Environmental temperature is something that presents a very real challenge to human physiology. External ambient air temperatures in a single location can vary widely and, across different locations can range from extremes of -94°C (Eastern Antarctic Plateau, Antarctica) to 56.7°C (Furnace Creek Ranch, CA, USA). Perhaps surprisingly given the above ranges, humans have a relatively narrow bandwidth of internal body temperature that we can endure without adverse effects. ‘Normal’ body temperature is generally seen as averaging 37°C, with clinical hypothermia (too cold) being anything below 35°C, and clinical hyperthermia (too hot) being anything over 40°C. This looks on the face of it to be a complete mismatch and very surprising that we can deal with even minor fluctuations in environmental temperature. Thankfully, evolution has given us a remarkable ability to regulate our internal body temperature to stay within this narrow goldilocks zone. In the remainder of this article, we are going to focus on the upper ends of these temperature ranges towards the ‘too hot’ end of the spectrum. We will look first at how our body regulates and expels heat, then we will look at what happens when this stops working. Finally, we will dig into two case study examples of performance in different extreme heats and how they overcome the rising mercury. Thermoregulation - dealing with the heat The part of our brain responsible for thermoregulation (temperature control) is the hypothalamus, which serves the purpose of an internal thermostat by monitoring signals sent from thermoreceptor nerves that detect both heat and cold internally and in the skin. This ‘thermostat’ is then able to affect and control the necessary actions that occur to regulate body temperature to stay within the normal range. There are 4 main direct methods through which we can lose heat: convection, conduction, evaporation, and radiation. Under normal, non-exercise conditions when the surrounding temperature is lower than the skin we lose heat by convection, conduction and radiation where heat is transferred to the air or objects we are in contact with (such as clothing). When we begin to exercise, or when the surrounding air temperature rises above skin temperature, we begin to lose more heat via evaporation (namely, sweating). When our body temperature rises, and this is detected by the hypothalamus, several things happen. Primarily, the body attempts to dissipate heat away from the core and organs by directing additional blood flow to the peripheral areas of the body and skin. This is achieved by vasodilation (expanding) of the skin’s blood vessels. Due to the high surface area of the skin, this allows much of the excess heat to be brought closer to the skin, whereby it can then be dissipated externally through the methods of convection, conduction, radiation, and evaporation. In the cases we will see in extremely high temperatures, this will mainly involve significant sweating. Additionally, our body reduces the backflow of blood to the core organs that do not directly need a high blood flow during exercise (eg: liver, stomach, digestive system) to prevent the continued rising of the core temperature. When the skin’s blood vessels dilate the heat is transferred into the water and fluids that surround the cells and blood vessels. This fluid is then excreted from the body through sweat glands. Once on the skin, this fluid (sweat) evaporates and so, the heat is taken along with it. Human’s ability to sweat so efficiently allows us to cope very well under fluctuations in external temperatures and lays a cornerstone for our ability to perform in extreme heat. What happens when this heat dissipation stops working? There are many reasons why this efficient process of heat dissipation might go wrong and we will see what happens when it does. As we use sweating as a primary method of removing excess heat brought to the surface of the skin, if we stopped sweating we would overheat rapidly. Sweat is produced largely by water and fluids within our body so if we become significantly dehydrated, we will lose the ability to produce more sweat and will be unable to dissipate heat as effectively. When someone is overheating, becomes dehydrated, and gets to the point that they are no longer sweating they’re in trouble unless they can cool down quickly via external means. Other issues that directly influence the areas of the brain and body involved with managing and monitoring temperatures can also stop our body’s thermoregulation from running correctly. When the thermoregulation processes go wrong or cannot cope with rising body temperature then there are a few different conditions that can develop and ultimately lead to death if left untreated. Heat cramps: When we sweat, we also lose a significant amount of salt and electrolytes through the fluids lost. In high heat and during physical work these fluid losses can account for 5-10 litres over a whole day, or significant amounts during a shorter period (eg: races, competitions, high activity levels in hot temperatures). These electrolytes serve an important purpose in facilitating muscle contractions throughout our body and so when the levels drop below a certain point we can begin to experience muscle cramps. These can range from small and inconvenient to largely debilitating and very serious. Heat collapse: This is the most common type of heat-illness that is seen in individuals who have become dangerously overheated. When the skin’s blood vessels dilate, they draw more blood towards the skin and away from the core. To account for this, the heart’s output also increases to supply the increased demand. However, this happens without there being a change in the total volume of blood circulating in the body. This shift in blood flow distribution causes overall blood pressure in your body to drop, especially towards your core. The resultant effects of this can include dizziness, vomiting, headaches, fainting/collapse, and neural symptoms (eg: slurred speech, confusion). Left unchecked and untreated this can become life-threatening. Indirectly, many of these symptoms could also place an individual at increased risk of other injuries and complications depending on the situation. Heat Stroke: This represents the most serious heat illness and is potentially life-threatening. It occurs primarily when the internal body temperature exceeds 40°C for extended periods and the body is unable to effectively regulate temperature any longer. Typically, heat stroke can occur in individuals with compromised circulation or in individuals who are physically exerting themselves in high temperatures for long durations. The main cause of heat stroke is a failure of, or severe disturbance to, the temperature regulation in the hypothalamus. In these instances, the body reacts oppositely; the blood vessels constrict and you stop producing sweat so that your skin becomes dry and hot. Blood pressure drops significantly and metabolic rates rapidly increase, all leading to severe symptoms of dizziness, headaches, and confusion. These can further lead to unconsciousness, coma, and eventually fatality. If external cooling is not provided (eg: shade, cool water, ice baths etc) then body temperature will continue to rise unchecked. Dealing with the heat: Extreme Real-World Examples Case Study 1: F1 Singapore Grand Prix (Hot, humid) The details of the race: * 61 Laps of a street circuit * Races here have never taken less than 1 hour 51 minutes, it is, therefore, one of the longest races on the competition calendar. * 23 corners per lap * The most of any track in the competition calendar. This exerts a huge physical demand on the driver’s bodies. * Singapore's environmental conditions during the race: Around 30°C, and 80% humidity. * Coupled with all of the very hot mechanical components in the car, this causes the temperature of the car’s cockpit t… This is a public episode. If you would like to discuss this with other subscribers or get access to bonus episodes, visit performanceattheedge.substack.com
