system
Given the following post, respond as if you're commenting on Reddit:
human
How long does it take for the body to replenish blood that is lost? And why does it take that long?

I have seen from various websites that for blood to fully replenish it takes 4-6 weeks. What I don’t understand is why it takes so long and what variation there is depending on the amount of blood lost I.e. 15% vs 30%?
gpt
Creation of RBC's. Millions of them are being made and dying every second but in more or less equal amount. When you give blood you lose more 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.The bone marrow produces stem cells, the building blocks that the body uses to make the RBC'S.

Your body makes about 2 million new red cells every second, so it only takes a number of 3-4 weeks to build up stores of them again.

Also a number of other messenger proteins also stimulate the production of WBC and platelets cells in the bone marrow, and over the next few days levels return to normal but quicker than RBC.