system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:after losing some blood, the body starts replacing that blood, how does it know that it has already finished replacing that lost blood so it could stop making more?
gpt
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 – 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.

Your body makes about 2 million new red cells every second, so it doesn't take long to build up stores of them again.

What about your white cells and platelets? A number of other messenger proteins also stimulate the production of these cells in the bone marrow, and over the next few days levels return to normal.