but won’t necessarily improve your acclimatisation or experience, instead, you get hypothermic and don’t enjoy it.
While hormesis is about manageable bouts, you must also pick when you undertake your swim, assess your current stress levels, beware of doing too much because trying to fit it all in around a packed schedule will just add to your stress; if your cup is already overflowing, it may not be appropriate to then undertake extra stressful hormetic situations. Do it when you feel like it, but don’t find excuses to avoid. This is another reason for doing it as a social group, they will help motivate you but if you really don’t feel like it then don’t, but you will still have got out and socialised. Also beware of certain rhythms and when you have low energy and may sensitise you to cortisol, the stress hormone.
Women should consider easing off just before their period, when your oestrogen levels drop and can lead to cortisol sensitivity (your stress hormone) so your body is susceptible to negative effects of additional stressors. There is also your circadian rhythm which we cover under Endocrine system.
The ideal is to combine cold water as your stressor but do you swim with a group but try and focus on your own swim, call it the golden triangle. You could do these separately but the compound effect makes them so much more potent together. In the words of Aristotle, “the whole is greater than the sum of its parts”.
WHAT ARE THE BENEFITS?
Studies have shown swimming in cold water benefits you physically and mentally in the following ways.
Swimming hard raises your heart rate and when your heart is about 80% maximum rate, it burns more calories than running because swimming is a whole-body exercise and water resistance is higher than air. However, swimming in cold water adds two further ways of burning calories:  your heart pumps faster in cold water and your body works harder trying to keep you warm;  shivering burns calories when you get out.
Your cold water response increases blood pressure from vasoconstriction and shunts blood from your periphery to your core, flushing your cardiovascular system and improving circulation and health.
Swimming is aerobic and makes your heart and lungs stronger, tones and builds muscles, and builds strength and endurance. Cold water increases this effect when it shunt blood from your limbs to your core, making your cardiovascular system work harder, including muscles whose efficiency is reduced by cooling.
Cold stress response also triggers your defensive fight or flight responses, helping boost white blood cell count which fight off viruses, bacteria, and improve general wellness. Repeated cold exposure builds this in.
Your brain is like a muscle, so it needs to be exercised to keep it in good condition. New experiences stimulate your brain, helping mental growth, inactivity results in withering. Triggering your stress response by cold water swimming stimulates your entire nervous system and a chemical soup of hormones. This results in neurological benefits from neurogenesis- generation of new neurons, and neuroplasticity – adaption of existing neurons from encountering challenges.
Cognitive abilities change across your life span. Exercise is know to improve cognitive abilities, whatever age you start exercising in life. It affects fluid type abilities, executive control functions, management processing type functions like planning, multi-tasking, spatial awareness. Exercise also leads to better sleep patterns. It also results in increased neurotrophins needed to regulate development, maintenance, and functions of nervous systems.
Swimming is a low impact exercise so good for you if unable to do weight bearing exercises. Hydrostatic pressure surrounding and supporting your body acts as a compression device, helping to reduce swelling. The water pressure around your chest means you work harder to breath, helping to develop stronger breathing capacity out of the water. Cold water triggers endorphins, your body’s natural pain killers, helping to relieve pain and inflammation.
A stress response releases various “happy hormones”, like endorphins, serotonin and dopamine, which help you cope with the cold but also leave you with positive feelings and improved mood that can last for hours or days.
If the water is very cold it is advisable not to swim alone. Meeting others combats loneliness and isolation and many swimmers build new lifelong friendships. Doing a challenge together and watching out for each other builds camaraderie. By providing a topic of conversation, cold water swimming can decrease social anxiety too. A useful tip: cake is always welcome after a swim, not only does it provide fuel to aid shivering and rewarming but offering it round helps break the ice and meet others.
WHERE CAN IT GO WRONG?
The two biggest threats to cold water swimming, are immediate cold water shock response and hypothermia. As hypothermia is gradual, it can be broken down by duration in the water and afterdrop when you get out. How quickly hypothermia develops will vary person to person, day to day, location to location and other factors which are covered later.
Cold-water shock is the immediate response when you first get in: increased heart rate, uncontrolled gasping if very cold, and panic to get out. This is your body’s automatic fight or flight survival response to any perceived threat, called an acute stress response. It can last a minute or much longer, depending on the water temperature. The danger is from jumping in and inhaling water from a gasp reflex. It can also trigger cardiac arrest or stroke in anyone with an underlying undetected heart problem. It is best to get in gradually, calm your breathing then begin swimming.
Having calmed yourself and stayed in, you are now rapidly cooling. Your body will have triggered various mechanisms to protect your core temperature. In particular peripheral vasoconstriction will narrow blood vessels to your skin to reduce heat loss. Your arms have a high surface to mass ratio compared to your body so they will cool quicker. This may cause cramps or loss of dexterity preventing you swimming back to safety or getting out when you reach land, or worse, make it difficult to stay afloat and keep your airways above the water. Swim failure can affect people without a significant drop in core temperature.
Many people do not realise how cold their core is getting, some even feel warmer. Sensory nerves in your skin called thermoreceptors detect the cold water, trigger a stress response warning you to get out. Thermoreceptors detect temperature change so if you choose to stay in there won’t be any change unless you find warmer or cooler areas of water. Your skin will have quickly cooled to the water temperature compounding the situation. So, your thermoreceptors focus will be on the external water temperature as a threat and not your internal cooling rate. Hypothermia is a creeping condition and as it worsens, your brain cools, and decisions become sluggish and confused, preventing you recognising how much your core has cooled. Eventually, reflexes disappear and your nervous system stops regulating your cardiovascular system and lethargy leads to comatose. In the worst cases, heart arrhythmia, heart attack, stroke or drowning may follow
Afterdrop is when you exit the water but your body continues cooling, even if you get out after a short dip. This is due to convective heat transfer between your warm core and your cold skin, which will have cooled to the water temperature. This cooling process will continue until your periphery begins to warm again. Afterdrop can be dangerous if your core continues to drop to a very low level. This is why it is advisable to dress quickly after getting out to halt afterdrop and help your natural rewarming processes.
Hypothermia should not be confused with the cold water shock, which is the immediate response when you first get into cold water. Hypothermia occurs gradually as your core slowly cools below its normal 37C. Most definitions say hypothermia is when your core cools to 35C or less. However, when your core temperature moves less than one degree below your norm it usually triggers responses, like shivering, so better to be cautious and if it looks like hypothermia then treat it as such.
Hypothermia develops slowly while your body is fresh and able to respond but your core may drop quicker as you tire. The colder the water temperature the quicker hypothermia will develop. To avoid hypothermia be cautious and keep swims shorter, particularly when air and water temperatures are very cold. Your core body temperature is about 37°C, below are general symptoms displayed as your core temperature drops.
- Mild Hypothermia (36-35°C)
- shivering (36°C)
- confusion, disorientation, introversion (35°C) sometimes referred to as the “umbles”: stumbles, mumbles, fumbles, and grumbles
- Moderate hypothermia (35-33°)
- amnesia (34°C),
- cardiac arrhythmias (33°C)
- Severe hypothermia (< 33°C).
- brain fog (33-30°C),
- unconsciousness (30°C), preventing an individual from maintain a clear airway causing drowning
- ventricular fibrillation (VF) (28°C), Less than 28°C, the heart may stop, or stroke develop.
- death (25°C).
Hypothermia is more difficult to recognize than it is to treat.
Mild hypothermia: Remove wet clothing. Dry off and dress in warm windproof clothing. Cover the head with a hat. Move to shelter from any wind. Insulate from the ground. Drink or eat some high energy food to fuel shivering. Go for a walk to help rewarming.
Moderate hypothermia: Do the same as for mild hypothermia, but motor skills and cognition may be affected, so help drying and dressing may be needed. Be gentle, overly rough care can shock cold heart tissue into an arrhythmia. Tell the casualty what you are doing and ask permission. If they are unable to reply then treat them, assuming implied permission has been given because of their state.
Severe hypothermia: Do the same as for moderate hypothermia. CALL 999 & GET A DEFIBRILATOR if one is close by for two reasons:  to help detect a pulse, which may be difficult because it is probably weak and hard to find on cold, vasodilated skin  the defibrillator will also tell you when to do CPR because unnecessary chest compressions can easily send a profoundly hypothermic patient into a potentially lethal arrhythmia.
Foil blankets: Using foil blankets is different for cold swimmers than for hot runners. A hot runner’s skin will radiate heat so a foil blanket will trap and reflect back radiated heat reducing rapid heat loss and them getting cold. A hypothermic swimmer’s skin will be cold so a foil blanket placed directly next to cold skin will just reflect back cold, increasing any afterdrop effects. So wrap them in dry clothing or towel then apply a foil blanket if you have one, as they make ideal windproof outer layer around warm clothing.
WHY DO WE COOL?
A warm object loses heat to something colder by heat transfer through radiation, or conduction, or convection. Evaporation increases convective heat loss. This applies to you too. The speed and way you cool differs between air and water. You cool quicker in water than in air and the way you lose heat changes too.
NOTE: a small amount of heat is lost as cold air breathed in is warmed to body temperature. Breathing rate may increase during swimming, so the amount of heat lost may be slightly greater.
- Radiation: 65% of heat lost
- Conduction: about 2% of heat lost
- Convection: 10% to 15% lost
- Evaporation: intense exercise may trigger sweating and cause heat loss of about 85%
- Radiation: Minimal loss – our skin surface cools within seconds to the surrounding water temperature (and why water stops feeling cold, so beware)
- Conduction: Most body heat is dispersed through conduction and convection. You heat a small layer of warm water around your body, which may be lost by convection.
- Convection: heat loss happens on two fronts: convection OUTSIDE as cold water flows round your body taking heat away, and convection INSIDE your body as warm blood flows to active muscles to provide oxygen and nutrients. Conductive heat loss in water is usually 25 times greater than air, however, due to this dual effect, the rate of heat loss is increased 1.5–4.5 times further. There are two kinds of convection: natural flow (eg currents) and forced flow (eg swimming). So, assuming no currents, convective heat loss can be reduced by standing still and pulling your limbs in close to your the body to reduce heat loss. This is the basis of the HELP (Heat Escape Lessening Position) position taught to sailors should they fall overboard. It protects the body’s three major areas of heat loss (groin, head/neck, and rib cage/armpits) by drawing knees up to chest and hugging them while keeping arms pressed to sides, easily done wearing a buoyancy aid.
- Evaporation: no heat loss from the submerged parts, some loss from those above the water, like arm during front-crawl, or wind blowing down your back.
- Radiation: radiated heat will be non-existent until your periphery begins to warm again.
- Conduction: you will continue cooling through conductive heat transfer from your warm core to your cooler periphery, called AFTERDROP.
- Convection: a cool environment and any windchill will prevent your body’s efforts to rewarm you and why you should dry and dress quickly
- Evaporation: wet skin and clothes will enhance convective cooling when standing on a cool ground, so wear insulated shoes and stand on a thermal mat
The following are other environmental and bodily factors that will affect the rate of heat loss from your body and causing you to cool quicker.
The colder the water the quicker you cool. Ambient conditions also affect cooling, in and out of the water. Radiated heat from sunshine will warm body parts above the water and warm you post swim. Air temperature will affect exposed body parts, especially if the air is very cold, when convection and evaporation will cool them quicker, especially if it is breezy. Wind chill is a term used in colder months, sometimes called “feels-like” temperature because it is how cold air temperature feels after wind removes the thin layer of warm air above your skin. Wind chill is calculated using factors such as air temperature, wind speed, and evaporation rate of bare skin. For example, if the air temp was 10C and wind speed 15mph the wind chill would make it feel 7C.
Heat lost to water will be quicker than in air because water has a higher thermal capacity, meaning its molecules are closer together so it is denser and quicker to heat up. As a comparison, running hard in an air temperature of 10C will probably keep you warm or even sweat. No matter how intensely you swim in the water of the same temperature, your core will eventually cool. You cool quicker in water because of the combined effect of water transferring heat quicker than air, plus two convective heat flows helping the process: cold water on the outside and warm blood inside flowing to working muscles, cooling near the surface and returning to your core. Swimming intensely to try and generate heat will likely result in you cooling quicker than standing still.
Cold water is more viscous than warm water because the molecules are wiggling less rapidly, so they are effectively stickier. Water viscosity increases effort and energy expenditure when swimming in cold water, but this effect is somewhat compensated by the increased pulling power per stroke due to the increased viscosity. Although the efficiency of each stroke may improve, the shivering and increased muscle tone during exercise in cold water may further reduce muscular mechanical efficiency due to increased shivering activity causing an antagonistic effect.
Your peripheral shell (skin, muscle, fat) is your natural insulation layer. Fat provides the most insulation, then muscle, then skin. Fatter people have more natural insulation than thinner people. Thicker muscle is not as good as fat but better than none.
A large surface area will lose heat quicker than a smaller surface area and a large body mass will retain more heat than a smaller body mass. Arms will cool quicker than legs because of their greater surface-area to mass ratio. Arms provide 90% of propulsive power in front crawl, legs 10%. In breaststroke, propulsion is 70% legs and 30% arms.
Muscle efficiency decreases as they cool. Peripheral vasoconstriction (blood vessels narrowing) will reduce blood flow to arm and leg muscles to protect your core temperature, reducing oxygen delivery for them to work properly. Blood lactate will also build from your muscles using locally stored glycogen in place of blood oxygen (swimming becomes less aerobic and more anaerobic), which can speed the onset of fatigue and mental confusion (blood flow to brain drops by 20% at 10C and cognition slows). Your peripheral muscles become less and less efficient, you get slower produce less heat and cooling speed up and eventually leads to swim failure. Never swim too far and save some energy for the return leg.
Cold tolerance is affected by gender where it affects body size, shape and composition. Women tend to have a higher proportion of body fat and their subcutaneous fat is thicker than men of comparable age. This provides them with better insulation so they are able to tolerate cold water longer without shivering. However, where men and women have similar fat thickness, women tend to have a greater surface area over which convective heat loss can occur, and smaller body mass to retain heat so core temperature falls quicker. In part this may be due to subcutaneous fat distribution around the body, which is driven by hormones and genetics, and therefore how well it then protects core temperature: women have more oestrogen, so subcutaneous fat is thickest in peripheral areas like bottom, hips and thighs, all places from where blood is shunted away from in cold water; men have more testosterone, so their subcutaneous fat is thickest in the arms, shoulders, abdomen, lower back which are better for protecting core temperature.
Both very old and very young people are especially vulnerable to the cold. There are several possible reasons. Vasoconstrictor responses to cold may be slower in older people than younger. Shivering may be less effective in older than younger people as metabolism slows or age-related loss of muscle mass. Older people lose their ability to sense temperature, which blunts behavioural responses to combat cold like shivering.
Acclimatisation can improve coping with cold by enhancing your fight or flight response to cold stress, developing a better shivering response, and quicker peripheral vasoconstriction response. Acclimatised swimmers experience a reduced core body drop and are able to tolerate longer swims than unaccustomed fatter pool swimmers.
Heat loss from any part of your body is largely dependent upon surface area and your head is no different. Your head is about 10% of your body’s total surface area, therefore only about 10% of total body heat is lost via your head. However, head submersion has a profound effect on core body cooling rate, speeding it on average by 42%. This is thought to be due to a redistribution of blood flow in response to stimulation of thermosensitive receptors and trigeminal nerves in the scalp, neck and face. Two thoughts on why:  quantity of blood vessels close to the surface of the scalp, cooling warm blood that returns to cool your core  shivering is not triggered when your head gets cold – shivering uses torso and peripheral muscles to generate heat and reverse cooling, so not shivering means you will cool quicker than normal.
Alcohol and other drug depressants affect metabolism and enhance heat loss. Although drinking alcohol can make you feel warm, it causes vasodilation (widen) so you lose heat quicker. It also blunts your shivering response to keep you warm, delaying its onset and reducing its duration. Alcohol also increases cold diuresis (increased urine production), reducing blood volume and your muscles working capacity. It also reduces cognitive ability, slowing thinking and increasing bravado reducing the ability to recognise dangers from cold water.
- Fitness: high fitness level enables greater heat production
- Fatigue: exhaustion results in decreased metabolic heat production
- Nutritional state: hypoglycaemia reduces shivering and increases cooling
WAYS YOUR BODY KEEPS YOU WARM
If you had no clothing to keep you warm, your body has two physiological ways it tries to keep you warm: heat loss reduction and heat production.
Acclimatisation helps improve the following physiological processes. Metabolic acclimatisation is enhanced thermogenesis, the ability to produce heat more effectively. Insulative acclimatisation is enhanced mechanisms reducing body heat such as vasoconstriction responses.
This is achieved by reducing blood flow to your skin to create an insulative layer to reduce heat loss to the colder outside environment. Basically, warm skin is sacrificed to maintain a warm core. The vasomotor centre area of your brain controls your blood vessel diameter. Vasoconstriction is when they are narrowed to reduce blood flow and heat loss when it is cold. Vasodilation widens them to increases flow and heat loss when it is hot. This process uses “smooth” muscles surrounding veins and venules (small veins), arteries and arterioles (small arteries), but not capillaries as they do not have a muscular wall. A small amount of blood flow is allowed to your fingers and hands via a hunting reaction to protect them from freezing. In fingers this is via transient increases in skin blood flow, in the forearm it is via muscle blood flow.
Your body generates heat from metabolic processes which can be divided into muscle contraction accounting for approximately 70% of total energy expended when exercising or shivering, and digestion and basal metabolism (ie breathing, heart beating) both of which use a small amount of energy. Approximately 70-95% of energy produced is released to the environment as heat.
While shivering is involuntary, we can keep warm by choosing some form of physical activity. This will generate the most heat because it uses larger muscles that generate heat when they contract. To contract muscles consume (oxidise) fuel in the form of carbohydrate and fatty acids stored in special cells in your muscles called mitochondria. These cells are like little factories producing adenosine triphosphate (ATP) to fuel muscles and releasing heat in the process. If exercise intensity is increased or prolonged, you need more oxygen to help produce ATP and why you breath harder and faster. If exercise intensity and efficiency are kept constant, heat production attains a steady state within about 10 min and continues while exercise continues. This is good if you are running as it keeps you warm, but you lose heat quicker in water.
Shivering is an automatic response when you get cold. It is a pattern of repetitive, rhythmic skeletal muscle contractions. Generally, it starts in torso muscles and spreads to your limbs. Shivering may start immediately or a while later, depending how quickly your core cools to trigger it. Depending on the intensity of your shivering, your metabolic rate can increase two to fivefold. Shivering can generate heat at a rate of 10 to 15 kJ/min. To sustain shivering after a swim and help rewarming, it is important to maintaining adequate blood glucose concentrations by eating or drinking some high energy food, like cake.
Non-shivering thermogenesis (NST) is metabolic heat production not associated with muscle activity. It is thought a substantial part of NST can be attributed to brown adipose tissue (BAT) amounting to about 5% of basal metabolic rate, which is very small anyway. It is possible other tissues contribute to NST to a lesser degree such as skeletal muscle, liver, brain, and white fat. NST contribution to keeping you warm is tiny but mentioned here for completeness. Of more interest is that it burn calories and helps weight loss.
Homeostasis is the term to describe your body’s drive to attain a healthy internal state. Mostly this happens unconsciously through your sensory organs. These detect a change in our environment, internally or externally, which triggers a bodily response to maintain equilibrium. Internal balances like your core temperature, blood pressure, hydration, respiratory rate, glucose levels, etc, or external threats like jumping into cold water. Mostly, the response from your systems is what is called a “negative feedback”, which counters the stimuli. For example when you feel hot your body sweats to cool you; if you feel cold your body shivers to warm you. A “positive feedback” is one that continues a stimuli, for example giving birth dilates the cervix which needs to continue and dilate further.
Next we look at interconnected systems within your body that interact to ensure homeostasis whether you are resting or when a threat is detected, like getting into cold water.
- Integumentary system (skin) is your interface to the outside world
- Nervous system is your brain, spinal cord and nerves throughout your body. Your brain receives information from your senses and tells your body how to respond
- Limbic system links memories with emotions and draws on past experiences to cope with stress
- Endocrine system supports your nervous system, controls human behaviour, including emotional and stress responses, and is the source of many of the rewards sought from cold water swimming, making it addictive.
- Cardiovascular system transports blood around your system, delivering oxygen, nutrients and hormones from your endocrine system, keeping tissue throughout your system alive and well.
This is your skin, it is a flexible, waterproof, protective layer, providing cushioning for your muscles and skeleton, and keeping your vital organs warm and protected. It is also your interface to the surrounding environment through special sensory receptors: hear, see, feel, taste, smell. Your skin is your biggest organ, it is approximately 15% of an average adult’s total body weight. Skin thickness varies across your body: the thickest skin is on your back, palms of your hands, and soles of your feet, where it can be up to 3mm thick. The thinnest skin is on your eyelids, where the epidermis measures about 0.05 mm. Skin thickness increases until about age 40, then it slowly thins, particularly in the dermis layer. Skin has three main layers: epidermis, dermis, and subcutaneous layer.
This is the visible, outer surface and the thinnest layer. It is a tough layer of skin cells in tightly packed sheets which are in a constant state of renewal while older outer cells slough off. It does not contain any blood vessels. The epidermis creates a barrier that helps slow down the evaporation of water, as well as prevents excess water from being absorbed into the skin while swimming. Merkel cells are one type of skin cell; they are touch receptor cells and most numerous in tactile areas like lips, fingertips and hair shafts. They secrete chemicals that relay information directly to the brain enabling you to feel the lightest touch.
This is the middle layer of your skin and gives skin its structure and elasticity. It is the thickest layer of skin, contains connective tissues, nerve endings, sweat glands, oil glands and hair follicles.
This is the lowest layer of your skin and connects your upper layers to muscles beneath and acts as a cushion, protecting bones and muscles. It is also called the hypodermis layer. This layer varies in thickness, the thickest is on buttocks, palms and feet. Thickness also varies by hormones: higher testosterone, your hypodermis is thickest in your abdomen, arms, lower back and shoulders; higher oestrogen, hypodermis is thickest in your butt, hips and thighs. It is mainly made of adipose tissue (body fat) it also contains blood vessels, nerves, and loose connective tissue. Adipose tissue can be classified as either white or brown fat
- WHITE FAT: is your energy store as large fat droplets around your body also providing insulation for your organs and helping to keep you warm. Too much white fat can cause obesity. It is particularly bad around the midsection (visceral fat) creating a higher risk of heart disease, diabetes, and other diseases.
- BROWN FAT: stores energy in a smaller space than white fat. It is packed with iron-rich mitochondria, which is how it gets its colour. When brown fat burns calories, it creates heat without shivering, called thermogenesis. Babies are born with it around their shoulders and neck because they don’t shiver for the first six months then tend to lose most of it. Scientists believe some white fat can be recruited as brown fat through cold exposure. Brown fat may help control blood sugar and improve insulin levels, decreasing the risk for type 2 diabetes
It is a mistake to think you can protect your internally core temperature by how your skin feels. It is an easy mistake based on your experiences. For example, on a cold day exposed areas of skin like hands and face feel cold or you feel the water with your hand before getting in for a swim, so you consider they reflect the ambient temperature, not your internal temperature. These are examples of a feedforward system. However, your protective autonomic thermoregulation controlled by your hypothalamus is based on feedback, not feedforward. To help understand the difference, here are a couple of examples:
- FEEDBACK: on a cold night a house thermostat detects internal air temperature cooling below a set level so triggers heating to switch on and rewarm the internal air. The feedback system monitors a controlled variable, in this case air temperature inside the house
FEEDFORWARD: a system detects the fall in outside air temperature then turn on the heating before the air inside cools. The feedforward system does not depend on the controlled variable, instead it monitors things that threaten it ie the outside environment.
Deep body temperature is relatively constant due to your autonomic systems maintaining homeostasis. Skin temperature will fluctuate due to external environmental effects and internal thermoregulatory changes: at any point your skin could be vasoconstricted or vasodilated, which is driven by core deep body temperature. Therefore, because skin temperature is not independent of body temperature, it cannot represent ambient temperature and be a feedforward measure of the environment.
So the best advice is to get out before you do feel cold.
Skin is capable of two opposite responses to help thermoregulation:
- abrupt cessation of heat loss by cutaneous vasoconstriction
- rapid dumping of heat by cutaneous vasodilation, occurs when the blood flow through these organs is increased (vasodilation) and directed to the anastomoses. Blood flow to your finger can increase by 500%
If you in a comfortable thermoneutral temperature, on holiday in warm location, your body will continue to thermoregulate your internal temperature by subtle adjustments of your skin, either vasoconstricting or vasodilating blood flow to if you become hot or cold.
These responses are your body’s first line of thermoregulation defence because of their efficiency: no loss of water through sweating or fuel burnt by shivering or non-shivering thermogenesis (a small amount is used for vasomotor control of smooth muscle when constricting or dilation blood vessels).
Glabrous skin (palms, underside of wrists, ankles, soles, forehead and lips) also act as biological radiators particularly good at dumping heat, covered below.
You have two types of skin:
- hairless (glabrous) palms, underside of wrists, ankles, soles, forehead and lips. Blood vessels to these areas are innervated solely by vasoconstrictor nerves ie they reduce blood flow
- hairy (non-glabrous) covers the rest of your body. Blood vessels are innervated by both by vasoconstrictor (reducing) and vasodilators (widening) nerves
Glabrous skin areas have common features other than being hairless, they have a dense mesh of blood vessels, and a large surface-to-volume ratio. This lends them to a unique role in thermoregulation, particularly heat loss, even though glabrous skin constitute a small percentage of total body surface area. Palms make up only five percent of a body’s surface area.
The specialised meshes of blood vessels are called arteriovenous anastomoses and where an arteriole directly connects with a venule instead of flowing through capillaries. Blood flow slows at these junctions which are close to the surface. As a result, these meshes not only help shed heat quickly from the body during vasodilation but as cooled blood returns to your core through the venous return systems it also cools your body further.
Palms in particular are a specialised organ, they are a biological radiators for heat loss because of their glabrous skin, network of blood vessels at the underside of the wrist and sweat glands that help heat loss further. If heat loss through glabrous skin doesn’t cool your body sufficiently then blood flow to non-glabrous (hairy) skin can help increase heat loss through your skin.
The above mainly covers shedding heat through glabrous skin. Two points in relations to cold water swimming: vasodilation would quickly stop heat loss while swimming in cold water; when you have finished swimming holding something warm in your hands and putting the sole of your feet on something warm, like a warm hot water bottle, will help speed up rewarming.
There are three phases of when you go cold water swimming that are represented by skin colour changes.
- Severe vasoconstriction reduces blood flow to your skin, so its pale white colour is due to virtually no blood flow to your skin.
- Your skin then typically becomes a purplish-blue colour (called acrocyanosis) as a reduced flow of blood through the skin returns (called hunting).
- When your blood vessels fully recovers, they dilate, allowing blood flow to resume and your skin may blush, becoming very pink or red.
Your limbic system is a set of tiny organs at the centre of your brain responsible for storing memories and linking them to emotions. These experiences are referred to later by your limbic system to decide how best to cope with a new situation, it then exerts control on your Endocrine System and Autonomic Nervous System directing an appropriate response. Your Limbic system has the following components and you will see from their descriptions how important and central your limbic system is to handling stressors, like cold water swimming.
- THALAMUS is gatekeeper for incoming sensory information passed between your spinal cord and cerebellum
- AMYGDALA is your master controller, assessing and regulating emotional responses, such as happiness, fear, anger, and anxiety based on sensory information (sight, taste, hearing etc). When it interacts with the hippocampus, it ties emotional meaning to your memories and decides where they are stored, based on a strong or weak emotional response, especially threatening experiences so a learned response is activated quicker (“fear learning”) for protection and survival. Learning about your surroundings from emotional experiences is better and more efficient, whether positive or negative emotions, because they are how we have evolved. Boring stuff does not affect survival so is ignored by your brain.
- HYPOTHALAMUS is directed by your amygdala and regulates basic bodily functions, such as blood pressure, temperature, sleep, hunger, thirst, pleasure, pain, anger and more by controlling your autonomic nervous system and your endocrine system.
- HIPPOCAMPUS is critical for cognitive functions, such as episodic memory formed in it then filed away into long-term storage throughout other parts of the cerebral cortex, helps spatial navigation and has been associated with learning and emotions, and is where new nerve cells are made.
- CEREBELLUM processes procedural memories: balance and posture from eyes and ears, movements learning and refinement eg swimming or riding a bicycle
- CINGULATE GYRUS provides a pathway from the thalamus to the hippocampus. It is responsible for focusing attention on emotionally significant events, linking memories to smells and to pain, and controlling autonomic motor function. It helps deal with fear and prediction and avoidance of negative stimuli, through monitoring the body’s response to past unpleasant experiences.
- BASAL GANGLIA are responsible for repetitive emotional behaviours, rewards and reinforcement experiences, and focusing attention. It regulates voluntary movements, including eye movements, helps with balance and posture.
- VENTRAL TEGMENTAL consists of dopamine pathways responsible for pleasure.
- PREFRONTAL CORTEX is closely linked to the limbic system. It is involved in thoughts about the future, making plans, and taking action. It uses the same dopamine pathways as parts of your limbic system so plays a part in pleasure and addiction.
That moment when you walk towards the water to get in, can you recall the emotion you felt? For most people it is a feeling of anxiety, some may have a different emotion. Whatever you fell, it is your amygdala preparing your body. It does this based on sensory information – is it windy and raining, does the water look choppy, even whether you slept well. Your amygdala will sift previous experiences, which are memories with an emotion attached, to decide how to prepare your body. Even if it’s your first time, it will base the response on something. Often it overprepares you, so the perception is often worse than the reality.
We are all different, so each of us will have a different emotional response, which will determine how well we perform physically. It is this emotional difference that results in a winner and a loser in a race between two equally physically matched people. We are the sum of our experiences, so earlier, formative years will also make a difference too.
Your amygdala is in control of your emotions, particularly anxiety, increasing feelings of stress and regulating your stress hormone cortisol. Even if you have chosen to do something, like cold water swimming, you will probably feel anxiety about the cold even if you’ve done it before. This is you amygdala preparing your body and placing internal systems on high alert as a protection against the perceived threat, which is based on past memories and associated emotions.
Being aware of why your body responds this way allows you to try and reframe your anxiety, applying higher level thinking over lower level responses. To avoid that spiral of anxiety, instead of seeing something as a threat, reframe it as uncertainty, calm yourself with a few deep breaths, and focus on preparation putting everything else out of your mind. Have faith in your ability to cope. This breeds confidence, which brings more confidence. Calming yourself and reducing feelings of anxiety helps you focus and perform tasks better too. Learning to keep calm and cope is transferable to other parts of your life making you less vulnerable to anxiety in other situations and help towards leading a better life.
Meditation is often misunderstood and considered to be something done with your eyes closed, sitting cross-legged and still. However, you can meditate at any time even when you are on the move, including swimming.
Meditation isn’t just about calming your body it also helps with neurological changes: it helps shrink your amygdala, which is the source of feelings of anxiety, fear and general stress, new experiences help grow your hippocampus, and stress causes neuron adaption. This rewiring and growing is called neuroplasticity and neurogenesis.
Mindful movement meditation is centuries old. You can choose to maintain awareness of your whole body as it moves or focus in on one part. Some people move slowly and deliberately, while others move faster.
Swimming is perfect for mindful movement meditation: it allows you to focus on your breathing, body position, or stroke together or individually. There is a meditative quality from the rhythmic breathing, synchronised with repetitive arm movements and rock and flow of your body. Being submerged also dulls sensory information, helping produce feelings of calm and focus. It also increases blood flow to the brain by as much as 14%, which helps to improve concentration. This frees your mind, leaving it open to whatever thought, sensation or stimuli comes along resulting in mindfulness.
It’s not all about speed and distance either, light swimming or floating can help to decrease anxiety, heal depression and improve your general mood.
So, next time you swim, instead of frantic thrashing, try to use the opportunity to slow your stroke down, finding a calmer more efficient length to it. Incorporating mindfulness in your training sessions will help you relax, be more in tune with your body, efficient as a result and ultimately increase your performance.
The basic unit of your nervous system is a nerve cell called a neuron. These are bundled together into nerves. This is similar to a rope which is made of lots of smaller strands. The neurons and nerves in your nervous system have different functions: receiving external information and sending it to your brain for processing or conveying messages from your brain controlling conscious and unconscious actions. We will look at each closer, starting with neurons.
Neurons have three essential parts: a cell body and nucleus, dendrites that receive signals, and an axon, which is a long tail with finger like endings. The axon is encased in myelin, a fatty substance which insulates electrical signals sent down the axon from interference by other neurons also signalling in the same nerve bundle. The electrical signal is converted to a chemical neurotransmitter at the axon ending. The neurotransmitter is released as molecules into a special cleft called a synapse at the end of each axon ending. Neurons do not touch each other so signals are passed by the neurotransmitter diffusing across the gap to receptors on a nearby neuron’s dendrites. This stimulates the receiving neuron to send an electrical signal down its axon so the message is passed on until the target is reached, such as a muscle to cause it to contract. There are three broad types of neurons:
- SENSORY NEURONS Sensory receptors are special types of neurons and wired to a target part of your brain or spinal cord to enable faster processing and responses. Sensory receptors nerves are located in specialised organs: eyes for vision, nose for smell, ears for sound, tongue for taste etc. Touch, heat, and light are physical inputs and smell and taste are chemical inputs Fingertips have more nerve endings than hands and arms, hands have more than legs and thighs. Heat receptors lie closer to our skin’s surface, cold receptors are located lower down in the dermis, therefore it takes fractionally longer for your skin to responds to cold than heat. You also have internal sensory receptors. For example, nociceptors detect pain inside and outside your body, so ice-cream head is thought to be due to pain receptors in your brain reacting to cold water on the back of your neck cooling blood flowing to your brain
- INTERNEURONS are found in your brain and spinal cord and receive sensory input, process it, then send responsive actions to motor neurons
- MOTOR NEURONS receive messages from your brain and pass them to muscles, organs, and glands all over the body, controlling voluntary and involuntary movements
A nerve is a bundle of neurons. For example, there are about 20,000 neurons in your ulna nerve that controls your fingers. In your brain there are about 86 billion neurons. This is just like an electrical cable in your home where lots of copper strands are wound together to make a larger, stronger wire for carrying working electrical currents.
Nerves run throughout your body, connecting your brain to every organ helping you to think, move, breath, digest, etc. Nerves can be of various lengths: your smallest nerve the trochlear nerve that enables your eye to move; your sciatica nerve is the longest running from your spine to your heel.
The nerves in your nervous system are categorised into the following subsystems based on their functions.
- Central Nervous System (CNS): These are the nerves in your brain and spinal cord, controlling your mental and physical functions.
- Peripheral Nervous System (PNS): These are the nerves outside your CNS but connect it to the rest of your body, passing messages back and forth. Your peripheral nervous system is divided into:  Somatic nervous system: these nerves control conscious actions you choose to make  Autonomic nervous system: these nerves control unconscious functions necessary to live ie breathing, blood pressure, heart beat, digestion etc. Nerves can also be categorised by the direction of messages: AFFERENT nerves send information TO you brain and are your sensory receptor nerves hear, see, taste, smell; and EFFERENT nerves that carry messages FROM your brain to target muscles and organs.
- Somatic Nervous System: These nerves control two areas: Sensory input: these nerves detect external stimuli through your sensory receptors and send messages to your central nervous system; Movement control: these nerves send messages to skeletal muscles and control deliberate movements.
- Autonomic Nervous System: This controls essential functions like heartbeat, blood pressure, breathing. It divides into two parts, called your SYMPATHETIC and your PARASYMPATHETIC Nervous Systems. These two systems share the same nerves, called dual autonomic innervation. The two systems have opposing roles, either accelerating or dampening your responses. Most of the time they work in harmony, tweaking things to maintain homeostasis. However, there are times when one system will dominate, for example under stress your sympathetic “fight or flight” system dominates, but after a heavy meal your parasympathetic “rest and digest” system dominates.
- Sympathetic Nervous System: This system looks externally, responding to threats and protecting internal organs. It is your “fight or flight” system and acts as a systems accelerator. It activates stress responses when a threat is detected, like raising heart rate, constricting blood vessels, releasing glucose for energy. The threat may be real or perceived but your system will still respond. Stepping in front of a moving car is real threat. Daily pressures sometime cause stress, which is a perceived threat and can become chronic when your sympathetic nervous system is left on for too long. Learning how to calm yourself helps switch it off.
- Parasympathetic Nervous System: This system looks internally, it is your “rest and digest” system and acts as a brake, dampening sympathetic nervous system responses, mopping up after any danger has passed, slowing heart rate and relaxing muscles. It is responsible for internal self-protection, regeneration, recovery, and reproduction. It manages food digestion, waste expulsion, and general maintenance, all functions best done in a state of relaxation.
The diving reflex causes physiological changes to preserve life by conserving oxygen when underwater. Seals and other mammals exploit it much better than us. The reflex response slows with age.
It is triggered by holding your breath and wetting your face with cold water, which stimulates the trigeminal nerve receptors in your face, nose, orbit, eyeball and scalp. Your trigeminal nerve sends a message to your vagus nerve, which is your longest cranial nerve, going from your gut through the heart and to your brain and communication with every organ. It’s main function is to control the parasympathetic nervous system. Your vagus nerve impact two distinct organ groups: the pulmonary (lungs) and cardiovascular (heart and blood vessels) systems. It causes bradycardia (a slow heart rate around 60 beats per minute) and re-directs blood to vital organs (heart, brain and lung) and away from your periphery, preserving oxygen during times of asphyxia. Splenic contraction occurs causing ejection of stored red blood cells into circulation to help move more oxygen around your vital organs.
It also regulates your gastrointestinal activity, trigger the release of neurotransmitters that help your body repair and reduce inflammation, and the release of acetylcholine to control muscles, dilate blood vessels, and slow down your heart rate.
By activating the diver’s reflex, you are able to calm yourself and reset a hyper-aroused nervous system. The dive reflex is considered to be the most powerful autonomic reflex and known to conflict with the sympathetic system’s drive to raise your heart rate as part of your fight or flight response. Another reason anyone with a heart conditions should take care in cold water.
In contrast to your nervous system, where neurons directly connect your brain to distant target organs, your endocrine system communicates by secreting chemical messages called hormones into your bloodstream to travel to other parts of your body and cause responses in other organs. Your nervous system is therefore faster acting and immediate while your endocrine system is slower but its effects last longer from hours to weeks before wearing off. Your endocrine supplements, moderates and extends nervous system responses.
Your endocrine system consists of the following glands. These automatically secrete hormones effecting functions essential for homeostasis, from metabolism to growth and development, emotions, mood, sexual function and sleep. Which hormones to release is automatically controlled through a gland called the hypothalamus (see limbic system above), which decides based on sensory information from your nervous system.
- HYPOTHALAMUS It is a tiny gland in your brain that keep your body in a stable, healthy state, called homeostasis. It is directed by your amygdala in your limbic system and coordinates your nervous system and endocrine system responses through messages it sends to your pituitary gland. For example, when you get cold your hypothalamus can initiate shivering to generate heat.
- PITUITARY GLAND is attached to your hypothalamus which tells your pituitary gland what response is needed and it secretes hormones into your blood that flow to and control other endocrine glands:
- corticotropin to adrenal gland to secrete adrenaline and noradrenaline
- endorphins that reduce feelings of pain in the body
- thyrotropin to thyroid gland to control energy levels
- antidiuretic sent to kidneys to control hydration
- growth hormone to tissues handling nutrients and minerals for growth
- for women: oxytocin for uterus contraction and prolactin for milk production
- PINEAL GLAND helps you sleep, receives light-dark information from eyes and secretes melatonin to pass darkness signal to other parts of your body.
- THYROID GLAND controls your growth and metabolism
- PARATHYROID GLANDS control calcium and phosphorus levels for bone health
- ADRENAL GLANDS release adrenaline and noradrenaline, your fight or flight hormones, and cortisol that helps manage this response
- PANCREAS in the endocrine it release insulin and glucagon for energy. It is also part of the digestive system where it secretes digestive enzymes.
- GONADS testes in the male and the ovaries in the female. These produce hormones oestrogen, testosterone, and progesterone are instrumental in sexuality and fertility.
CORTISOL: Cortisol is secreted from the outer core of your adrenal glands. It is considered the primary stress hormone. It helps refine and regulate your body’s stress response, increases sugars (glucose) in the bloodstream, enhances your brain’s use of glucose and increases the availability of substances that repair tissues. Cortisol also curbs functions that would be nonessential or harmful in a fight-or-flight situation, such as digestion.
Cortisol levels rise and peak about 20 mins after the onset of stress and can remain elevated for several hours before returning to normal. This means cortisol helps the immediate stress response and beyond it, eking out your body’s abilities to handle stress from the start to some later finish point. For example, when a stressful event is over, the rush of adrenaline will have consumed sugar reserves, leaving you shaky and hypoglycaemic, your blood pressure and heart rate will be high, and your processes to digest and repair tissue will have stopped while you fight to survive. Cortisol is released to counterbalance the adrenaline released. Cortisol is your body’s natural steroid, activating antistress and anti-inflammatory responses, and is also an immune suppressant. Cortisol encourages the breakdown of glycogen (stored glucose) and production of new glucose from fat in the liver. Cortisol also triggers the release of dopamine in your brain to help you focus and think your way out of the problem.
People suffering chronic stress with continuously high levels of cortisol can take up to six months to rebalance them. Too much is bad and causes chronic stress and diabetes.
SEROTONIN: Serotonin is a neurotransmitters in the brain. It one of the ‘happy hormones’ along with dopamine, being associated with feelings of happiness, focus and calm. It has an inhibitory effect, meaning it balances out any excessive excitatory effects of other neurotransmitters, such as those released during a stress response. It also stimulates the pineal gland in your endocrine system which control sleep and wakefulness. Aerobic exercise, like swimming triggers the release of serotonin in the brain
ENDORPHINS: Endorphins are released by your pituitary gland and hypothalamus. Endorphins are your body’s natural painkillers and are “feel-good” hormones which make you feel better and put you in a positive state of mind.
Catecholamines are a set of hormones with a common molecular structure, released by your adrenal glands, in particular noradrenaline, adrenaline, and dopamine
NORADRENALINE: Noradrenaline (aka norepinephrine) is a neurotransmitter released from sympathetic neurons attached to skeletal muscles, causing them to contract. It is also released as a hormone from the adrenal glands into your blood flow to continue a fight or flight response. It can cause feelings of happiness and euphoria but also lead to panic attacks, raised blood pressure, and hyperactivity.
ADRENALINE: Adrenaline (aka epinephrine) is secreted into the bloodstream from the medulla core of your adrenal glands. It flows through your blood to organs controlled by your sympathetic nervous system, giving them an extra boost to handle the immediate threat. Its main effects are:  increases heart rate and force of contraction raising output and blood pressure  stimulates the liver to increase glucose levels in the blood for extra energy  causes pupil dilation and improved visual acuity.
DOPAMINE: Dopamine is a neurotransmitter in the brain where has two main roles:  it is considered one of the ‘happy hormones’ along with serotonin  it is your motivator, driving behaviour to seek rewards, being productive, and heightened emotion like desire or aversion. Outside the central nervous system, dopamine functions primarily as a local messenger.
- motor control and smoothing movement
- blood vessels: it inhibits norepinephrine release and acts as a vasodilator
- kidneys: it increases sodium excretion and urine output.
- pancreas: it reduces insulin production.
- digestive system: it reduces digestion and protects intestinal mucous.
- immune system: it reduces the activity of lymphocytes (white blood cells)
Circadian rhythm is your natural 24 hour cycle of physical, mental, and behavioural changes linked with the sleep-wake cycle. A major output pathway of the circadian clock is the endocrine system which allows for a systemic coordination of various physiological target systems according to the time of day. It is controlled by your hypothalamus, by responding to daylight. Daylight stimulates retinal receptors causing your brain to send signals triggering various hormones, which fluctuate throughout the 24-hr circadian cycle, such as cortisol and melatonin, which affect your sleep-wake cycle, metabolism, stress response, and circadian balance.
- CORTISOL level peaks in the morning, declines throughout the day, is lowest in the evening, and slowly rises overnight. A disruption in cortisol rhythm can lead to restless sleep, inadequate stress response.
- MELATONIN is lowest in the morning, increases throughout the day, is highest in the evening, and slowly declines overnight. Melatonin release is by light exposure: as the sun sets, your hypothalamus signals your pineal gland to produce more melatonin triggering sleep. In the morning your pineal gland slows down melatonin production so you wake up. Beyond sleep, melatonin is an immune-supportive antioxidant that possesses anti-inflammatory and neuroprotective benefits. Melatonin and cortisol have an inverse relationship.
Normally you have a circadian rhythm that is slightly longer than 24 hours. Every day, morning light and other behaviours reset the sleep-wake clock to a 24-hour schedule. Without light and this clock resetting, people’s sleep time will drift later and later. A recent discovery is that every organ, cell and DNA has its own 24 hour clock, each working independently while also working collectively.
There are three core rhythms (sleep, eating and activity) that connected and influenced by your lifestyle and health, mainly it is at night, when you are asleep. Disruption to these systems can compromise the optimal function of any one organ or system, for example insomnia. Your body is designed to be active throughout your waking hours. The body’s best time for exercise is in the morning before breakfast or mid to late afternoon (around 16:00), which is also when we are at our peak active performance. In fact any physical activity that expends energy will improve your circadian rhythm and therefore your sleep cycle.
Your cardiovascular system is your most important system. In the homeostasis league, it is protected before all other systems. Without a deliver mechanism to feed the tissues of your body, particularly your brain, other systems become irrelevant.
Cardio means heart, and vascular refers to blood vessels. Your circulatory system ensures blood reaches all your body’s tissues so they can function. Blood is the medium by which cells throughout your body receive oxygen, nutrients, hormones and it also carries waste products and carbon dioxide away. It plays an important role in meeting the demands of activity, exercise, and stress and helps maintain body temperature. Your cardiovascular system also engages in resource allocation because there is not enough blood flow to distribute blood equally to all tissues simultaneously. So, during exercise, more blood is directed to skeletal muscles, heart, and lungs. After a meal, more blood is directed to your digestive system. Only the brain receives a more or less constant supply of blood whether you are active or resting. Your cardiovascular systems divides into three important circulatory systems
- Coronary circulation are the small system of blood vessels wrapped around the heart whose circulation provides the heart with oxygenated blood so it can function properly.
- Pulmonary circulation moves blood between the heart and the lungs. It transports deoxygenated blood to the lungs to absorb oxygen and release carbon dioxide. The oxygenated blood then flows back to the heart.
- Systemic circulation moves blood between the heart and the rest of the body. It sends oxygenated blood out to cells and returns deoxygenated blood to the heart.
Your cardiovascular system forms a closed loop, like a circuit, that begins and ends at your heart. Your body contains about 60,000 miles of blood vessels. Blood vessels are tubing of various diameters that carry blood around your body. The largest is your aorta artery and can be up to 2 cm wide, the smallest capillaries range from 2 to 12 micrometres, which is less than the diameter of a human hair. Each type of blood vessel serves a different function:
- Arteries: carry oxygen-rich blood from your heart to your body. They are strong, muscular blood vessels to handle high pressure blood flow squeezed from your heart. However, they don’t carry a large volume of blood: only about 10% to 15% of your body’s blood is in your arteries at any time.
- Arterioles: arteries branch into smaller vessels called arterioles. Both arteries and arterioles are very flexible and their diameter can reduce (vasoconstrict) to help maintain blood pressure.
- Capillaries: have thin walls that allow oxygen, nutrients, carbon dioxide and waste products to pass through, to and from the tissue cells.
- Veins and venules: venules receive blood from capillaries and pass it to veins. Veins and venules only have thin, less elastic walls to help them handle high volumes and low pressure. Because they carry blood back to your heart upward and against gravity, most veins have one-way valves that open and close to stop blood running back downwards. About 75% of your blood is in your veins.
Your heart’s main job is to pump blood round your body. It normally does this 60 to 100 times a minute, 24 hours a day, and circulates about 2,000 gallons of blood every day. Your heart has four chambers, the two upper are called atria, the two lower are called ventricles. An internal wall of tissue divides the right and left sides of your heart. This wall is called the septum. The right side handles blood returning from your body and low in oxygen. The left side handles oxygenated blood from your lungs and pumping it out to your body.
As your heart relaxes between beats
- On the right side, blood flows into your right atrium from your head and body and down into the ventricle below.
- On the left side, oxygenated blood from your lungs flows into your left atrium and down into the ventricle below.
As your heart contracts
- On the right side, blood is pumped out to your lungs to exchange oxygen for carbon dioxide waste.
- On the left side, oxygenated blood is pumped out to your head and body through your aortic valve.
Blood pressure is when blood presses on the internal walls of your arteries. Many factors can raise blood pressure: stress, exercise, standing from a sitting position etc. We measure blood pressure using two pressures systolic/diastolic, a normal blood pressure level is less than 120/80 mmHg.
- SYSTOLIC PRESSURE is when your heart contracts or beats and the pressure is greatest. The optimal systolic blood pressure is 120 mmHg.
- DIASTOLIC PRESSURE is when your heart relaxes. The optimal diastolic blood pressure is 80 mmHg.
Blood pressure is affected by two factors
- Cardiac Output (CO) which is a combination of
- heart rate – how quickly your heart beats
- stroke volume – the amount of blood squeezed out each beat
- Systemic Vascular Resistance (SVR) or total peripheral resistance (TPR), which is affected by
- Diameter – this is the size of hole blood travels down. It can be affected quickly by sympathetic nerves causing vasoconstriction or longer term by plaque (deposits of fatty substances, cholesterol_
- Length – this is a longer term change ie a bigger body from increased muscle or fat
- Viscosity of your blood this long term too eg taking IPO to increase blood platlets
Under normal conditions, the main areas of your body receive the following amounts of blood:
- Brain – 15%
- Coronary circulation – 5%
- Gut & intestines – 25%
- Renal (kidneys) – 20%
- Skeletal muscles – 20%
- Skin – 5%
A stress event triggers a change to three of the blood pressure variable mentioned above through sympathetic nerves of your fight or flight system:
- increasing heart rate
- increased stroke volume
- narrowing arterioles to skin and gut via vasoconstriction.
By reducing the blood flow to your skin and gut, it means there is now 30% more blood available which will increase blood pressure and volume of blood flowing to important areas of your body, delivering more oxygen and nutrients needed to handle a fight or flight situation.
Baroreceptors are special sensory nerves monitoring blood pressure sensing the stretch and change in diameter of major blood vessels.
- High-pressure baroreceptors measure pressure as blood is pumped from your heart to your body.
- Low-pressure baroreceptors measure blood as it return to your heart from your body.
Normally, blood flow from your heart is the same as blood flow back into it. If blood is returning faster than it is being ejected, the high-pressure receptors will trigger sympathetic nerve firing to increase cardiac output until homeostasis is achieved. The opposite is also true. This mechanism is referred to as the atrial reflex. It happens within a couple of beats of your heart.
When blood pressure rises too high, your baroreceptors fire at a higher rate, which triggers:
- Increase parasympathetic stimulation of the heart via your vagus nerve to slow it and cardiac output falls.
- Decrease sympathetic stimulation of arterioles allowing them to relax, resulting in vasodilation and increased blood flow and reduced pressure
When blood pressure drops too low, your baroreceptor firing decreases, which triggers:
- Increase in sympathetic stimulation of the heart, causing cardiac output to increase.
- Increase sympathetic stimulation of arterioles causing vasoconstriction, which restricts flow rate and increases pressure.
Your body may need to adjust your blood pressure when you
- Change your body position, such as when you stand quickly or when you exit from swimming, particularly when it is cold water and blood flow is more sluggish.
- Experience something that threatens you that triggers your stress response, such as cold water swimming
- Quickly change from doing something sedentary to more active, like swimming hard when cold water swimming.
Your heart pumps blood out to your body with tremendous force and speed. As blood flows through to your capillary beds, the rate of movement slows dramatically, to about one-thousand times slower than when it exited your heart. Usually a fluid should travel faster as it is forced through a narrower diameter tube. However, while your aorta diameter is much greater than each subsequent individual arteriole and capillary, the internal volume of them all combined is far greater than the diameter of your aorta. The result is the rate of blood flow slows as it progresses through your cardiovascular system.
This is handy because your blood has to service every individual cell in your body as it passed through tiny capillaries, so its slow speed allows gas and nutrient exchange with each cell at this microscopic level. As blood leaves your capillary beds to return to your heart, your venous return system has some clever ways using muscle contractions to help it on its way, covered later.
Vasoconstriction is when the internal diameter of your arteries or arteriole are narrowed to increase blood pressure as blood is forced through a narrower gap. In a stressed situation, like cold water swimming, as it also reduces blood flow and volume as a consequence, your heart beats faster to ensure adequate supply of blood to major organs and muscles. Constriction is done by smooth muscles in your blood vessel walls, the muscles are like fingers wrapped round a hose pipe and squeezing. Vasoconstriction is all done automatically by sympathetic nerves, triggered by the cold water shock as your get in.
Arteries and arterioles have muscular walls to handle higher pressure blood flow. They are the main blood vessels involved in vasoconstriction. The large veins to the heart from the body (the vena cava) do not change much under vasoconstriction as they don’t have as much smooth muscle surrounding them because they don’t have the same pressure to handle. Capillaries are tiny, thin-walled blood vessels that can’t constrict as they don’t have any smooth muscle around them.
As you have a limited amount of blood, your body must ensure sufficient blood gets to active/important areas by reducing flow to inactive/unimportant areas.
Blood flows from each arterioles into a networks of capillaries, called capillary beds, which feed individual cells with oxygen and nutrients. Precapillary sphincters allow your body to precisely control when capillary beds receive blood flow. A sphincter is a ring-like smooth muscle able to contract and stop blood flowing. Just like turning off a radiator in your house. If the sphincter is open, blood flows into the connected capillaries. If all of the sphincters are closed, blood bypasses the capillary beds in an area and flows from the arteriole directly to the venule through a vascular shunt channel called a metarteriole. At any given moment only about 5-10% of our capillary beds actually have blood flowing through them.
During cold water swimming, blood to your skin and digestive system is reduced by vasoconstriction and more blood is diverted to other and muscles through vasodilation. After eating a meal your parasympathetic system dominates, so most of your blood is diverted to your stomach by vasodilation of digestive system blood vessels and vasoconstriction of blood vessels elsewhere.
Everyday body temperature control
The most efficient way to maintain body temperature is cutaneous vasoconstriction. It is your first line of defence; it takes little energy and no loss of water. Restricting blood flows retains heat and increasing it loses excess heat. These are normal everyday subtle changes in skin blood flow to maintain normal body temperature. Cutaneous reflex vasoconstrictor response slows as you age and the reason why older people are susceptible to hypothermia.
After exiting capillaries, blood pressure has diminished considerably. This means veins and venules are under much lower pressure than arteries, so their walls are much thinner and their internal diameter are larger allowing more blood flow with less resistance. Many veins, particularly those in your limbs, contain valves to assist the unidirectional flow of blood back to your heart. This is critical to help blood flow from your arms and legs which becomes sluggish due to the lower pressure and effects of gravity. In addition to valves, which prevent blood slipping back, your body has two pumps to help increase blood pressure in your veins and venules.
RESPIRATORY PUMP: As you breath in and out so the pressure in your chest (thorax) goes up and down. As you breath in your chest expands and causes air pressure in your thorax to drop and blood pressure in your thoracic veins decrease and falls below the pressure in the abdominal veins. Higher pressure in veins outside your thorax cause blood to flow into your thorax. When you breath out, air pressure inside your thorax increases, pressure in your thoracic veins increases, pushing blood flow into your heart. One way valves in your veins prevent blood from flowing backward from your thoracic and abdominal veins.
SKELETAL MUSCLE PUMP: During upright posture, to help blood flow upward against gravity and return to the heart:  surrounding skeletal muscle contractions as you walk or run, squeezing veins and venioles that pass through them, causing an upward pressure. Veins and venioles have one way valves along their length.  Valves above the contracting muscles open so blood flows through, simultaneously, valves below the contracting muscles close to stop blood seeping back downward toward your feet or hands. If blood pools in lower limbs rather than returning to the heart your brain will not receive enough oxygenated blood and you become dizzy or may faint losing consciousness. Like when you stand after sitting for a while and why it is important to get up and move about instead of sitting for long periods causing blood to pool in the extremities
Calf muscles: Every time your calf muscles contract, valves open and up to 70% of the blood in your legs is propelled back to your heart. When your calf muscles relax, valves tighten to close, and prevent blood from flowing backwards down your leg. Your calves ensures venous return back to the heart works when you use them, particularly important when you are upright walking or running. With swimming, you do use your calves when you flex your feet to kick but your horizontal position, plus hydrostatic pressure also helps venous return. However, this can cause dizziness when you get out, covered later.
There can be lost of causes of dizziness, two common to cold water swimming are mentioned below.
When you exit the water, you may experience feelings of dizziness or feel about to faint; it is the same feeling when stand quickly from after sitting or lying. This is called orthostatic hypotension or postural hypotension. Gravity pulls blood downwards into your legs, blood pressure drops, and less blood will flow back to the heart. Less blood entering the heart means your heart does not expand as much and will not contract as strongly and cause lower cardiac output. Baroreceptors nerves sense this and trigger changes to counter the effect, as quickly as a couple of heart beats.
Orthostatic hypotension can be triggered by mild dehydration, low blood sugar or overheating. It can also be triggered by cold water swimming:
- hydrostatic pressure from surrounding water naturally assists venous return, so losing that mechanism can exacerbate the situation.
- swimming will decrease effectiveness of your lower extremity muscle pumps that help venous return, which will be exacerbated by cold water effects.
- endurance exercise in general can result in vasodilation, further decreasing blood pressure.
When cold water makes direct contact with the eardrum, it changes the density of a fluid called endolymph in the semi-circular canal, which is part of your inner ear. This sends faulty signals to your brain that cause sensations of head turning and vertigo. Wearing ear plugs is a good way to prevent this and also prevents surfers ear (a swim cap can might cover your ears but often they ride up letting cold water in)
Swimming induced pulmonary edema (SIPE) is a life threatening condition that occurs when fluids from the blood leak abnormally from the small capillaries passing through your lungs into the airspaces (alveoli). The causes are incompletely understood but it seems to be a perfect storm from heightened stress causing high heart rate and blood pressure, in response to [1[ cold water  participation in an event, like a triathlon or other endurance swimming event  swimming faster than used to doing  horizontal body position  hydrostatic tension from the water combined with wearing a wetsuit increasing internal pressure. Symptoms include:
- Shortness of breath out of proportion to effort being expended.
- Rapid, heavy or uneven breathing, or uncontrollable coughing.
- Crackles, rattling or ‘junky’ feelings deep in the chest associated with breathing effort – usually progressively worsening with increasing shortness of breath and may be cause for a panic attack
- Cough, usually distressing and productive or not of a little pink, frothy or blood-tinged sputum
- The wetsuit may feel as though it is hindering breathing ability.
- Confusion or apparently irrational behaviour.
YOUR STRESS RESPONSE
Let’s look at how your internal systems work together to handle a stressful situation, such as when you swim in cold water. It is the same stress response irrespective of which senses detect the threat. It is also a simplification of what happens. In reality, your stress response happen in an instance and much quicker than the following makes out. In fact, so quick your conscious brain often has to catch up with your automatic reaction.
Your limbic system handles memories and how you reacted previously to this or similar events. The components of this system work together to determine if something is stressful and an appropriate response. Your thalamus (input gatekeeper) passes the information to your amygdala, which messages your hippocampus (the part of our brain that stores memories) to recall emotive memories to check how you coped before. The prefrontal cortex is not part of the limbic system but helps with stress by processing information on a more intellectual level rather than pure emotion. This is where your conscious effort is focused, but stress can disrupt the process of making good decisions. Your amygdala decides it is a threat and based on past memories, how much of a threat, and triggers the stress response and your hypothalamus carries it out. Your hypothalamus then triggers your sympathetic system and endocrine system
Your hypothalamus is connected to major organs by nerves in your autonomic system. It is continually monitoring them through feedback mechanisms and able to tweak things to maintain homeostasis. This means adjusting through the sympathetic nerves or the parasympathetic nerves. These each have opposing actions allowing your hypothalamus to adjust and keep your metabolism in a balanced range eg blood pressure, temperature, respiration, digestion etc. However, when a stress response is required it immediately sends signals through your sympathetic nervous system, your fight or flight system, so it quickly dominates your bodily functions to handle the threat.
Your endocrine system supplements your sympathetic system’s stress response. As the initial surge of adrenaline subsides, your hypothalamus activates glands in your endocrine system: your hypothalamus, pituitary gland, and adrenal, known as the HPA axis, which together ensure hormones are secreted to help continue your sympathetic nervous system’s fight or flight response.
Your hypothalamus triggers your pituitary gland by releasing Corticotropin Releasing Hormone (CRH), down a blood vessel linking them together. Your pituitary gland then releases AdrenoCorticoTropic Hormone (ACTH) into your bloodstream to flow to other endocrine glands. In particular, it reaches your adrenal glands, which sit above the kidneys. The outside of your adrenal glands (outer cortex) release cortisol (aka the stress hormone) and the inside (medulla) releases adrenaline into your bloodstream. Adrenaline gives you extra strength and endurance. Cortisol helps spread the effect so you can stay in a high alert state longer.
These hormones flow to other organs important for survival, like heart, lungs and muscles. These have receptors on their cells which are sensitised and stimulated by adrenaline. This causes your heart to beat faster, pushing blood to larger muscles, lungs, and other vital organs. As your pulse rate and blood pressure go up you also breathe more rapidly, small airways in the lungs open wide, so you can inhale more oxygen, not only for muscular needs, but extra oxygen is sent to your brain to increase alertness. Adrenaline also triggers the release of blood sugar (glucose) and fats from temporary storage sites in the body, flowing through your bloodstream to supply energy to all parts of the body. Also, sight, hearing, and other senses become sharper. Your pituitary gland and hypothalamus also release endorphins, which are your body’s natural painkillers and make you feel better, putting you in a positive mind.
When the stressful situation is over, in your case you exit from your cold water swim, it takes between 20 to 60 minutes for your body to return to its pre-arousal levels. Cortisol levels fall and your parasympathetic system is activated to countering the sympathetic system and dampens the stress response, bring your metabolic rate down by lowering your heart rate, dilating blood vessels, and returning more blood to your skin and gut.
Berta Vogel Scharrer helped to found the scientific discipline now known as neuroendocrinology by studying cockroaches and showing how they rapidly adapted to a changing environment. They have a small number of neurons in their nervous system yet achieve huge flexibility handling new situations. This is by secretion of hormones which facilitate neuron adaption. This systemic modification enabling survival through adaption is applicable across the whole of animal life.
Neuroplasticity is the ability of the brain to form and reorganize synaptic connections, especially in response to learning or experience
So, while the modern world may be complex, your behavioural responses are rooted in survival instincts, especially heightened when you experience a stressful situation. These responses are the same set triggered by other stressful situations, whether chosen or not, like a cold swim to slipping on ice. Your internal systems work together to protect you: your sensory system detects a threat, your limbic system draws on past experiences to determine a suitable response, quickly engaging your sympathetic nervous system (aka flight or fight system) and supplements it with a flood of hormones. You experience this reaction as an emotional response from fear to euphoria, all of this and how well you coped is stored away for reference on another occasion.
So when you next go a short dip to acclimatise yourself to cold water swimming, on the surface it may appear a relatively simple and straightforward regular dip. However, inside your body there is a whole lot more going on.
Hopefully this article provides a better understanding of how you handle new experiences and adapt to them, including cold swimming or whatever you choose to do. Being aware means you are better placed to apply higher level thinking to help overcome lower level responses, bringing some control and calm to whatever you are doing, reducing any stress and making it more pleasurable.