Thermal comfort. Why, when your feet are cold, do your throat, joints, and kidneys hurt?

Hypothermia by itself does not lead to illness. BUT! Hypothermia, even local, quickly reduces immunity, and with weakened immunity, it is much easier to catch an infection. And not just a cold..

Let’s try to understand why this happens. To begin with, here are a few simple facts about the heat exchange of the human body:
1. Even local overcooling can lead to serious heat loss.Our body is made up of 70% water, and has almost the same high thermal conductivity The thermal conductivity of the skin and subcutaneous fat is almost half as much, which helps to retain heat, but does not fundamentally affect the situation. A layer of good fur with warm undercoat would drastically reduce our heat loss, but what isn’t there, isn’t there.. Blood circulation, and blood being an excellent heat carrier, further increases the rate of heat exchange, and therefore — the amount of heat loss.
In other words,if a part of your body cools down more than the rest, for example, if you spent the whole day fishing in water up to your knees, then the entire body will lose heat. Of course, different parts of the body lose heat at different rates, but still quite quickly. In the “extreme case,” in icy water close to 0°C, a person without special protective gear can endure literally 15-20 minutes. Beyond that, death from hypothermia. And here’s an example from everyday life: it’s nice to sit under a fan after a shower in hot weather. However, the risks of catching bronchitis afterwards are real, and if you’re unlucky, even pneumonia.

2. The body’s reserves for heat production and compensation of heat loss are very limited, to say the least, extremely scarce. The daily energy requirement we consume through food, which is around 2500 Kilocalories, is only enough to heat 68 liters of water from 0°C to 36.6°C (68 liters*36.6°C*1 Kilocalorie/liter (specific heat of water) = 2488 Kilocalories)
And now imagine what will happen if you remove a pot of hot water from a warm stove and place it on a cold stone floor.
Correct, the water in the pot will cool down much faster.
The same applies to the human body. Even slight local overcooling can lead to serious heat loss on a whole-body scale. The only way to compensate for such losses is to eat more food. Often, much more, as 2500 Kilocalories per day is not as much as we just saw. You can, of course, deplete some of the existing fat reserves, but that only postpones the problem of finding additional food for later. Throughout the last 100,000 years, the human diet hasn’t been very abundant. And all this time, our genetics have been tuned to energy conservation. Therefore,

3. As a rule, the natural reflex reaction of the human body to overcooling, even of individual body parts, is not additional heating, but rather a reduction of heat loss wherever possible.More accurately, the natural reaction to local overcooling is a reduction of heat loss. In reality, as is often the case, it’s much more complex. Shivering, for example, increases heat loss. But these are secondary level adjustments.

4. In practice, the reflex reduction of heat loss primarily means the narrowing of peripheral blood vessels (remember, blood is an excellent heat carrier?).

5. The narrowing of peripheral blood vessels leads to a reduction of blood supply to our “external contour,” and consequently, to a reduction in the capabilities of the immune system where blood supply is limited, as a significant portion of protective factors enter tissues along with the blood.

6. Additionally, as temperature decreases, the majority of chemical reactions slow down. And immunity is precisely a complex biochemical reaction, highly sensitive to temperature. When you’re already sick, and your body activates a “boosted” mode to fight the infection, body temperature increases, including to enhance immunity. As you’ve probably experienced, an additional 1-2°C is usually sufficient for effective disease fighting.
Most pathogens are much more tolerant to temperature, and that’s how they survive.

So why does the throat suffer when the feet are overcooled?

If the skin has an external layer of hardened epidermis as independent antimicrobial protection, the mucous membrane of the throat lacks such protection. As a result: wet feet — blood vessel constriction as a reaction to heat loss — weakened local immunity in the throat — microbes feast on your mucous membranes with impunity, while you deal with tonsillitis.

Can local overcooling affect internal organs?

From a physics standpoint, more likely yes than no.
Firstly, as mentioned earlier, the thermal conductivity of the human body is quite high, meaning that with local cooling, the zone of active heat loss can spread to a significant depth.
Secondly, the body’s thermal conductivity is not uniform. Different tissues conduct heat differently. Therefore, the zone of active cooling may represent a “cold bridge,” located, for example, along an old injury, through which heat from the internal body quickly escapes outward, locally cooling some unlucky organ, and slowing down its immune reactions.
Add to this the sedentary lifestyle of the majority of humanity, where due to an inconveniently placed desk, you may end up overcooling the same body part for many hours day after day.
So, I wouldn’t be too surprised if after sitting with your back against a cold wall for 10 years, an office clerk develops, among other things, chronic kidney or joint disease.
To be fair, I haven’t been able to find any truly representative studies on this topic.

By the way, the opposite is likely true as well. With the help of localized controlled heating, diseases seemingly unrelated to the application site of heat can be treated. A whole range of traditional methods of therapeutic warming, a separate section of physiotherapy dedicated to treatment through heating (thermal therapy), Eastern methods — from warming massages to treatment with hot stones, and of course, saunas, all provide tangible evidence of this.