system
Given the following post, respond as if you're commenting on Reddit:
human
Why do RBC's get bigger or smaller depending on whether Vitamin B12 or Iron is deficient?

I have studied that in Vitamin B12 deficiency, there is macrocytic (big celled) RBC production, whereas when there is a lack of iron, there is a microcytic (small celled) RBC production.

I know that vitamin B12 is required for DNA synthesis, and iron is required as a part of the heme group of hemoglobin, but why do the deficiencies have such different outcomes?
gpt
Well a red blood cell is in essence a cell which is filled to the brim with hemoglobin.

If you lack hemoglobin but can produce cells at a regular rate you have less hemoglobin per cell -> smaller cells. If you have a regular amount of hemoglobin, but can't produce enough cells because you're lacking in DNA synthesis you're ending up allocating more hemoglobin per cell -> larger cells.