Why do we sweat?

We all know that we sweat to refrigerarnos.

In this article I will try, in a very simple reason that need.

When performing any activity, and everyone is all, your body needs energy. Even when we sleep we use energy.

And what do we get?

Drawing on our waiver, those cupcakes we ate yesterday.

Using the oxygen we breathe and sugars of cake, there is what is known as oxidation reaction, which, sugars and O2, producing CO2, water and an amount of energy released as heat.

Well, is that energy which helps us to perform any activity, even sleep as I said.

Can you quantify how much energy is required for each activity?

Yes is the oxygen consumption data that will allow me to calculate how much energy I generate in each activity, and therefore, I will know a very important fact and used: metabolic heat or metabolic rate (M). Its value is expressed in watts (In another article, I will explain how to calculate this)

But what interests us now is to know some of the standardized values ​​of this data. I put some data presented in ISO 8996.



Metabolic rate



Metabolic rate


Walking on flat 2 Km/h


Running at 15 Km/h


Walking on flat 3 Km/h


Ice skating at 12 Km / h


Walking on flat 4 Km/h


Ice skating at 15 Km / h


Walking on flat 5 Km/h


Ice skating at 18 Km / h


Walking uphill at 3 km / h and 5 ° tilt


Clean a house

100  to 200

As you can see, stronger activities generate higher metabolic rate because they involve more mechanical work for our body breathe more intensely and more use sugars to produce the oxidation reaction with the corresponding energy generation becomes the concrete mechanical stress.

And here appears a little problem; performance.

If all the energy generated in the combustion is transformed into mechanical work would not have overheating problem. Not so.

Not all the energy I generate results in activity. To our surprise, only 20% and at best 25% of the energy generated is used to perform mechanical stress itself. I can also raise it like that for an activity, should produce 75% of the energy needed to do it for the loss of it.

If we symbolize as mechanical work W done involving consumption of 25% of the energy generated, we can understand that the net heat that I can stay in the body is MW

So what do I do with the "extra" energy?

Dissipate as possible.

And here, allow me to introduce the concept of heat by convection and radiation, while my body which generates heat in an activity, this temperature difference with the environment I'm in, can produce heat exchange. Let me explain.

Human beings have a body temperature of 35-37 ° C and the environment where I generate my activity is in this case at 10 ° C. Nothing prevents excess heat in my body to dissipate into the environment (apart from the clothes that can act as a barrier, of course. This is another thing that I will discuss in a moment).

It is a spontaneous process of energy transfer.




But what happens when I perform the activity is very strong and I am in a thermal environment of about 30 º C?

I find that we have to dissipate more heat and the ability to dissipate this heat energy in the form of convection or radiation is diminished by the decrease of the temperature difference between ambient / body.

To put it another way, it is a process "not spontaneous" or "less spontaneous" (not correctly use the strict sense of spontaneity, I use it only for clarification).


Although contributions have greatly simplified heat loss, I can understand that you have created the need for the body to find a different way to dissipate heat: sweating.

What is sweat?

Sweating is a colorless liquid with a certain concentration of electrolytes. Its key feature is now its high latent heat of evaporation (CLE). A high latent heat of evaporation means requires much energy, heat in this case, so that the sweat in the liquid state to a gaseous state pass and thus can emerge through the skin pores to the exterior of the body. (CLE = 2427 KJ / Kg).

That is, when my body is a high amount of heat generated in an activity, my body is used to heat the sweat that goes from liquid to gas goes out of the body through the channels of the skin. Only evaporating can emerge.

I achieved so consume a large amount of metabolic heat that could be stored in a dangerous increase in body temperature and simultaneously cool the skin with the sweat.

To end, I'll put a numerical example so you can see what he is capable of sweat.

A man who works with an oxygen consumption of 2.3 L / min will produce a net metabolic heat (M - W) of about 640 W.

This attempt to dissipate energy by radiation, convection, breathing, decreased blood flow .... (concepts that we have not explained), and sweating. What is clear is that if not dissipated, increase your body temperature to dangerous levels ..

If all this energy ONLY eliminated by sweating, efficient evaporation of perspiration about 16 grams per minute (approx. 1L / h), would get the speed of the heat loss can match the speed of accumulation, so that the internal body temperature to remain stable.

I can not conclude without commenting on various factors influencing the amount of sweat produced by an individual:
• It depends directly on the number of glands and sweat glands size. The fact that some people sweat more than others can mainly be attributed to it.
• Heat acclimation is another important factor that determines the production of sweat.
• With age, accentuates the decreased production due to aging of sweat glands.
• Sweating is affected by factors such as insufficient hydration heat.
• The evaporative heat loss depends on the gradient of the water vapor pressure between the human skin and the surrounding air. Thus, a high environmental humidity and the use of thicker or waterproof garments, limit heat loss by evaporation, while the dry air, air flows over the body and clothing fine porous facilitate evaporation.
• In a labor intensive major perspiration occurs, but the evaporative heat loss can be limited by the organism's ability to produce sweat, which maximum is between 1 and 2 l / h.
• Even in a comfortable environment, there is loss of small amounts of sweat.

Nothing more for now. Gradually we will explain new concepts related to this article.

A greeting.