ABSTRACT
The primary function of an epithelium is to limit the uncontrolled leak of various solutes across the barrier, which allows selective absorption/secretion of molecules across the barrier with the aid of various transcellular processes. Each epithelium specializes in selective and controlled translocation of specific ions (e.g. Na, Cl, HCO3, K, Ca) and various hydrophilic (e.g. nucleosides, nucleotides, sugars, amino acids, peptides, and proteins) and lipophilic (e.g. substrates for multidrug resistance transporters) solutes in an orchestrated and unique fashion under elaborate regulation by neurohumoral factors. Epithelial water transport is also thought to be tightly controlled in a similar fashion.
