The goal of human metabolism is to maintain a consistent body temperature. Metabolism creates heat (as a byproduct of doing useful work) that needs to be shed, this is in addition to the natural heat flux.
Heat flux is proportional to temperature difference, and is solely in the direction of hot to cold.
Let us suppose that at 27 C, the natural heat generation is shed, and let us also suppose that the core body temperature is 37 C. (The actual values don't matter, but this feels approximately correct and gives nice round numbers). In this case, if the temperature is below 27 C, then we are losing too much heat and our body temperature will decrease. If it is above 27 C but below 37 C, we're not shedding our body heat fast enough, so body temperature will increase. If it is above 37 C, then the environment is actually adding excess heat into our body instead of cooling it--and we still have natural metabolic heat to shed.
So we have three main mechanisms we use to regulate body temperature.
The first mechanism is to change the heat flux: by changing our clothing, we can control heat loss. This is obviously more effective for cold temperatures than warm temperatures.
The second mechanism is to change our internal heat generation. For example, staying more active in cold conditions or being less active in hot conditions. The response of shivering is in fact an attempt to generate extra heat to mitigate heat loss.
The final mechanism is to use evaporative cooling. Converting liquid water to gas requires energy. So if you coat yourself in water that gets evaporated, it actively sucks heat away from you in the process. The wet-bulb temperature is a measurement of the coolest you can make something in this method... and that's why wet-bulb temperatures near body temperature are lethal: it's the point at which you just cannot shed any heat whatsoever.
4) Increase/decrease the blood flow towards our skin and arms/legs.
This is why in hot conditions, blood vessels may widen, the heart pumps harder to increase core <-> skin heat exchange, and you see people blushing.
In turn, this "heart pumps harder" (or failing to do so) may worsen other conditions (especially in older people), and eg. contribute to kidney failure (and that in turn, to blood poisoning).
Heat flux is proportional to temperature difference, and is solely in the direction of hot to cold.
Let us suppose that at 27 C, the natural heat generation is shed, and let us also suppose that the core body temperature is 37 C. (The actual values don't matter, but this feels approximately correct and gives nice round numbers). In this case, if the temperature is below 27 C, then we are losing too much heat and our body temperature will decrease. If it is above 27 C but below 37 C, we're not shedding our body heat fast enough, so body temperature will increase. If it is above 37 C, then the environment is actually adding excess heat into our body instead of cooling it--and we still have natural metabolic heat to shed.
So we have three main mechanisms we use to regulate body temperature.
The first mechanism is to change the heat flux: by changing our clothing, we can control heat loss. This is obviously more effective for cold temperatures than warm temperatures.
The second mechanism is to change our internal heat generation. For example, staying more active in cold conditions or being less active in hot conditions. The response of shivering is in fact an attempt to generate extra heat to mitigate heat loss.
The final mechanism is to use evaporative cooling. Converting liquid water to gas requires energy. So if you coat yourself in water that gets evaporated, it actively sucks heat away from you in the process. The wet-bulb temperature is a measurement of the coolest you can make something in this method... and that's why wet-bulb temperatures near body temperature are lethal: it's the point at which you just cannot shed any heat whatsoever.