Schematic representation of the passive-gradient models: Spatially lumped (SL) and spatially distributed (SD).

<p>In the SL passive-gradient model, iron is released from the cells to a SL extracellular domain. In the SD gradient model, iron is released from the cells to a SD extracellular domain. The driving force for iron release in passive-gradient models is a concentration gradient of ferrous iron across the plasma membrane. Ferroportin acts as a selective channel through which iron diffuses passively in both the passive-gradient models. Reactions in the EC fluid are identical to those of iron kinetics in solution in both versions of the model.