WATER PERMEABILITY OF THE FETAL ERYTHROCYTE.

The rate constant for the diffusion of tritiated water across the fetal erythrocyte membrane has been measured as 0.056 +/- SD 0.013 msec.(-1) The equivalent pore radius calculated by the method of Paganelli and Solomon for the fetal erythrocyte is 3.9 A. The effect of a normal distribution of channel sizes within the erythrocyte membrane is discussed and the theoretical effect of variation of the distribution on the diffusion to bulk flow ratio is evaluated. Since membrane channels of 4 A characterize a variety of membranes, it is suggested that permeability differences may be associated with slight variations in the statistical distribution of the channel sizes within the membrane. The limits of +/-0.5 A standard deviation that qualitative experiments place on the expected variability of channel size will not account for the lower rate of water diffusion in the fetal cell.