A wave of heat, such as the one currently facing our country is characterized by extremely high temperatures which can reach up to 40 °C, on several days and nights. This type of episode affects our daily lives : we are more tired, less energetic, to the limit of overheating...

In France after the heat wave of 2003, a national plan for heat wave has been put in place. It is based on four alert levels, and provides, in particular, of the communication actions to recall that the "preventive actions individual to implement" : drink plenty of water, shelter from the heat, not to do intense physical exercise... in Short, to protect, to the extent possible in order to avoid a heat stroke which could put the lives in danger.

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But, specifically, how the human body makes it in the face of these conditions of extreme heat ? Why are they so dangerous ? Which of our organs are the most affected in the event of temperature rise, and when things spoil ?

Stay cool

The human body temperature normally fluctuates between 36 and 38 °C. within this range, the biochemical reactions are able to occur normally, the sine qua non for the proper functioning of our cells and our organs. To adapt to the environmental changes that could make it out of this comfort zone, as during a heat wave, the human body is well equipped.

The changes in abnormal body temperatures trigger a physiological response in our body. It aimed to reduce our internal temperature to more normal levels. This thermoregulation can be compared to a thermostat domestic : if the temperature deviates too much from the setpoint temperature, the heating systems or air conditioning are in motion to reach the desired temperature.

In the human body, this thermostat is located at the base of the brain in a region called the hypothalamus. It is at this location that the information provided by temperature sensors located in our peripheral organs, such as skin or muscles, is integrated and processed, triggering a physiological response when needed.

Recycling

once the response is triggered, the first physiological response (and most important) is the production of sweat. In evaporating, it helps dissipate the heat, at the level of the skin and extremities of the body (hands, feet).

This system is very effective to refresh the body, but it can take up to 2 litres of water per hour in extreme heat. For this reason, as we shall see later, during the extreme heat, the body manages its water more than just, recycling it to the maximum.

However, in order to maintain the capacity of the thermoregulation of our body, it is essential to drink for refilling the tank. If you don't drink the water not enough, the risk is of running out of water, and thus lose the ability to sweat and cool off, which can lead to overheating of our bodies. By drinking, we absorb also electrolytes and salts that are necessary to maintain the pH of blood and the proper functioning of our cells.

voltage Drop

To understand what can go wrong in the event of heat stroke, let's see how the response thermoregulatory affects the functioning of our various organs, and how they respond to extreme temperatures.

The cardiovascular system is one of the first to be affected. In order to be able to sweat, it is necessary that the blood flow moves from the central organs to the peripheral organs so that it can cool down. One of the visible consequences is that, often, people who suffer from the heat blush. The loss of water by transpiration, and redistribution of blood flow causes a drop in blood pressure. To try to compensate, in order to maintain the blood flow through the vital organs, the heart beat accelerate.

If the blood flow redistribution is accompanied by a too great loss of water, blood pressure drops dangerously, which can cause fainting, signs of a heat stroke. If this voltage drop is not treated, it can cause, in more serious cases, heart failure.

Alterations in emotional

The brain is another vital organ that suffers from the stress in case of strong heat. The rising temperatures disturb the communication between nerve cells and can even damage them or even cause their death. In fact, the heat affects the structure of DNA and proteins, as well as the integrity of cell membranes.

dehydration also causes electrolyte imbalances that can disrupt the communication between nerve cells and muscle cells. The higher the overheating lasts for a long time, the consequences can be serious. Pathways cognitive can be deregulated, which can lead to alterations in emotional, such as increased anxiety, headache, impaired judgment, etc

remarkably, the brain is in particular cooled by the respiratory system. In the event of overheating, the body increases the frequency of breathing, cooling the blood to the brain and back, by mechanisms of surface cooling and heat exchange. This system can literally be considered as a natural air-conditioning. However, it has a negative effect : it increases the blood pH, due to the decrease of the pressure in CO2, which could put in danger the cellular functions of other organs.

Less urine

Another important organ that receives less blood during the extreme heat, because of its redistribution to the periphery of the body : the gut. This loss impedes its proper functioning and, in extreme cases, causes nausea and vomiting.

Finally, the loss of water and salts through sweat can also affect the urinary tract. Under the influence of a hormone-specific produced by the brain (the antidiuretic hormone), reabsorption of water and salts is stimulated, in order to compensate for the loss of blood pressure in the cardiovascular system.

therefore, our kidneys produce less urine. It is concentrated, which is manifested by its colour more brown. We are going less often to the toilet ; when periods of high temperatures may be prolonged, and that one is dehydrated, the kidney tissue can be damaged, and the kidneys do not work properly.

A system that has its limits

The system a temperature regulator of our body is particularly well suited, which allows us to cope with conditions of extreme heat. In addition to the physiological response, the heat also triggers a behavioral response. When the temperature rises, our thirst increases and we tend to look for places cooler and more comfortable.

However, in the case of the episode heatwave, our body is subjected to intense stress, and its temperature regulation system may reach its limits. A body temperature above 40 °C causes the system to its limits, even beyond its self-healing capabilities. In this case, the risk of loss of control of the regulation of the temperature is real, which can compromise the functioning of the organs.

The body is the most vulnerable in this regard is probably the brain. Heat stroke, and dehydration which accompanies it, causes a reaction systemic inflammation which, in turn, leads to irreversible brain damage or even death if no action is taken very quickly.

Listen to your body

people who don't listen to their body, do not drink water and ignore the advice given by the health authorities in the period of the heatwave bring their body to the limits of human physiology. They are in danger of exhaustion or heat stroke, which can have potentially fatal consequences in case of failure of multiple organs.

the same goes for people at risk such as the elderly and patients who have a history of cardiovascular disease. In addition, older people may be less aware of the dangers of the heat, because their sensors body heat to function less well than those of the young.

babies and toddlers, they depend on the vigilance of their parents, who must be alert to take the necessary measures for their protection.

Finally, it is important to limit the consumption of beverages containing alcohol or caffeine, since these substances themselves have the drying effects.

in The end, the tips to remember are simple : drink water, cool off from time to time, avoid the highest temperatures, and follow the recommendations of the authorities. And, of course, take care of those who are the most vulnerable during these episodes are exceptional.

* Pieter Vancamp is a post-doctoral fellow, national Museum of natural history (MNHN)