Abstract
The foetal vascular bed of the rabbit placenta was perfused in situ with plasma. From the rate of diffusion from perfusate to mother the permeability of placenta to 14C-urea, 14C-erythritol, 3H-mannitol, 14C-sucrose, 14C-inulin and 131I-albumin was estimated. It was found that except for mannitol, the permeability of the placenta decreased with increasing size of the molecule. Using the equations of Renkin (7) the dimensions of the hypothetical pores in placenta were calculated from the placental permeability to water (4), urea, erythritol, sucrose, inulin and albumin. The calculated values of the pore radius and the pore area per unit path length, however, varied greatly with the dimensions of the molecules used. It was therefore concluded that the rabbit placenta does not behave like an isoporous membrane. The experimental data fitted fairly well a model of a membrane containing two populations of pores of different radii and different areas per unit path length.
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