The red cell's 120 days

Untreated iron deficiency: the marrow is short of iron, so it makes too little hemoglobin and the cells come out small and pale. The spleen's stores are nearly empty.

Pick untreated or IV iron · two days a second

Red cells, youngest at leftMade short of ironFerritin storesIron on transferrinA red cell lives about 120 days. Bone marrow makes about two million a second; when they wear out, macrophages, mostly in the spleen and liver, eat them and recycle the iron, about 20–25 mg a day in a typical adult, far more than the 1–2 mg the gut absorbs. Iron leaves the macrophage on transferrin and goes back to the marrow.That's why ferritin and hemoglobin run on different clocks after IV iron. The macrophages take up the dose within days, so ferritin (a measure of stores) jumps, peaking around day 6 in this model. Hemoglobin rises only as the marrow turns iron into new cells, over the next four to eight weeks, which is why bloodwork is rechecked at 6–8 weeks.A typical pattern for one modelled woman (Hb 105 g/L, ferritin 8 µg/L, 1000 mg IV), from Fig 3's model; not a prediction for any person. Ferritin reads high for weeks after IV iron, so it isn't a reliable store measure in that window. The cells are drawn far fewer and larger than life; one drawn cell stands for a normal day's output. Time runs at about two days a second.