Fig. 6.
The effect of increasing the length of lateral spaces as in layered epithelia or the lens. To maintain equality of salt transport, we set πr2jA = 2πal jBL by reducing jBL in proportion to 1/l, with jA constant. (A) Fluid transport as a function of l when apical and basolateral membrane water permeabilities are maintained constant. Because the boundary condition effect at the mouth of the tube reduces net transport (see text describing Fig. 4A), the longer the cleft the less important this effect and transport increases. However, because increasing cleft length also has the effect of increasing total basolateral membrane water permeability relative to apical, eventually apical water transport becomes rate limiting and water transport never exceeds 92% isotonic. (B) Fluid transport as a function of l when apical and basolateral membrane water permeabilities are maintained equal by reducing PBL as 1/l. In this situation, the mouth of the tube effect becomes negligible at long l, but apical flow does not become rate limiting, hence water transport approaches its isotonic limit.