What is physically happening when a body grows acclimated to temperature extremes?

My gym has a cold plunge that I sometimes enjoy using after a workout. When I haven’t used it in a long time, it makes my ankles hurt like blazes, but only for a few days. After regular exposure, the pain goes away almost entirely and I can settle into the water just fine.

So what’s the deal? Is it purely psychosomatic, or is there a physical mechanism that inures me to the cold? The only thing that seems reasonable is that the cold plunge causes pain because I have a lot of inflammation in my feet, and regular cold therapy lowers the inflammation somewhat. That’s an absolute WAG, though. I don’t even know if chronic inflammation would cause pain of that level in a cold plunge.

Maybe when its hot, your blood thins to accomodate.vice versa.

Since we really don’t sense something as hot or cold but instead sense heat transfer in or out of our body you’re really just sensing the extreme changes in that transfer rate.
I’m guessing the initial pain is the shock of your body adjusting via thermoregulation to the massive suck of heat out of you. Once it can regulate and control the proper flow to its liking I’d guess it calms down to make you feel more comfortable.

That doesn’t explain why there’s no more initial pain after a few days of regular use. What’s happening to change that? And if the pain is just an expression of sensing the transfer rate, why is it localized in my ankles?

One thing that could be happening is that habitual exposure to cold could either increase the amount of brown fat in your body or improve its ability to increase your body temperature.

The Wikipedia article states that the other way that the body produces heat when exposed to cold is the shivering of muscle cells. It could be that the pain in your feet is caused by the shivering, which could be needed less when your brown fat response is improved.

One thing that I don’t understand is that different parts of the body readt differently. My face doesn’t really feel cold till it gets to 0F. My legs are okay down to maybe 10F, but my arms feel chilly already at 60F. Is it nature or nurture?

What could happen is similar to the story of a frog placed in a pan of room temperature water that is slowly brought to a boil. For the frog, everything seems fine . . . until it isn’t.

Here’s a previous thread that contains some useful sources

I’m pretty sure that when you grow acclimated to cold weather in a serious fashion, your metabolism actually ramps up to keep your body temperature where it should be. But both of these would be more when you’re living outside constantly- like what a soldier might experience in the field.

Most of us just get “used to” cooler temps these days, and I don’t think our metabolisms are really adjusting, because even when we are in winter, we spend most of our time more or less at room temperature. Same thing with hot temps- I think when you really acclimate, your body gets better at sweating, produces less heat overall, etc… But a lot of it is just transition between being used to one temp and getting used to another. It’s a question of how it feels, not how your body is necessarily reacting to it.

There is a classic movie that takes place on a banana plantation. The guy who has recently arrived asks the regulars how much longer until he gets acclimated. One of them says something like, “You never get acclimated. It’s a myth. But it’s the only way we can get fresh workers if they believe it.”