Fluid transport: a guide for the perplexed.

Readers of the physiological literature may be excused if they feel that fluid transport has become a complex and confusing field that is difficult to understand and to assess. The major theories of fluid-transporting epithelia are examined here with respect to their ability to explain quasi-isotonic fluid transport and its modulation by salt transport, osmotic permeability and basal tonicity. The basics of each theory are set out concisely, and their pros and cons are made explicit. Finally, a comparison is made in table form indicating their overall performance in relation to the problems of this difficult but important field.