How does the body know when to produce more blood? Is there a difference between quickly producing more blood to make up for an injury vs slowly producing more blood for a body gaining more mass?
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You can think of it in terms of rates of things.
Currently, your body has a rate of producing the components of blood, and a rate of breaking down and expelling the remainder.
When the amount of blood in your system is less than the usual, the rate of removal will automatically go down (less blood to clean = less waste products = less removal), but your rate of blood production does not take a hit, so, for those brief periods, your body will produce more blood than it will retire.
Note: not a biologist, so there may be more complex biology at work too, but even if your body chooses to do nothing, you will end up producing more blood because of how dynamic equilibria work.
Your body is always producing blood cells (as long as you have healthy marrow) and old blood cells are continuously dying off. Without injury or growth, they reach an equilibrium where the rate of death equals the rate of production.
If you do grow or suffer an injury, some of your cells in your kidney will notice that they aren’t getting oxygen as quickly as they used to. They’ll create a hormone called EPO (erythropoietin, which is also used by athletes as a performance enhancing drug). Your bone marrow reacts to EPO by going into overdrive and creating blood cells much faster than usual to try to recover.
To explain your question properly, we need to first establish that blood has 4 main components. Plasma, made of water and electrolytes, white blood cells, red blood cells and platelets. When you are injured and lose blood, pressure receptors in your carotid sinus, as well as your glomeruli, the filtering unit in the kidney, will notice that they aren’t getting as much *volume* as usual. This will cause veins to constrict (tighten), displacing more blood to the arterial part of the circulation, as well as release adh, which has the effect of decreasing the free water in your pee and making you thirsty. This will replace the VOLUME, by increasing plasma and diluting evertything else.
After that is done, in a few hours, the body will have had time to notice that less oxygen is coming through to the glomerulus (actually the juxtaglomerular apparatus), which will trigger the release of erythropoietin, stimulating the bone marrow to ramp up production of red blood cells.
When you increase body mass (by getting fat and or muscular) the change in blood volume is very minimal, so the same mechanisms will apply in a much smaller scale. Since blood cells are in constant renewal and are kept in balance by the mechanism I explained above (along with the spleen, which filters out old cells), production will accompany demand.
Eli5: when you lose blood, you get thirsty and pee less to increase the water in your blood, the cells come in later, more slowly. When you increase in size, the change is very small, and slow, so the same mechanism happens, on a smaller scale.
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