Model for water and electrolyte transport through simple epithelia. The model comprises four compartments which are (1) the lumen of the organ (e.g. the gut), (2) the lateral intercellular cleft (LIC) which is assumed to be homogeneous with respect to electrolyte concentrations, (3) the cytoplasm of the cell and (4) the interstitium. In the lumen a given concentration of electrolytes is assumed. The tight junctions (TJ) separate the lumen (1) and the LIC (2) and are assumed to be impermeable for the electrolyte and permeable for water with a permeability coefficient K
. The concentration of electrolytes in the cytoplasm is assumed to be constant c
. ATPases are assumed to pump the electrolyte through the lateral membrane into the lateral intercellular cleft. The interstitium is assumed to display a constant electrolyte concentration c
which is maintained through the blood vessels located here. The important compartment is the LIC. Water enters this compartment through the TJ or from the interstitium. Ions enter through the lateral membrane due to the ATPases and leave the LIC due to diffusion and due to the water flux φ
, flushing the lateral intercellular cleft. The width of the lateral intercellular cleft b is dependent on the binding activity of the 7D-cadherins, which in turn is dependent on the extracellular Ca2+-level.