This paper by Eugene M. Renkin investigates the diffusion and molecular sieving through porous cellulose membranes, building on the theory developed by Pappenheimer and his colleagues. The study aims to provide experimental evidence supporting the validity of Pappenheimer's theory, which describes restricted diffusion and molecular sieving in living capillaries. Renkin measures ultrafiltration rates, molecular sieving during ultrafiltration, and diffusion rates of various molecular species through inert porous membranes. The results are compared with predictions from Pappenheimer's theory, and estimates of membrane pore radii and diffusion areas per unit path length are checked for consistency. The materials and methods include diffusion experiments using tritium-labeled water, urea, glucose, antipyrine, sucrose, raffinose, and hemoglobin, and ultrafiltration experiments with water and aqueous solutions of these substances. The membranes used are Visking "nojax" cellulose sausage casing, Du Pont uncoated cellophane sheet, and Viscose wet gel. The diffusion rates are calculated using Fick's law, and ultrafiltration rates and compositions are measured. Additional physical measurements, such as membrane thickness and water content, are also taken. The results show that the experimental data closely agree with the predictions from Pappenheimer's theory, yielding consistent values for pore radii and diffusion areas per unit path length. However, estimates of pore radius based on the widely used calibration method of Elford and Ferry are found to be significantly inaccurate. The paper also discusses the theoretical basis for restricted diffusion and molecular sieving, and provides a new method for membrane calibration using direct diffusion of isotope-labeled water. The study concludes that the experimental data support the methods and theory used by Pappenheimer et al. in their studies on living capillary walls, and provides valuable insights into the behavior of porous cellulose membranes.This paper by Eugene M. Renkin investigates the diffusion and molecular sieving through porous cellulose membranes, building on the theory developed by Pappenheimer and his colleagues. The study aims to provide experimental evidence supporting the validity of Pappenheimer's theory, which describes restricted diffusion and molecular sieving in living capillaries. Renkin measures ultrafiltration rates, molecular sieving during ultrafiltration, and diffusion rates of various molecular species through inert porous membranes. The results are compared with predictions from Pappenheimer's theory, and estimates of membrane pore radii and diffusion areas per unit path length are checked for consistency. The materials and methods include diffusion experiments using tritium-labeled water, urea, glucose, antipyrine, sucrose, raffinose, and hemoglobin, and ultrafiltration experiments with water and aqueous solutions of these substances. The membranes used are Visking "nojax" cellulose sausage casing, Du Pont uncoated cellophane sheet, and Viscose wet gel. The diffusion rates are calculated using Fick's law, and ultrafiltration rates and compositions are measured. Additional physical measurements, such as membrane thickness and water content, are also taken. The results show that the experimental data closely agree with the predictions from Pappenheimer's theory, yielding consistent values for pore radii and diffusion areas per unit path length. However, estimates of pore radius based on the widely used calibration method of Elford and Ferry are found to be significantly inaccurate. The paper also discusses the theoretical basis for restricted diffusion and molecular sieving, and provides a new method for membrane calibration using direct diffusion of isotope-labeled water. The study concludes that the experimental data support the methods and theory used by Pappenheimer et al. in their studies on living capillary walls, and provides valuable insights into the behavior of porous cellulose membranes.