How does heat affect the human body?

The Physics of Freezing - How to Keep Warm

Image source: Frank E. Kleinschmidt - Library of Congress Prints and Photographs Division, Washington, DC 20540

Winter is approaching and with it it will be cold. How does the human body react to this, which mechanisms help it to keep its body temperature at operating temperature and how can you keep yourself warm in winter? To answer these questions, today's advent calendar entry deals with the topic of warmth and people. What is heat, how much heat does the human body produce and how does it give off heat?

The human body can be viewed as a thermodynamic machine that produces heat with its movements and molecular biological processes (thinking also requires energy). You can specify this heat output with about 150 watts, i.e. about the output of two and a half classic 60 watt light bulbs (or about 15 new LED filament lamps). In general, this heat has to be released to the outside. The human body can demonstrate some ingenious techniques for this:

  1. Heat dissipation through the skin. Every body gives off heat to its surroundings (air) via convection. The air near the warm skin is heated and absorbs the warmth of the human body. Fresh (cooler) air comes to the skin through air currents and can thus absorb more heat. This heat transfer can be imagined in detail as follows: When a cool air molecule hits a warm human skin molecule (the skin molecule sways strongly according to its temperature), the air molecule is literally thrown away and has a much higher speed and thus higher energy than before ( it is warmer).

  2. Radiant heat. Every body emits heat rays (so-called infrared radiation) with different frequencies depending on the temperature. We know these completely harmless rays from thermal imaging cameras, in which they can be made visible and objects can be recognized by their heat emission. Humans cannot see these infrared rays, but they can feel them, just think of a so-called infrared cabin.

  3. Material output. Humans take and give up and down matter (food, excretions) through their metabolism. The substances released are usually warmer than those ingested. Just think of the air you breathe.

  4. Sweat. This is an ingenious method of cooling the body, which is used when the outside temperature or human heat production (during physical exertion) is so high that the heat given off by the air and radiation is no longer sufficient. When people sweat, they excrete water, which then evaporates on the surface of the skin. In order to make water evaporate, it needs a lot of heat, which the human body can give off (2.4 kilojoules per milliliter of sweat)

Why does the human body actually need an operating temperature of approx. 36 ° Celsius? What happens when it gets colder?

Many processes in humans need a certain temperature to run properly so that chemical and molecular biological reactions can run properly. The most sensitive human organ is the brain, which has functional impairments outside the temperature range of 35 to 40.5 ° Celsius.

Humans have numerous temperature sensors about one square millimeter in size, so-called cold and warm points, which detect cold (8 ° to 37 °) or heat (37 ° to 45 °) based on temperature-dependent chemical reactions. The lips have the highest sensor density (25 sensors per cm²) and the calves the lowest density (5 sensors per cm²). Below 8 ° Celsius, humans can no longer distinguish between the temperature, but feel pain.

How does the human body react when it detects cold and what happens when it can no longer maintain the temperature of 36 ° Celsius?

There are two strategies for responding to the cold:

  1. Isolation. First of all, one can try to reduce heat loss by better insulating oneself. In the skin surface, the small veins that run through the skin and are actually there to give off heat to the skin surface constrict. The skin appears whitish to blue due to the reduced blood flow and acts as an insulating layer. If the body cools down even further, the reduced blood flow to the outer areas (arms, legs) leads to the fact that two blood flow areas are created in the body, an inner, warmer core and an outer, cold, poorly perfused shell. The body accepts that limbs can partially freeze off, while the most important organs in the core remain protected. A remnant from those times when the ancestors of humans still had fur is shown by the so-called goose bumps. When it gets cold, the body tries to set up its no longer present hair in order to create an insulating volume of air in its no longer present fur. This isolation technique can be seen well in cats.

  2. Heat. The second method consists in the additional generation of heat through unconsciously controllable tremors (stimulation of the sympathetic nervous system) or an increase in heart rate and breathing. If the human body does not manage to maintain its body temperature despite insulation and heating measures, both cognitive and physiological abilities are impaired. Attention decreases and one increasingly loses the ability to speak. At lower body temperatures of 33 ° Celsius, the metabolism is reduced, which can lead to a sugar shock, as the cells can no longer metabolize the sugar present. With further increasing cooling, people increasingly lose the ability to move their muscles (the metabolism required for this can simply no longer function). This means that breathing and the heartbeat also slow down until an insufficient supply of oxygen leads to death (around 25 ° Celsius body temperature).

What can you practically do to protect yourself from the cold?

The best method is certainly to dress warmly enough, i.e. to reduce the body's heat dissipation to the outside. Above all, you should make sure that you do not get wet (leaky shoes), as drying clothing removes an immense amount of (evaporative) heat from the body. If your feet or hands are cold, you can try to increase blood flow through movement (clapping, exercise, warm-up exercises, etc ...).

Why does the rescue blanket work? It actually doesn't feel very warm.

The rescue blanket basically consists of a metal foil that reflects infrared radiation and thus prevents heat loss through radiation. Furthermore, it seals the wrapped body airtight and prevents the escape of warm air (e.g. by wind), which acts like an insulator between the film and the body.

Since December 1st, the Caritas cold phone has been back in operation on 0676/89 85 27 90 20 in Carinthia. If you see someone lying helpless on the side of the road, please call the hotline.

What is warmth Heat is understood to be the average energy of a body (object) that is stored in the movements of atoms or molecules (combination of atoms). Imagine a box with air. Air is made up of many small molecules (oxygen, a great deal of nitrogen and other gases) that move at a relatively high speed. The average speed of an air particle at room temperature (25 ° Celsius) is around 1400 to 1800 km / h, which is even more than the speed of sound. The particles do not fly far because they hit other particles relatively soon and thus transfer their energy to other molecules or atoms. This wild mess and constant bumping into each other (you might think of a mosh pit) is commonly referred to as warmth. The average kinetic energy stored in the body or gas corresponds to the thermal energy.

What is temperature The temperature, on the other hand, is a measure of the state of thermal equilibrium in a body. Bodies at the same temperature do not exchange heat energy. A very large amount of energy can be stored in a body. This material property is known as heat capacity, which describes how much heat a body can absorb. For example, water can store far more heat than air. This is shown by the fact that the temperature at the sea does not fluctuate as much as inland, or that it takes much longer to heat a lot of water than the same amount of air (compare sauna with heated swimming pool).


About the author

Gerhard Dorn is a theoretical physicist and works as a university assistant at the Technical University of Graz, where he studies quantum effects in electrical transport through molecules.