How do humans (mammals) maintain body temperature when the ambient temperature is higher than body normal?

Sweat.

Mechanisms of heat exchange between your body & the environment: Radiation, Conduction, Evaporation, and Convection. Only 1 of these 4 still works to shed heat when ambient is hotter than body temperature: Evaporation. So our skin persperates – a trait not shared by all mammals, btw – which evaporates water to lose heat. Still, in a hot environment you’re at risk for hyperthermia & heat stroke even if you produce a river of sweat, especially with no wind and little air circulation.

In addition to sweat, a lot of evaporative cooling happens in the lungs, where the tissues are moist and there’s a rich blood supply. Even when there’s no wind, this works. Mammals that can’t sweat rely heavily on this mechanism.

You won’t sweat much if the humidity is high. High humidity combined with high temperatures feels very uncomfortable as you can’t cool down.

@flutherother I think you mean to say that you’ll sweat like crazy, but it won’t evaporate.

@thorninmud You are right. The sweat can run down you in rivers but if it doesn’t evaporate it won’t cool you.

The next time I’m in 112F heat, I’m going to put a thermometer under my tongue and watch the temperature drop as that (relatively) cool blood flows past the bulb. This still ranks as one of the great miracles of physiology to me – it’s amazing that evaporative cooling could work so well in living beings. Nice touch with the info on cooling in the lungs – 112F air goes in and cooler air comes out? A great example of all this is the kangaroo, which literally slobbers all over its forearms to provide additional means of evaporative cooling.

By burning calories like an AC consuming electricity.

Actually, the centre of our temperature regulation system is in our brain. It is part of our hypothalamus. http://www.bbc.co.uk/schools/gcsebitesize/science/add_ocr_pre_2011/homeostasis/bodytemprev1.shtml

As usual, it is a bit more complicated than… ‘Oh… we sweat.’ No… we don’t just sweat. Our nervous system is involved. When we are cold, our nervous system sets us shivering and then does an amazing job to try to conserve our energy by constricting blood vessels in our extremities. The opposite when we are hot, our blood vessels expand. These reactions to hot and cold are controlled by our autonomic nervous system.

@cazzie While I think we can all admire the complexity and overall wonderfulness of the human mind/brain, I believe the core of the issue w.r.t. dumping excess heat is, in fact, answered by the ‘evaporative cooling’ responses. Yes, the nervous system is clearly the thing doing the sensing and the triggering of the various bodily responses, but in terms of addressing the original question, it is more of a ‘pure physics’ kind of phenomenon. Even understanding evaporative cooling, it is still quite amazing (to me, at least) that the body can dump excess heat into an environment that is hotter than the body itself. I understand how a refrigerator does it, but the body is not a refrigerator.

Even evaporation fails to cool the body if the humidity is high enough. At some point “higher than normal ambient temperature” turns into “roasting in an oven” lol.

I worked one summer at Disney World in Florida, typical temp 99F, humidity 99%. One of the “characters” (a guy in a suit mingling with people) recorded a temp inside his head of 130F. The characters all routinely took salt tablets and were limited to 15-minute shifts outdoors, then back down to underground air-conditioned staging areas.

@thorninmud makes a good point about respiratory evaporation. Exhaled gas is always saturated with water vapor at body temperature. Heat exchange between the body and ambient air takes place both at the skin surface and in the lungs. Hyperventilation should increase the rate the heat loss to some extent. Which is stimulated by hyperthermia – another feedback loop for physiologic regulation.