A pump-leak model of epithelia reveals the roles of paracellular pathways in water transport & the importance of boundary conditions

A pump-leak epithelial transport model shows paracellular pathways are crucial for maintaining electroneutral water flux, enabling continuous Na+-powered flow with small osmotic gradients.

• Why it matters: Epithelial water transport depends on how ions and water move together, but boundary conditions and paracellular electroneutrality constrain whether steady flux is possible.
• What they did: The authors model water as a growing sink compartment, derive sink voltage calculations and analytical solutions, and use growing boundary conditions to allow water and ion fluxes in steady state.
• The result: With Na+ pumps on the basolateral membrane, water flux direction matches Na+ when apical Na+ conductance and basolateral K+ conductance are high, and flips when conductances are reversed.