"The thermometer nailed to a post reads 110 degrees F., but in the shade, with a breeze and almost no humidity, such a temperature is comfortable, even pleasant." -Edward Abbey
Devils Golf Course in Death Valley
I will have to respectfully disagree with Mr. Abbey on this point. I've spent much time in the desert, and not once have I found 110 degrees to my liking under any circumstances. Perhaps an exaggeration on his part? As even animals that evolved here avoid those sorts of extremes at all costs.
I recently found myself in this condition though. Hot desert sun baking me, bare rock beneath my feet radiating that heat back towards my body. Another point I will contest Abbey on is where he exactly expects one to find shade, in many parts of the hottest deserts there is none. In my case the shade of boulders existed ephemerally, disappearing and shifting as the sun danced across the sky. What is one to do when no amount of liquid can quench your thirst? When drenching yourself with provides only a moment's relief, and when the sun becomes your worst enemy? Scramble for shade. Get wet. Get indoors are the solutions which come to mind.
More importantly for our purposes, is the question of what is happening inside our bodies and cells when we (or any other animal) are tested with these extremes in temperature.
First lets focus on the brain. Our most important organ, the control center for our body. What happens to our cognitive abilities during heat stress? Well it turns out that it depends on a number of factors. In general, when you are challenged by heat, your cognitive performance will decrease. The degree to which it is negatively affected depends on many things including difficulty of the task, gender (females are perhaps less susceptible to cognitive decline during heat stress), and hydration (the better hydrated you are the less steep the decline when challenged with heat). This cognitive decline is notable because your ability to make smart decisions decreases when you are challenged with heat, potentially leading to mistakes which could make a bad situation worse.
What about in your muscles? As you work in hot conditions, your muscles become fatigued more quickly. You will be unable to exert as much force following long periods of exercise in hot conditions. This affect on muscles seems to be mainly seen during long periods of exertion, short term physical exertions are not influenced as much unless they come immediately following a period of prolonged exercise in hot conditions.
What about at the cellular level? Well, the cellular response to heat stress is perhaps one of the most well preserved responses throughout evolution. Following exposure to high temperatures your cells will start producing a special type of "heat shock proteins". Organisms needed a way to deal with the consequences of stresses like heat, and these special proteins could be evolution's answer. These proteins are recruited to make sure that your cells don't succumb to the often deadly affects of heat. That much is clear, but more remains to be understood regarding their mechanism of action within a cell.
There may be some temporary benefits of being challenged by heat. Rabbits that were challenged by heat had hearts that were less susceptible to other environmental stresses 24 hours later. It seems that dealing with heat, may equip your body to deal with other environmental stressors within a short time frame. Though this advantage seems to disappear the longer an organism is removed from the heat.
Decreased brain function, muscles that fatigue more quickly, and cells functioning differently than they would otherwise. All are consequences of heat exposure. Stay safe, stay hydrated, and stay cool.
References and further reading:
Hancock, P. A., and Ioannis Vasmatzidis. "Effects of heat stress on cognitive performance: the current state of knowledge." International Journal of Hyperthermia 19.3 (2003): 355-372.
Gopinathan, P. M., G. Pichan, and V. M. Sharma. "Role of dehydration in heat stress-induced variations in mental performance." Archives of Environmental Health: An International Journal 43.1 (1988): 15-17.
Morimoto, Richard I. "Regulation of the heat shock transcriptional response: cross talk between a family of heat shock factors, molecular chaperones, and negative regulators." Genes & development 12.24 (1998): 3788-3796.
Currie, R. W., R. M. Tanguay, and J. G. Kingma. "Heat-shock response and limitation of tissue necrosis during occlusion/reperfusion in rabbit hearts."Circulation 87.3 (1993): 963-971.
Nybo, Lars, and Bodil Nielsen. "Hyperthermia and central fatigue during prolonged exercise in humans." Journal of applied physiology 91.3 (2001): 1055-1060.
Lindquist, Susan. "The heat-shock response." Annual review of biochemistry55.1 (1986): 1151-1191.