I vaguely know about how blood is made, but what stops the body from making way too much? And if the person isn’t done growing, how does the amount of blood increase until adulthood then not keep going? I’m probably overthinking this, I just have a lot of time to think at work. Thank you.
Edit: Thank you everyone for answering my question. I really appreciated the mix of simplified and more detailed responses. This was such a great opportunity to learn from experts, and you made me want to learn more. Also, you answered my follow up questions before I got around to asking them, so thank you for that, too.
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Think of blood like fuel. Your body is only going to produce as much fuel as it needs to function properly.
Not overthinking it at all, it’s hugely complex. The short answer: negative feedback loops.
The more of something there is, the less signaling the body receives to make more. Once there’s enough, the signal to make more is gone. This is how virtually every normal process in the body works.
The slightly more involved answer starts with understanding that red blood cells only last about 3-4 months, so this feedback loop occurs many thousands of times between childhood and adulthood. Now how can the body know if it has enough red blood cells, they’re little particles flying around the blood stream and occasional bleed onto the floor? Many ways, but at least one is detecting oxygen supply in our end-organs (like muscles in arms and legs) – not enough oxygen, those cells produce lactic acid, the bone marrow easily detects these changes and ramps up red blood cell production. Plenty of oxygen? Bone marrow de-escalates to just enough production to replace what will wear out today.
When this feedback loop breaks, you get anemia (not enough) or polycythemia (too much). Lung problems causing a low oxygen level for other reasons (COPD/emphysema), you guessed it, they tend to have higher blood levels than people with healthy lungs. In fact much of what we know about our bodies comes from studying these broken states that we call disease.
It sounds like you think you stop making blood when you finish growing, but actually your body is always making new blood! Red blood cells only last around 120 days so your body is continually replacing them, you’re constantly adding liquid to your blood when you drink and your kidneys are continually removing liquid as part of filtering the blood.
It’s a constant process and your body is continually monitoring things like the blood pressure, oxygen content and saltines of your blood (you need that to live) and adjusts the rates of making new blood / removing old blood to keep things in balance.
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Your kidneys sense if blood volume is too high or low and will produce more or less urine to compensate
Your kidneys also produce a hormone that tells your bones to make more red blood cells in case they’re low as well.
Red blood cell production is very dependant on a number of factors and minerals you need in your diet, otherwise you can get any number of different types of anaemia (when you don’t have enough red blood cells)
Maintaining blood volume is also a massive headache too, because it involves huge flowcharts’ worth of positive and negative feedback loops in terms of how your body detects it and then acts on it
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