Relationship of water diffusion to osmotic flow

As stated earlier, the permeabiUty of a given barrier whether it is a single membrane or a tissue to water can be measured either under steady-state conditions or as a bulk flow. The difference between these two types of measurements is not merely one of 

In many cases, the difference between Pf and P can be attributed to the presence of an unstirred layer. On the other hand, in the case of human red cell membrane the difference cannot be accounted for by the presence of an unstirred layer [14]. Additional support for this difference between Pf and P j comes from the studies dealing with the effect of antidiuretic hormone (ADH) on water movement across epithelia. It was found that in the presence of ADH, the osmotic permeability coefficient of this tissue to water is 120 times greater than the diffusional coefficient (see Table 9.5 in [23]). 

In classical terms, any value of Pf/P =g greater than unity may be taken as evidence for bulk flow through channels, and the equaUty sign holds for truly 

Permeability coefficients of representative membranes and tissues to water under both osmotic and diffusional flows  

Cell Under osmotic flow Pf-10 (cm/s) Under diffusional flow Fj -10 (cm/s) 8=P,// d 

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