Concentration Polarization in an Ultrafiltering Capillary

Concentration polarization, the accumulation of retained solute next to an ultrafiltering membrane, elevates osmotic pressure above that which would exist in the absence of polarization. For ultrafiltration in a cylindrical tube, use of the radially averaged solute concentration results in an underestimate of osmotic pressure, yielding an effective hydraulic permeability (k) less than the actual membrane hydraulic permeability (k(m)). The extent to which k and k(m) might differ in an ultrafiltering capillary has been examined theoretically by solution of the momentum and species transport equations for idealized capillaries with and without erythrocytes. For diameters, flow velocities, protein concentrations and diffusivities, and ultrafiltration pressures representative of the rat glomerular capillary network, results indicate that the effects of polarization are substantial without erythrocytes (k/k(m) = 0.7) and persist, but to a lesser extent, with erythrocytes (k/k(m) = 0.9), the reduction in polarization in the latter case being due to enhanced plasma mixing. In accord with recent experimental findings in rats, k is found to be relatively insensitive to changes in glomerular plasma flow rate

Ethanol Production by Saccharomyces cerevisiae Immobilized in Hollow-Fiber Membrane Bioreactors

Saccharomyces cerevisiae ATCC 4126 was grown within the macroporous matrix of asymmetric-walled polysulfone hollow-fiber membranes and on the exterior surfaces of isotropic-walled polypropylene hollow-fiber membranes. Nutrients were supplied and products were removed by single-pass perfusion of the fiber lumens. Growth of yeast cells within the macrovoids of the asymmetric-walled membranes attained densities of greater than 10(10) cells per ml and in some regions accounted for nearly 100% of the available macrovoid volume, forming a tissue-like mass. A radial distribution of cell packing existed across the fiber wall, indicating an inadequate glucose supply to cells located beyond 100 μm from the lumen surface. By comparison, yeast cell growth on the exterior surfaces of the isotropic-walled membranes resulted in an average density of 3.5 × 10(9) viable cells per ml. Ethanol production by reactors containing isotropic polypropylene fibers reached a maximum value of 26 g/liter-h based on the total reactor volume. Reactor performance depended on the fiber packing density and on the glucose medium flow rate and was limited by low nutrient and product transport rates. The inhibition of ethanol production and the reduction in fermentation efficiency arose primarily from the accumulation of CO(2) gas within the sealed reactor shell space

The Clearance of Protein-Bound Solutes by Hemofiltration and Hemodiafiltration

Background. Hemofiltration in the form of continuous venovenous hemofiltration (CVVH) is increasingly used to treat acute renal failure. Compared to hemodialysis, hemofiltration provides high clearances for large solutes but its effect on protein-bound solutes has been largely ignored. Methods. Standard clinical systems were used to remove test solutes from a reservoir containing artificial plasma. Clearances of the protein-bound solutes phenol red (CPR) and indican (CIN) were compared to clearances of urea (CUREA) during hemofiltration and hemodiafiltration. A mathematical model was developed to predict clearances from values for plasma flow Qp, dialysate flow Qd, ultrafiltration rate Qf, filter size and the extent of solute binding to albumin. Results. When hemofiltration was performed with Qp 150 mL/min and Qf 17 mL/min, clearance values were CPR 1.0 ± 0.1 mL/min; CIN 3.7 ± 0.5 mL/min; and CUREA 14 ± 1 mL/min. The clearance of the protein-bound solutes was approximately equal to the solute-free fraction multiplied by the ultrafiltration rate corrected for the effect of predilution. Addition of Qd 42 mL/min to provide HDF while Qp remained 150 mL/min resulted in proportional increases in the clearance of protein-bound solutes and urea. In contrast, the clearance of protein-bound solutes relative to urea increased when hemodiafiltration was performed using a larger filter and increasing Qd to 300 mL/min while Qp was lowered to 50 mL/min. The pattern of observed results was accurately predicted by mathematical modeling. Conclusion. In vitro measurements and mathematical modeling indicate that CVVH provides very limited clearance of protein-bound solutes. Continuous venous hemodiafiltration (CVVHDF) increases the clearance of protein-bound solutes relative to urea only when dialysate flow rate and filter size are increased above values now commonly employed