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krammark12 t1_j29k2q2 wrote

The veins in your foot are relatively close to the surface, making heat transfer from your inner body to the outside temperature much easier. The blanket (or socks) are isolating your feet, making this transfer more difficult.


SirDuke6 OP t1_j29kxgp wrote

Sometimes the transfer of heat almost seems instantaneous, is that really how fast the blood travels through the body on a regular basis?


Akward_Tortoise t1_j29oquk wrote

It's instant because of the temperature difference between under the covers she outside the covers. You haven't lost a whole lot of heat, but you can feel the temperature difference in your foot immediately.


SirDuke6 OP t1_j29p22z wrote

Sorry, I should've been more descriptive. I meant an instantaneous change in parts under the blanket still. Not the foot out of the sheet.

I'm pretty dumb but I'm not THAT dumb lol.


AsoHYPO t1_j29y1ih wrote

Do you mean a feeling of instant relief like how you instantly feel less thirsty after drinking some water despite it needing a little while to enter your bloodstream?


SirDuke6 OP t1_j2a3u8g wrote

Kind of but more of a physical feeling in other parts of the body like the other leg or torso being cooler as opposed to a psychological feeling of being less thirsty.


nstickels t1_j2a53d3 wrote

Blood moves at roughly 3 feet per second through the body. So assuming average height, it’s approximately 1.5 seconds for blood from the foot to go back to the heart, where it is pumped out again throughout the body. This is repeated every single beat your heart is making. Now that cooled blood will be mixing with warmer blood in the heart, but each and every beat after that first 1.5 seconds will be mixing in slightly cooled blood from your foot over and over.


SirDuke6 OP t1_j2a70yg wrote

That's awesome. I had no idea how fast blood moved but that's pretty incredible.

I mean, I know how powerful it was based off of things like how far blood squirts out of an artery/vein when cut but didn't have an actual speed to comprehend it at.


iNd3xed t1_j2avjsv wrote

Although the above comment is not wrong about the high speed of blood this is only the case close to the heart.

In smaller blood vessels, and especially in veins, blood flows much slower, as low as a couple of centimeters per second in the index finger []

Not that this helps answer the original question by much, but this adds some nuance to the answer


fiendishrabbit t1_j2ay0ih wrote

If I remember my biology right, on average it takes 45 seconds for blood to make a full round-trip. Very fast in the aorta, much slower elsewhere.


MalleableCurmudgeon t1_j2dhns2 wrote

I served in Iraq during the ‘00’s and on particularly hot days, after patrols and convoys, we’d go to the medics for a bag of IV fluid they’d keep slightly chilled. Holy holy! The relief of cool fluid directly into the veins and feeling it move from my arm throughout my whole body was awesome!


Crepuscular_Oreo t1_j2fdd4p wrote

>I had no idea how fast blood moved but that's pretty incredible.

The speeds of various bodily processes are interesting. I have nerve damage. I was lying on the table at the doctor's office while they were testing the speed that signals travel from one part of my body to another. Being bored while the tests were going on, I did the math and calculated that the runners in the 100-meter dash at the Olympics run faster than my nerves send signals through my body. That seemed strange to me; I always thought of nerve signals being instant. It was several years ago so I don't remember the exact numbers.


Fat_Doinks408 t1_j2b45zj wrote

3ft per second?! Thats a lil hard to belive, ima have to look that one up.


d4nowar t1_j2bqmbb wrote

You've seen the squirting blood trope in movies, right?


jpwanabe t1_j2d53it wrote

Holy shit. Did not know it moved that fast


ImReverse_Giraffe t1_j2al66t wrote

It's a psychological effect. Just like the drinking water provides "immediate" relief even though it really doesn't. It's your brain rewarding itself for doing something it decided it needed to do to survive. The brain is funny like that.


Taverdi84 t1_j2cb2ia wrote

What I THINK you may be referring to is an inner-thermostat connected to your nervous system event. The sensation of relief from temperature change has a lot to do with the mind and expectations. A good little experiment is to hold one hand onto a cool surface (wooden table works best) hold that one hand there for about 20-30 seconds. Then switch hands in that exact same spot. The spot where your first hand felt cool on the table the whole time will now feel warm to the other hand. This is especially sensitive with hands and feet because they’re like little probes for the world around you.


CanISellYouABridge t1_j2d1oo8 wrote

Well you warmed the table up by pressing your palm into it for 20-30 seconds, of course the spot will feel warm to your other hand.


KnitYourOwnSpaceship t1_j29r6jk wrote

I've sondered the same, and my theory is that you're also disturbing the air under the blanket when you move your foot. Sticking part of yourself outside the blanket gives the chance for colder air from the room to move under the blanket. It's not a complete change of all the trapped air, but enough to make a difference pretty quickly.


SirDuke6 OP t1_j2a3y4a wrote

Makes sense, especially because if you were to lift your leg at all, the blanket rising would create a vaccuum that sucks in colder air.


macedonianmoper t1_j29qsl7 wrote

Keep in mind that if you have one foot out the blankets are most likely not perfectly closed so air from outside can come in so it's not just the cooler blood circulating in your veins, it's also that the moment you put your foot outside the entire air below the blankets also cooled


biff64gc t1_j2aj08v wrote

While a round trip can take nearly a minute, your foot is at one extreme. It's only path available is back towards your core so it does happen within a couple of seconds.

The effect happens similar to a coolant line with a radiator. Your foot expels a lot of heat quickly because of the temperature difference, cooling the blood off. That now cool blood travels back up to your core, immediately absorbing heat from your much warmer core area. So what happens to your foot, the reverse happens in your core with the same blood.

The greater the difference in temp at your foot, the faster it expels heat, the cooler the blood, the faster it absorbs heat from your core once it gets back up there.

So you don't need to wait for the blood in your core to reach your foot to feel cooler. You just need cooler blood to enter your core.


ImReverse_Giraffe t1_j2akyis wrote

Yes. If you're ever really hot, run cold water on the inside of your wrists. Those veins are near the surface and lead directly to your heart. It'll cool you down so fast it's crazy.


SirDuke6 OP t1_j2ano69 wrote

Life pro tip for free? Thank you!


Scoobysmith44 t1_j2c9jv1 wrote

Just to piggy-back, basic first aid to quickly cool someone suffering from heat stress/exhaustion, use ice packs or cold, wet towels on the head, neck, groin, trunk and under armpits.

More info from OSHA


LandoChronus t1_j2cwcga wrote

Ok, I threw some icepacks in the back of their car, now what?


4tehlulzez t1_j2dp6m3 wrote

Now put them basically everywhere besides the feet like the commenter said.


Conflucius t1_j2e4zjs wrote

It said on, not in. Quit being weird and strap them to a tree like the rest of us.


borg286 t1_j2e6cvc wrote

Where is this trunk on a person? Honest question.


jolloholoday t1_j2dggsx wrote

Just tried this, now my bed is soaked. Thanks for nothing.


bkydx t1_j2e6ucd wrote

Palms and feet and forehead all work better.

Glabrous skin is designed for heat transfer and only located in these 3 spots.

The skin on your wrist does not transfer cold as effectively beneath the skin surface.

Other suggested locations are Neck/arm pits because they are thinner cutaneous skin similar to wrist but the blood flow is direct to your brain.

De-oxyginated blood from your wrist goes to your heart and not your brain and there would be little cooling effect and less benefit then using any of the medically recommended cooling areas.


cherrybounce t1_j2ayzsz wrote

The feet are ideally suited to helping us keep a stable body temperature, for a few reasons.

They - like the hands - have a large surface area as well as specialised blood vessels which can be opened up to pass high volumes of blood through them and therefore offload heat quickly when required.

When not required, the blood vessels are constricted.

This, coupled with the fact that the feet (and hands) are at the end of our limbs and don't have much muscle (which produces heat) means that they cool down much more than other regions of the body.


sheevum t1_j2d18gq wrote

This is the right answer. It’s not just about draining heat, but the key is that they can drain and regulate temperature very effectively. So sticking one foot out is enough to regulate your temperature, which it can’t do effectively when your whole body is hot. It’s an adaptive radiator when stuck out.


rusty-lewis t1_j2c8tvz wrote

Your blood makes a complete circulation three times every minute. This includes your extremities. I have always attributed the instant cool feeling to this fact coupled with the blood vessels close to the surface. It’s also the reason you can cool down really quickly by putting you hands in cold water in the summer time.


Adonis0 t1_j2d59br wrote

It’s perception

Your brain goes, I’m too hot right now but that will be fixed in a few moments so we’re going to make it feel fixed now so we don’t go overboard

If you only felt comfortable when everything was right you’d be stuck in a yo-yo of toss blankets off, get too cold, rug up, get too hot. So your brain makes it feel fine when you’ve done enough to fix it


bkydx t1_j2e5yqb wrote

Glabrous skin is located in 3 parts of your body, the sole of the foot, palms of your hands and forehead.
It lacks the hair and pigmentation and has extra pores for heat transfer.

The skin on the rest of your body is thick and well insulated and designed to keep heat in and everything else out.


Adonis0 t1_j2d5c8a wrote

Also, your foot is the furthest from your heart, and so cooling there helps it radiate a lot of heat from the body as in the return trip it cools your leg and torso a little


cbeck23 t1_j2cpie2 wrote

Correct. That is why when it is hot , I put rubbing alcohol on the tops of my feet, sides of my neck, and back/inside of my knees


DonkeySilver6051 t1_j2dysvv wrote

But isn't that very fleeting? Or is it enough to cool down? Apologies for my ignorance.


cbeck23 t1_j2elr1n wrote

When it is that hot, any relief, however fleeting, is appreciated. Alcohol will flash off, but takes some of your body heat with it. It is enough to help if you are having problems falling asleep, and if you're awake, slather on some more Alcohol when it dries.


quitegonegenie t1_j2b39hl wrote

To quote Moby-Dick:

"We felt very nice and snug, the more so since it was so chilly out of doors; indeed out of bed-clothes too, seeing that there was no fire in the room. The more so, I say, because truly to enjoy bodily warmth, some small part of you must be cold, for there is no quality in this world that is not what it is merely by contrast. Nothing exists in itself."


FcBe88 t1_j2cagkk wrote

What a wonderful excerpt of the human experience of sticking your foot out of the bed when it’s cold.


abat6294 t1_j2avb17 wrote

People talking about heat transfer at your foot, but I think the bigger affect is how when you stick your foot out, you've created an opening in the blanket that allows warm out to leave and for cooler air to enter.


BuildANavy t1_j2b2fzb wrote

100% this. Heat transfer from the foot itself is definitely a factor, but I don't think it's the biggest one. While by no means sealed, the air within the blanket becomes pretty stagnant and holds heat while you're underneath it. When you stick your foot out you are creating a significant opening as well as air currents and turbulence from the movement that allows a large exchange of warm air under the blanket with cold air from the room.
You can easily test this by very gently poking your foot out of the covers while keeping them tightly wrapped around your ankle. You don't get the same cooling effect (I just tried it while I was writing this). P.s. if I'm hot in bed I just kick my leg up high, which lifts the covers and lets all the warm air out/cold air in. Don't need to leave my foot stuck out to be quickly cooled down.


bkydx t1_j2e7pse wrote


Soles and palms are made of glaborous skin design to let heat out.

Cutaneous skin keeps heat in.

A fan or creating airflow is airflow has absolutely nothing to do with biology.


DonutCola t1_j2cur1m wrote

That’s silly dude there is no fan circulating air out of your blanket besides your asshole


tryinadosomethingher t1_j2dwddw wrote

Don’t need a fan.. do you o own what happens when hot and cold air touch? When a cold front hits a hot front? It’s creates rapid air movement :) this can be felt when opening a bathroom door after a hot shower


DonutCola t1_j2cuszs wrote

How tf does cooler air magically enter? You’re not really creating a viable convection current dude. It’s just like a brain illusion thing.


abat6294 t1_j2dviq9 wrote

It doesn't rush in, but now there is an interface where cold/warm air can exchange. Having 1 small opening for air to flow is significantly different than having 0.


tryinadosomethingher t1_j2dwugh wrote

Heat rises. When hot air from the blanket open and touch the cold air.. we’ll you know what happens when a cold and a hot front meet. Ever open the door after a steamy shower and see the air move fast? Same principle with the hot air under your blanket just not as drastic


ThePhoenixBird2022 t1_j29njw6 wrote

If you look at your foot, you can see more veins near the surface of the skin than you would if you look at your lower leg because there is far less fat or muscle. With all those blood vessels near the surface of the skin to help transfer heat/cool between the blood and skin surface, feet are great temperature regulators.


SirDuke6 OP t1_j29p7tt wrote

So would it be pretty much anywhere that has blood vessels close to the skin? Pretty much anywhere that doesnt have much fat/muscle underneath the skin?


bkydx t1_j2e845c wrote


You have 2 types of skin.

Cutaneous - Thick, harry, pigmented, less pores, Doesn't transfer heat in or out of your body well.

Glabrous skin - Hairless, pigment less, Lots of pores. Transfers heat well.


Glabrous skin is located on your palms/soles and forehead.


Nyaos t1_j2abntf wrote

That’s interesting. I probably have under appreciated how much sandals can help keep you cool.


jerpha t1_j2a1qhd wrote

It is about the flux. If you are storing heat, you will feel hot. If you are loosing heat, you will feel cold. Temperatures almost don't matter.

Take a steaming hot shower, when you're done get away from it, you will feel cold even though your place is ar room temperature. You're actually loosing some heat.

Take a very cold shower, you will feel hot because you're storing it.

If you're close to not loosing neither storing, you will just feel neat.

Putting your foot outside the blanket just makes you store less heat. Plus yes feet have a lot of blood flows that helps the thing.


BuildANavy t1_j2b32fa wrote

Not really. When you get out of a hot shower and move into another room you feel cold because there's a lot of water evaporating off your skin, taking latent heat with it. Also, if you have ever been very cold you will know that just sitting in front of a fire doesn't immediately make you feel warm; even though you are warming up straight away it takes time for you to feel warm.


bkydx t1_j2e8mxy wrote

It's biology

Cutaneous skin is bad at heat transfer.

Glabrous skin is good at heat transfer.

Any other explanation is wrong.


royalsilk t1_j2dnabx wrote

3 main heat dump locations on your body, your palms, your upper cheeks and forehead, and your bottoms of your feet. Combine that with venting an already hot and contained area and you get immediate relief.


bkydx t1_j2e8bul wrote

Glabrous skin vs cutaneous skin.


Spiritual_Worker3062 t1_j2dhfq9 wrote

Because blanket provides a kind of safety and the leg controls the temperature, so you feel safe and comfortable.

Blankets are warm, soft, cover(“protect”) you. By being under a blanket you feel safety, comfort. And it separates you from the outside world, making it easier to fall asleep. But despite all the positive effects of being under a blanket, temperature can’t be controlled.


SpectralMagic t1_j2auzzn wrote

The soles of your feet have plenty of blood circulating through them, this combined with our feet's double-water pores and increased amount of pores per cm^2 makes it easier to shed heat through perspiration. Water is incredibly good at temperature regulation because of the energy required to heat and cool it to different states

Our palms and soles have an increased amount of sweat glands to provide greater friction with surfaces, our soles with ~600-700 pores per cm^2.

Edit: can't find a source for the "double water sweat glands", but I recall seeing something about it in a PBS documentary comparing humans to other primates, more specifically how sweating more efficiently may have given us the evolutionary advantage


wee-g-19 t1_j2d58cf wrote

I enjoy doing this it feels good. I work outside so most times wrap up warm, when overheating I pull my beanie up over my ears and I start to cool down without removing layers. Still warm I then roll up sleeves. Do whatever before removing a layer.


Tobirama4374 t1_j2d6vta wrote

Coz they are literal heat sink. Think of hairless skin patches on body like foot soles, palms and facial area that don't grow hair as heat sinks in machine that need to throw away extra heat. Body is never not burning calories and thus constantly creates heat which is also why balls are outside so sperm don't die of heat.


DonkeySilver6051 t1_j2dzwey wrote

This begs the question though, if said live sperm enters the female body it must be extremely hot in the uterus area during eg a heat wave. Just wondering.


courageous_salmon t1_j2d82op wrote

Forget the scientific explanations, it’s about tension and release. It’s the same reason a foot outside of the covers going back in also feels great.


bkydx t1_j2e97by wrote

Science has the exact unarguable answer but lets just ignore it and be idiots.

Glabrous skin is good at transferring heat.

Cutaneous skin is bad at transferring heat.


lazerdab t1_j2bz9m8 wrote

Most of the skin on you're body fluctuates how much blood is near the surface to regulate temperature. The bottom of your feet doesn't do that so much so blood stays relatively close the the skin surface so your feet are great out sending out heat.