eli5: If your leg gets amputated, how does your body know to not produce enough blood to replace what was lost?
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Think about it this way: When you get a cut that *doesn’t* remove a limb, how does your body know how much blood is lost even then? How would it know how much flowed out and therefore how much to replace? You can see that even without losing a limb, it’s impossible to know how much volume of blood needs to be added, because there’s no way the body can know or measure blood loss by volume.
So what it actually does is measure a loss of blood **pressure.** When you lose blood, your body makes more until the system is filled back up to its usual pressure. This works whether a limb is lost or not!
* If you lose 3L of blood, your body notices a loss of blood pressure and makes 3L.
* If you lose 3L of blood and a leg that used to hold 2L of blood, your body makes 1L, notices you’re back up to full pressure, and stops there.
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