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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The answers so far are crappy.
Yes the kidneys regulate blood pressure, and if that drops due to losing blood, they will simply stop producing urine to keep all the water inside the body and dilute the remaining blood to keep the pressure up, so it still goes places.
But that’s not actually the body replacing the blood. That just the body going oh shit and throwing a lot of water at the issue as a stop gap.
The kidneys also produce a hormone called erythropoietin (called epo when talking about doping in sports). They produce more of there’s less oxygen in the bloood. Which happens when the blood is diluted, because you lost blood, and it wS simply filled back up with water.
This hormone goes to the bone marrow and causes it to produce more red blood cells.
Once the kidneys sense there’s enough oxygen in the blood, they will go back to the basic maintenance levels of producing erythropoietin, and the bone marrow just produces as many new red blood cells as naturally get recycled after 3 months.
Since this mechanism is tightly regulated, you only produce more blood than needed if some disease messes with these systems, or you artificially inject EPO.
That’s pretty much how everything in our bodies is regulated.
Organ A senses some property, and release an amount of a hormone that’s proportional to the level of the property, which tells some other organ to produce XYZ which fixes the issue.
Which is also why we develop tolerances to very many drugs: if you change the balance at the wrong point, the body will just adjust its messaging to compensate for the drug messing with its balance.
If the drug goes beyond the capabilities of the body to balance things, it’ll work permanently, or if it makes it so the body doesn’t notice the balance has shifted.
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