To add to this we can tell on lab tests if it's chronic or acute anemia through red cell morphology. A slower chronic condition genetally results in red cells smaller than average size with less hemoglobin (microscopic hypochromic) where in an acute bleed situation the cells are normal sized and filled properly, just not enough of them. We can also tell how hard the bone marrow is working to replace cells by the presence and number of nucleated or immature rbcs.

I know you didn't ask, but I don't get to nerd out about this stuff often and I find it fascinating.