how does your body know to produce more blood after blood loss? How does it know when there is enough?
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From [this](https://www.blood.co.uk/the-donation-process/after-your-donation/how-your-body-replaces-blood/) website:
*When you give blood you lose red cells and the body needs to make more to replace them. Special cells in the kidneys, called peritubular cells, sense that the level of oxygen in the blood has decreased (due to the loss of red cells) and start secreting a protein called erythropoietin. This passes through the bloodstream until it reaches the bone marrow (the soft fatty tissue inside the bone cavities).*
The bone marrow produces stem cells, the building blocks that the body uses to make the different [blood cells](https://www.blood.co.uk/why-give-blood/how-blood-is-used/blood-components/) – red cells, white cells and platelets. The erythropoietin sends a message to the stem cells telling more of them to develop into red blood cells, rather than white cells or platelets.*
Tldr; your body basically measures blood oxygen levels and produces till its fine again, excess resspirces get put back into the bone marrow iirc.
Fun fact your pee and saliva is technically blood too. Saliva for instance at its base is blood plasma. So excess gets kinda flushed out over these systems too. But most of the ressources can more or less be pulled back.
So stay hydrated because your body and all its cells rely on this water cycling.
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