how does our body react to heat?

Exposed to excessively high temperatures for several days, the human body is strained to maintain itself at 37 ° C. How does he do it? At what price? Since Wednesday, a large part of France has been placed on heat wave alert. Subjected to high temperatures including at night for several days, the bodies are put to the test. A wave of lethargy seems to have descended on the populations, and the health services are on the alert. How much can hot weather affect our bodies?

What temperatures can the body handle?

Man is a homeotherm, which means he needs to keep his body at a constant temperature (around 37 ° C) to function properly. Generally, for a human being, the comfortable temperature of the outside air should be between 20 and 27 ° C, and the humidity between 35 and 60%. Above these thresholds, the body is forced to adapt by modifying its functioning.

If the heat episode (above 27 ° C) is brief, the body exerts a little effort for a while, then resumes its cruising speed without a hitch. But if the heat wave continues, the body is at risk of "heat stress", which causes general discomfort, can lead to serious problems and even death.

What are the means of dissipating the heat?
The human body constantly analyzes its internal heat level using temperature sensors located on the skin and in the brain, in the hypothalamus, which serves as a "central thermostat". In case of excessive heat, the body will do everything to lower its temperature by evacuating the excess: this is thermolysis.

For this, the body has four main tools: radiation, conduction, convection and evaporation.


1. Radiation allows the body to naturally release heat to its surroundings by radiation from the skin. When it is too hot, all the blood vessels in the skin dilate to carry heat to the extremities and then out, in the form of light rays. It is this radiation that notably allows infrared cameras (or thermal cameras) to film someone even in the middle of the night.

2. Conduction allows the body to release heat into a cooler object when in direct contact with it. This is why, for example, you will be able to cool off faster lying on cool tiles than on hot sand, but after a few minutes, the tiles in contact with your skin will have become lukewarm.

3. Convection allows the body to release heat into the air or water around it, when it is colder, by transferring energy. This heat transfer increases with the speed of movement of the air or water (this is why the air seems "cooler" to us when it is set in motion, by a fan or through the window. moving car, for example).

4. Evaporation allows the human body to dissipate heat through perspiration. By constantly maintaining a thin film of water on the skin, sweating helps cool the body because to evaporate perspiration, it will have to burn calories, and therefore evacuate heat (up to 600 kcal for a liter of sweat). Although the sensation can be unpleasant at times, it is therefore important not to completely dry yourself off when you sweat.

Why does it work less well when it's really hot?

When the ambient temperature remains very high for a prolonged period, the four "tools" of the body are saturated and no longer manage to guarantee homeothermy:

- The sun's radiation, much more powerful than that of the body, rejects much more heat than the body can reject itself by radiation.

- The body can only come into contact with a few cold objects, most of which are also strongly heated by the sun.

- Ambient air, which is warmer than the body, does not allow enough heat to be evacuated by convection.

- Perspiration evaporates quickly thanks to the heat of the ambient air, without fully playing its role of thermoregulator.

In the end, for the body, evacuating a lot of heat in an environment that already contains too much of it, or a lot of perspiration in air already saturated with humidity, quickly becomes complicated. (Moreover, we withstand the dry heat of a sauna, which can reach up to 100 ° C, much better than the humid heat of a hammam, which will never exceed 50 ° C.)

Do we all react the same way?

No. Each person reacts differently, depending on several factors:

- Their overall state of health (certain pathologies such as hypertension make them more sensitive to heat because they weaken the cardiovascular system, which is strained in the event of a heat wave)

- Its weight (being overweight generally leads to a decrease in thermoregulatory capacities, because of the insulating properties of fat, but also of the additional effort required to move around and the larger volume of body mass to be cooled)

- Its age (the elderly and children under 10 are more sensitive to temperature variations because they sweat less)

- Gender (with equal factors and temperatures, women generally produce less perspiration than men and are therefore more sensitive to high temperatures)

- Its origins (the genetic factor plays an important role in our ability to manage temperatures)

What happens in our body when the heat is not dissipated enough?

Prolonged exposure to heat, especially during heatwave episodes, ends up weighing on our body, which ends up in hyperthermia. The regulation of the body temperature then becomes the priority of the body, which puts to the background all the rest of its mechanisms: the heart rate increases, the blood vessels dilate, the blood is sent in priority to the extremities of the body, the digestion slows down, the brain is (a little) less irrigated ...

This often results in loss of appetite, but also a decline in concentration and the ability to perform physical or intellectual work, as well as difficult sleep. This fatigue is usually accompanied by increased irritability.

A long and intense episode of heat can also cause

- edema (swollen ankles and feet)

- a miliary rash, which manifests as red patches on the skin, caused by inflammation of the sweat glands when you sweat too much

- muscle cramps and pain (the body having lost a lot of salt, or sodium, by sweating)

- dehydration or a deficiency in mineral salts, leading to general exhaustion, headaches, nausea, etc.

- heat syncope, due to a drop in blood pressure in the rib cage (since blood rushes under the skin)

What is heat stroke?

If the body can no longer regulate itself at all, in the event of prolonged exposure to excessive temperatures (in direct sunlight, in particular), then "heat stroke" occurs, which requires prompt medical attention. It is manifested by a significant rise in body temperature (over 40 ° C), intense headaches, vomiting and / or loss of consciousness of varying length.

There are two forms of heat stroke:

1. The "classic" heat stroke, which mainly affects fragile people (mentioned above) and can occur even at rest and even in the shade.

2. Exertional heat stroke, which can affect even the most enduring of us, and usually occurs during physical activity or strenuous work or in direct sunlight.

Heat stroke is not to be taken lightly: it requires immediate medical attention, otherwise it can lead to injury or death.

This is because heat directly damages the body's cells. The danger increases with the temperature reached and the duration of exposure to this temperature. Beyond 45 minutes of hyperthermia at more than 41 ° C, the lesions can be irreversible and permanently weaken vital organs, in particular the kidneys or the liver.


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