system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how does the body regenerate blood after donating?

Also when the blood that has been donated is regenerated why/how does the body know it is back to its needed amount?
gpt
After you donate blood, the number of red blood cells in your blood decreases. Red blood cells are responsible for carrying oxygen and this decreased oxygen in the blood is a stimulus for the kidney to secrete the hormone erythropoietin. This then travels to the bone marrow and stimulates the production of more red blood cells. This is a great example of a negative feedback loop.