08.11.2018

The science of freezing - How to keep warm

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

Winter is coming and it is getting cold. How does the human body react to cold, what mechanisms help the body to keep its operating temperature and how can you keep warm in winter? To answer these questions, today's Christmas calendar entry deals with the topic of warmth and humans. What is heat, how much heat does the human body produce and how does it give off heat?

The human body can be regarded as a thermodynamic machine that produces heat during its movements and molecular biological processes (thinking also requires energy). This heat output amounts approximately to 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 must be emitted to the outside. For this purpose the human body provides some sophisticated techniques:

  1. Heat dissipation via the skin: Each body emits heat via convection to its environment (air). The air near the warm skin is heated and absorbs the heat of the human body. Air flows always bring fresh (cooler) air to the skin and can thus absorb further heat. This heat transfer can be imagined in detail: If a cool air molecule collides with a warm human skin molecule (the skin molecule swings 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) at different frequencies depending on the temperature. One knows these completely harmless rays from thermal imaging cameras, in which these can be made visible and one recognizes objects by their heat output. People cannot see these infrared rays, but they can feel them, just think of a so-called infrared cabin.

  3. Emission of matter: Human bodies absorb and release matter (food, excreta) through his metabolism. The released substances are usually warmer than the absorbed ones. Just think of the breathing air.

  4. Sweating: This represents an ingenious cooling method used by the body when the outside temperature or human heat production (during physical exertion) is so high that the heat emission via the air and radiation is no longer sufficient to cool the body. When sweating, the human being emits water, which then evaporates on the surface of the skin. In order to make water evaporate, it requires a lot of heat, which the human body can then emit (2.4 kilojoules per millilitre 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 be able to run properly so that chemical and molecular biological reactions can run properly. The most sensitive organ in humans is the brain, which suffers functional losses outside the temperature range of 35 to 40.5° Celsius.

How does the body actually notice that it is too cold?

Human beings have numerous temperature sensors on their skin that are about one square millimeter in size, so-called cold and heat points, which detect cold (8° to 37°) or heat (37° to 45°) due to 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, people can no longer distinguish 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 reacting to cold.

  1. Isolation. First, you can try to reduce heat loss by insulating yourself better. The small veins that run through the skin and are actually there to give off heat to the surface of the skin constrict. The skin appears whitish to blue due to the reduced blood circulation and functions as an insulating layer. If the body cools down even further, the reduced blood circulation in the outer areas (arms, legs) leads to two areas of blood circulation in the body, an inner warmer core and an outer cold, less circulated skin. The body accepts that limbs can partially freeze, while the most important organs remain protected in the core. A remnant from those times where the ancestors of humans still had a fur shows up in the so-called goose bumps. When it gets cold, the body tries to set up its no longer existing hairs to create an insulating air volume in its no longer existing fur. In cats one can observe this isolation technique well.

  2. Heating. The second method consists in the additional production of warmth by not consciously controllable trembling (stimulation of the sympathetic nervous system) or an increase of the heart rate and the respiration. If the human body does not manage to maintain its body temperature despite insulation and heating measures, both cognitive and physiological abilities are affected. Attention diminishes and one increasingly loses the ability to speak. At lower body temperatures of 33° Celsius, the metabolism is reduced and a sugar shock can occur because the cells can no longer metabolise the sugar present. With further increasing cooling the human being loses increasingly the ability to move its muscles (the metabolism necessary for it can simply no longer proceed). This also slows breathing and the heartbeat until an undersupply of oxygen leads to death (about 25° Celsius body temperature).

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

The best method is certainly to dress warm enough, i.e. to reduce the heat emission of the body to the outside. You should make sure that you don't get wet (leaking shoes), because drying clothes take a lot of (evaporation) heat out of your body. With cold feet or hands you can try to increase the blood circulation by movement (clapping, sports, warm-up exercises, etc…).

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

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

The Caritas cold telephone has been back in operation in Carinthia since 1st of December on 0676/89 85 27 90 20. If you see someone lying helplessly at the roadside, please call the hotline.

What is heat? Heat is the average energy of a body (object) that is stored in the movements of atoms or molecules (the union of atoms). Imagine a box with air. Air consists of many small molecules (oxygen, a lot of nitrogen and other gases) that move at relatively high speed. The average speed of an air particle at room temperature (25° Celsius) is about 1400 to 1800 km/h, even more than the speed of sound. However, the particles do not fly far because they soon collide with other particles and thus transfer their energy to other molecules or atoms. This wild mess and constant colliding (one could think of a moshpit) is commonly called heat. The average kinetic energy stored in the body or gas corresponds to the heat energy.

What is temperature? Temperature, on the other hand, is a measure of the state of thermal equilibrium in a body. Bodies with the same temperature do not exchange heat energy. There are bodies than can store a much higher amount of energy than others. This material property is called heat capacity and describes how much heat a body can absorb. Water, for example, 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 in the 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).

Quiz

About the author:

Gerhard Dorn is a theoretical physicist and works as PhD student at Graz University of Technology where he studies quantum effects in electrical transport through molecules.


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