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Akward_Tortoise t1_j29oquk wrote

Reply to comment by SirDuke6 in ELI5: Why does putting one foot out from under the blankets bring so much relief of heat while laying in bed? by SirDuke6

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.

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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.

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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?

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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.

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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.

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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.

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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 [https://pubmed.ncbi.nlm.nih.gov/11284002/]

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

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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.

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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!

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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.

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Fat_Doinks408 t1_j2b45zj wrote

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

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d4nowar t1_j2bqmbb wrote

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

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jpwanabe t1_j2d53it wrote

Holy shit. Did not know it moved that fast

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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.

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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.

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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.

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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.

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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.

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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

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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.

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