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 kidneys sense blood pressure and try to keep that in a tight range, rather than the actual volume of blood. If pressure doesn’t drop in the remaining body (i.e. bleeding is kept to a minimum during the surgery), they barely register the change at all.
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.
Also stimulus to breathe is not to get oxygen, but to exhale carbon dioxide/lactic acid/etc. Regulated by the feedback in the brain.
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.
>If your leg gets amputated, how does your body know to not produce enough blood to replace what was lost?
If you lose a leg, you also lose the long, marrow filled bones in which the blood is produced. A missing limb means less need for blood and less capacity to produce blood, so it balances out.
Blood is a combination of cells and water (plasma). The percentage of cells in blood gives blood its viscosity. Special cells in the kidneys are able to sense the oxygen carrying capacity of the blood, if that decreases the kidneys release a hormone called erythropoietin that stimulates the production of blood cells by the kidneys.
TLDR: The amount of blood after all is controlled by the kidneys based on the blood’s ability to transport oxygen.
To put it simply: The production of blood, specifically RBC, is triggered by the availability of oxygen. No problem in oxygen availability? Then, the body will only produce enough “RBC maker.”
“RBC maker” is an erythropoietin which triggers the bone marrow to produce RBC.
When it comes to plasma, which consists mostly of water, the brain senses if we need more water or not. If we don’t need water to create plasma, then no need to do so.
In summary, our bodies have indications if we need something or not. Since, after amputation, our body is still a closed system, hence, our organs live in harmony.
Your bones are what produce blood, I would imagine the bone lost is proportional to the amount of blood you no longer produce.
The kidney is your oxygen sensor. If the tissue inside the kidney has low oxygen the kidney produces erythropoietin. Erythropoietin makes the bone marrow produce more red blood cells. The control of blood volume is a separate process, based on brain and kidney released hormones (anti diuretic hormone, renin, angiotensin and aldosterone).
The body responds to its own needs. Blood is composed (mainly) of water, cells and proteins. The amount of red blood cells an its hemoglobin content depends on oxygen consumption of the body. If your leg gets amputated, you will lose certain amount lf blood, and with it you’ll have less red blood cells and less oxygen transport capacity, but you will also lose muscle and tissues that consume oxygen. If there’s more oxygen consumption than delivery via red blood cells, your body (the kidneys) will sense this difference and make a hormone called erythropoietin that will stimulate bone marrow to make more red blood cells and more haemoglobinto correct this, until there is no difference between oxygen consumption/delivery.
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