What's the mechanism behind 'losing heat through your extremities'?

Question

I can understand how if you are not circulating enough (warm) blood, by not moving much while in a cold environment, your extremities themselves lose heat, and become cold - but I don't understand how they (the hands, feet, and head) can make the rest of your body lose heat too.

'Drawing the heat out of your body' (as people have termed it to me) doesn't make sense itself - what is the actual mechanism(s) behind the (exposed, relatively uncirculated) extremities, 'drawing' heat out of the body?

Is it that it merely draws the need to heat those (poorly insulated, circulated) extremities from the existing distribution of heating the other parts of your body, thus lowering how warm those other parts are?

If so, how does that happen? In the case above, where you're not moving the body to circulate warm blood to the hands (or using other warming stimuli like friction), how would 'heat transfer' to the hands occur then? Or does it just indeed slowly 'lose heat' and get colder and colder (which would then lead me back to my question, how would that explain the rest of your body becoming colder as a result of the extremities becoming colder)?

Answer 1

Your extremities lose heat at a faster rate than the rest of your body because they have higher surface area per unit volume. Heat exchange happens at surfaces, so if you have more surface, you transfer heat more quickly. Imagine you have a sphere of water with 1L volume, and a long cylinder of water, also 1L, lets say 1cm wide, the sphere will hold heat longer because it has less surface area per unit volume.

Why does losing heat through extremities cool the rest of your body? Because of your blood. As blood runs through your core, it takes heat from surrounding tissues. The blood carries that heat to the extremities, where cooler tissues pull it away from the blood. It's kind of like how the coolant in your car takes heat from the engine block to the radiator.

When it is very cold, your body will reduce blood flow to the extremities to save heat, making your fingers vulnerable to frostbite. However, it cannot stop bloodflow, those tissues still need oxygen, so you still lose some heat.