system
Given the following post, respond as if you're commenting on Reddit:
human
If our bodies can produce more blood, what stops it producing too much? How does it know when we have enough blood?
gpt
Erythropetin (EPO) is the primary regulator for the production of red blood cells (RBCs), and it's regulated by feedback loops.

EPO is produced in response to ow tissue oxygen. A protein called HIF is expressed in all cells, but it is degraded when oxygen levels are above a certain level. When oxygen drops, however, HIF forms a dimer and is moved into the nucleus. There, it binds to DNA and stimulates the expression of certain genes. One of those genes is EPO.

Additionally, EPO is kind of "soaked up" by healthy RBCs. If there are lots of RBCs, EPO gets soaked up and has less effect.