Why and where do red blood cells die? Textbooks commonly state that red blood cells are removed by the liver and spleen. Do those organs destroy the red blood cells within capillaries that course through those organs or are they transported into the liver/spleen cells to be broken down into their constituents? What causes them to be destroyed/removed from the circulation?

RBC's lifespan is about 120 days. RBC's don't have a nucleus or endoplasmic reticulum, but they do have enzymes in the cytoplasm, capable of producing ATP from glucose. This energy is used mostly to maintain the structure and flexibility of the cell. The erythrocyte is a biconcave disc, that is ~7 micrometers in diameter, but the cell is pliable and it should be able to squeeze trough narrow spaces without breaking the cell membrane. Over time these metabolic systems become less effective and damaged, and the cell slowly becomes more fragile and prone to rupture. Spleen has special trabeculae (bands of connective tissue) in its red pulp, that the blood passes trough. The spaces between them are narrow and "old" RBC's tend to rupture while trying to squeeze trough. Then macrophages "catch" any cell fragments down the stream. This can happen anywhere in the body, but spleen and liver are kind of filters for blood contents.