The blood's filter and bench. The spleen culls worn red cells, mounts immune responses to blood-borne threats, and holds an emergency reserve of blood.
Two pulps in one organ: red pulp — cords of macrophages between sinusoids, where aged erythrocytes are inspected and dismantled; white pulp — lymphoid sheaths and follicles around arterioles, an immune organ for the bloodstream. The iron from culled hemoglobin is recycled to marrow via transferrin; the heme ring is excreted as bilirubin through the liver.
Metabolic Pathways
Iron recycling: the spleen recovers most of the iron the body reuses each day — heme oxygenase doing the extraction.
Bilirubin export hands pigment to liver bile — the spleen-liver axis of red-cell disposal.
Extramedullary hematopoiesis: in disease, splenic tissue can resume blood-cell production.
Regulation
Filtration is physical: rigid or antibody-coated cells cannot pass the cords' gauntlet.
Immune response tunes to opsonized particles — the spleen is the encapsulated-bacteria response organ.
Contraction can expel stored blood — the baroreceptor reflex's minor reserve.