Now that the planet is experiencing unprecedented heat waves, temperatures above 37°C are becoming commoner. In such a situation, when the surroundings constitute a heat source at a temperature higher than the body temperature, it is natural to expect (based on the Second Law of Thermodynamics) that our bodies will absorb heat, and so heat up indefinitely. This does not happen. Why not?
It does not help to argue that our skin is at a temperature lower than 37°C. (It only makes matters worse, for then we would be absorbing heat at a still lower temperature.)
Hyperthermia is the condition of our bodies not able to expel excess heat. As the outside temperature rises, the body switches methods to expel heat, either evaporation, radiation, etc..
I disagree that the Earth's climate is warming up; nevertheless, if you are in an environment that does not allow adequate heat to be expelled, then death is imminent. But, people are able to live in many extreme environments, deserts, etc... Wind allows evaporation, hence, the requirement of much water in a dry, hot environment. If a person cannot find shelter or relief from the blazing sun of a Sahara desert, then death is imminent. Clothing also can modulate and aid is expelling excess heat.
Ultimately, given your thesis of a warming Earth, then people will have to migrate to cooler spots on the globe, otherwise, face death. Of course, this and many other reasons cause populations in dry, hot environments to be low. There are communities that live in the middle of the Sahara desert, but these are very small communities and have ready access to water, etc...
We have buffers, mostly sweat. 10 g of water evaporated are enough to decrease the temperature of the whole human body by 0.1-0.2°C. And we have insulation (especially the fat guys and the curly guys) but air is the worst heat conductor itself so hot air shouldn't be that dangerous as, e.g., strong sun. I would try to find the highest possible temperature a human can stand as the balance between water we evaporate and heat transfer through the air (and I can say immediately that the answer will depend not only on air temperature but also on humidity, wind velocity, body weight, body skin area and hairiness).
I wonder how long can a human can stay alive under water that is 40°C hot. In water, our buffer is not effective and our main insulation - the air - is not present. Do we die immediately? If not, then we must have other buffers.
Here's more:
https://www.quora.com/What-is-the-highest-temperature-a-human-being-can-survive
The key to understanding how the body survives any temperature is to understand that we must maintain a temperature gradient, in other words, the body is an engine, a heat engine; and, if the body is denied a gradient, it begins to shutdown. Of course, the other extreme, hypothermia, presents another problem all together: the body cannot create enough heat to maintain a healthy temperature, hence, one succumbs to death.
Thanks to both of you for your data. (By the way, I do not totally support the thesis of global warming; I was just wondering about the thermodynamics of the human body.)
@Raul,
Here's an interesting truth about thermoregulation of the human body: what would you expect is the effect of a coat? Many believe that a coat keeps you warm; but research showed that the human body releases about the same amount of energy with or without a coat, hence, a coat only makes you feel warm. In other words, the body must expel a certain amount of energy per unit of time, otherwise, it begins to breakdown.
The question entails that the body is a passive physical entity. However, the body takes a very active role in temperature regulation. To my knowledge, the regulation operates primarily through the hypothalamus, and sweating is the main cooling mechanism.
That being said, the last paragraph of the question poses an interesting thermal problem, namely, if the temperature of the body skin is say 37C and the environment is above that, say 39C, then there should be a heat flux going into the body. The solution to the paradox is evaporation. The heat flux evaporates the sweat. It is the same physical mechanism that defines the wet-bulb temperature. The catch: you need to drink water continuously. It also helps to wet yourself, like taking a bath or a shower. A nice trick I discovered is to wet the back of your ears, because there is a lot of blood irrigation there. Also, when the environment id dry, evaporation works much better. The worst scenario for body cooling is hot and humid, like Rio de Janeiro in summer.
The human body can only lose heat at ambient temperatures above 37 C by expelling latent heat in the form of the evaporation of water through sweating. It is not the sweat that carries the heat away. It is the evaporation of sweat from the surface of the human body which causes the cooling. Thus air flow which encourages evaporation also aids cooling.
From the POV of AGW, it should be noted that evaporation will only happen if the air is not already saturated. In other words, when the dew point (wet bulb temperature) rises much above above 37 C then we, like many other mammals, die.
http://www.nytimes.com/2015/06/07/opinion/sunday/the-deadly-combination-of-heat-and-humidity.html?_r=0
"Now that the planet is experiencing unprecedented heat waves, temperatures above 37°C are becoming commoner. " Do not believe to the "global warming story", the activity of the sun displays maxima and minima, well known may hnudert of years ago...if the temperature is about 37 C you must drink more water ...:-)
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