Heat tolerance

Danger

Hi, all;
I haven't visited Biology before, so forgive me if this has been addressed already. It's a pretty unimportant question, but it's been nagging me all day.
I've always noticed that people's mouths can stand higher temperatures than their hands, and just got to wondering why. Do you figure that it's because of saliva, purely coincidental, or could it be a trait that evolved after the discovery of fire to keep us from burning our digestive tracts on something that we've just cooked? Do other primates share it?

Ouabache

There's a presumption in your comment that a person's mouths can withstand a higher temperature than the rest of our body. I'm not convinced that this is necessarily true. I can handle some seriously hot water when taking a bath. Also when I transferred culture media fresh from the autoclave, or when pouring 6 dozen petri plates, my hands could withstand some serious heat. How about people who can walk across hot coals without hurting themselves?

Our bodies are adaptive to changes in temperature. Our skin will thicken and callous in response to irritation (including heat). I believe the skin of your mouth may be equally responsive to irritation as your hands. Either way it is an adaptive response. In some sense this may be described as an evolutionary adaptation. Without doing any comparitive anatomy or physiology, I would say yes other primates (and vertebrates), share this adaptation.

Danger

Hmmm.... maybe it's just something in my family then. I know that I can eat chicken wings (bar snack type) if I hold them by the extreme ends, but the grease burns my fingers badly if I grab the centre. Also, I can't dip my finger into tea or coffee that's at a comfortable drinking temperature. Neither could my parents. I haven't actually asked anybody else about it, though.

Renge Ishyo

It could be because when you are holding the chicken statically with your fingers the heat is concentrated in one area whereas when you put it in your mouth, you are "swirling" the heat around and keeping it from building up at one point? Try this, measure the time it takes when you hold a piece of hot chicken in your hands until it gets too hot to hold. Then place another hot piece of chicken on your tounge or in some other position in your mouth but DO NOT chew it or swirl it around (try to keep it as static as you possibly can, at one place as if you were "holding" it in your mouth the same way you would hold it in your hand). Measure the time until the burn happens. I am no expert in err...mouth thermodynamics, but theoretically your two measurements should be about the same.

I can imagine that you are able to eat the hot chicken because it is constantly being moved around in this way (keeping the heat from building in any one area). If you tossed the hot chicken back and forth from hand to hand you would probably be able to hold it much longer than if you hold it in your hands for an extended period of time. Incidentally enough, this is the exact same thinking that is used for people that walk on hot coals (notice that you will see them moving the different parts of their feet constantly on and off to spread out the heat as they move across; very rarely do you see anybody STAND on hot coals for an extended period of time, because that would concentrate the heat on their foot and they would get burned).

somasimple

Hi All,

That is quite untrue.
Conductance in the mouth is better than skin.
Tongue is very sensitive for heat and we feel "it is hot" when reaching 43°C and we can support more for hands (near 60°C).

BTW, this sensitivity decreases in oesophagus and it is why it may be hot in mouth but no more "after" (but danger remains...)

Danger

I really thought that you had it nailed down, Renge. Your post makes a lot of sense. Unfortunately, the experiment failed. I have before me a cup of coffee that is a bit cooler than I like to drink. When I stick my finger in, it becomes too hot to handle after about 10 seconds (and uncomfortable sooner). Conversely, I can hold it on my tongue, under my tongue, or in either cheek, and it remains too cool to be decent coffee.
Maybe it's just a physical characteristic that I inherited, possibly in regard to my thermal maintenance system. My hands and feet get terribly cold in winter, due to bad circulation and almost no insulation fat.

somasimple

There is also accomodation/habit. Japanese people are habituated to drink hot tea but many people can't.

Cooks could manipulate hot dishes and plates. It is the same thing.

It it certainly the same thing for you, Danger.

Moonbear

I wonder if that's it. Most people I know are the opposite, where you can touch things with your hands that are too hot to put in your mouth (like that steaming, gooey, pizza that burns the roof of your mouth). This is usually because our hands have thicker skin, including callouses, that provide more insulation from temperature extremes, probably because they are exposed to the environment the most. That's why they tell new parents to test the baby bottle temperature on their wrist, not with their hands, and the same for baby's bathwater, because what feels cool to your hand is often too hot for any other part of your body.

Then again, temperature is also relative. If you just come in from outside on a cold winter's day (every day in Alberta, right Danger?), and your extremities are chilled, even tepid water can feel scalding if you run your hands under it to wash them. In contrast, the inside of your mouth has been kept the same warm temperature all along (unless you walk around with your mouth gaping open), so tepid water still feels cool in your mouth relative to your body temperature.

Ivan started a thread a while back on acclimation to different temperatures, in the context of living in warmer and colder climates and over time, not needing to bundle up so much in a cold climate, or not feeling like you're dying of heat in a hotter climate. I don't think we came to any good consensus of a mechanistic explanation for that at the time. We have new members since then, so maybe one of them knows more about that and can shed more light on it than we've previously sorted out.

matthyaouw

Another thing to think about- A large cup of coffee will stay warm for quite a while, where as the small amount in your mouth will cool to the average temperature of your mouth quite quickly. I can take sips out of a drink thats way too hot for me to stick my finger in, but I wouldn't fancy taking my chances gulping it down in one go.

Monique

How about sticking your tongue into it for 10 seconds, or take a big gulp.. which I wouldn't recommend (as wouldn't matthyaouw above).

As somasimple mentioned, the Japanese have a culture where they drink very hot tea and eat very hot rice, they have a high incidence of mouth cancers due to the heat irritation.

DocToxyn

What about vascularization and how quickly heat can be drawn away from its source via blood flow? Would a higher density of blood vessels in the tongue/mouth explain this phenomena. Anecdotally I would say from my experience that mouth wounds bleed more profusely than those on my hands, but I have no controlled experiments to prove this.

Perhaps specific heat shock proteins can lead to activation of angiogenic mechanisms in the mouth to provide this degree of protection/acclimation. Whereas the hands/fingers are better suited by a thickening of the dermis, you certainly wouldn't want this in the oral cavity.

Renge Ishyo

It could be a factor, but this would lend towards the idea that the hand would heat up much faster than the mouth (which is what Danger was saying, but goes against the idea about sensitivity that was brought forth by Soma simple).

Basically, if you accept that the blood (or the water) is responsible for resisting changes in temperature (this ignores any sensitivity differences of the actual organs) then places with more blood will heat up or cool down slower than places with less blood. I know in the cold with Vasoconstriction, the blood is pulled away from the hands, feet, ears, etc. quite rapidly because these areas seem to have low amounts of blood flowing through them to begin with so when you constrict the blood vessels, the blood is pushed away from this area rapidly into inner areas (I say seem because I am no physiologist). If that was the case it could be that since these are the areas that "cool down" most rapidly that they also "heat up" most rapidly? I'm going to do the "hot coffee test" a little later this morning and I will post my results too. Curious to say the least...

EDIT: Ok, I tried this. What I did was I used boiling water (no coffee), 1/4 cup, and stuck my finger in. As expected, I lasted maybe 1 second before I got that strong signal and had to pull it out. Next, what I did was I took a small sip and I was able to tolerate it quite nicely (didn't need to spit it out at all in fact). Next, I tried to take a much larger gulp and I got the same response as I did with my finger in it (maybe even faster), and had to spit it out immediately. My body is hardly a sensitive measuring device, so I can't say which reaction was stronger based on this alone. I don't recommend trying this either, but as far as I am concerned the following post seemed to be a factor at least:

This experiment would be much better to do with the spicy chicken itself since volume wouldn't be as much of a factor. Also, I am going to go back on the idea of vasoconstriction playing a large part in heat sensitivity compared with the sensitivity of the organs themselves. Suffice to say, i got to thinking about it and err...even though there is more "blood" in a certain male reproductive organ, I know from going into a cold swimming pool that the sensitivity to temperature changes is noticeably stronger there than it is with the feet or fingers (and before any of you ask, NO, I am not curious enough to try the boiling water experiment for the "love of science" in this instance...).

Danger

Wow! I never expected such an elaborate set of responses.
Unlike Renge and Matt, though, t'was no small sip that I took of my caffeine. It was a good slug, and although it did cool in my mouth, the initial contact was perfectly comfortable.
Doc, your notion about the vascularization might have validity here, but it's something that I don't know about. Basic thermodynamics would indicate that increased bloodflow would distribute the heat over a larger area, and thus 'dilute' it.
Monique, I just put the kettle on and will try the tongue experiment with my new coffee.
As for acclimatization, I wonder if that could be accomplished by eating spicy (chemically 'hot') foods rather than thermally hot ones. I go through a fair bit of jalepenos and little dry chilis on my pizzas and natchos.

Renge Ishyo

This I do know a little something about (indirectly). A past biology teacher explained to me that the chemical "spicyness" is relayed via signal receptors. What happens if you eat a lot of spicy foods is that your body gets used to it, and, rather than torture you, the body inactivates or withholds the chemcial receptors (she wasn't specific) so that the spicy food cannot relay the pain signals as effectively as it did before you "built up a tolerance."

She said that this is why it isn't really "macho" for people to eat super spicy food, because if they are used to it they won't feel pain because the receptors for this are not present in the same way that they would be for someone not used to eating such spicy food. I wonder if a similiar "resistance" to signaling can be built up for simply heat sensing (as opposed to chemical signaling). Certainly, when I took a pretty big gulp of boiling water it didn't take very long for me to have the exact same reaction as when I stuck my finger in the cup. At the same time, spicy food tends to "hurt" in my mouth because I tend not to eat it much. I suppose in this regard, I am not the best test subject anyhoo

DocToxyn

It's difficult to work that particular organ into this discussion, since for one it's incredibly dense in sensory nerves and therefore, without taking this point too far away from PC, would be expected to be very sensitive to many stimuli including temperature. Plus, the vascular control in the regionsis also different since that is what controls one of its functions, it may not be more vascularized, just that the vasculature is specially controlled to retain blood once it enters the organ in question. Having said that my whole vasculature theory for temperature control was simply that, a theory, and the feedback is appreciated. Perhaps I'll do some searching this weekend...

DocToxyn

Since the spicy foods/chemical receptor system involves thingsl ike capsaicin and substance P receptors, you're right that you can get receptor down-regulation which will decrease their density following long-term activation. The heat tolerance issue doesn't directly work through a receptor system, although as I stated in my earlier post there are protein systems that can react to heat and set off cascades similar to what one might trigger with receptor-based systems. Yeah...I'll have to do some googling to see what I canfind on this subject.

DocToxyn

Danger, good to have you here. You've opened up with a good one and we won't let it go until we've thrashed it thoroughly.