Solute Transfer across Dialyzing Membranes

Copyright 2007 by Academic Press. All rights of reproduction in any form reserved. 

TABLE 6.1 Measurements Made in Assessing Drug Elimination by Different Routes 

Measurements Renal elimination Hepatic elimination Hemodialysis 

In this equation./ A is the surface area X is the thickness of the dialyzing membrane/ and D is the diffusivity of a given solute in the dialyzing membrane. Solute diffusivity is primarily determined by molecular weight. 

EIGURE 6.1 Plot of dialysis clearance (CL]j) vs. dialyzer blood flow (Q). The theoretical curves were fit to experimental data points to obtain estimates of the permeability coefficient-surface area product (P-S) for each solute. Flow-limited clearance is indicated by the dashed line. The data were generated with a Kolff-Brigham type hemodialysis apparatus. (Reproduced with permission from Renkin EM. Tr Am Soc Artific Organs 1956 2 102-5.) 

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In the previous section, mess tmasfer was discussed for a single point on (he membrane For practical applications, this analysis must be extended to describe irises transfer for the dialyzer as a whole. Neglecliug convective contributions, we can write the following equation for the mass of solute dm transferred across an element of membrane of length dx tied area dA per unit time (Fig. 21.1 -S) ... 

As an example, let us consider a simple one-compartment model for the prescription of treatment protocols for dialysis by an artificial kidney device (Fig. 1.1). While fire blood irrea concentration (BUN) in the normal individual is usually 15 mg% (mg% = milligrams of the substance per 100 mL of blood), the BUN in irremic patients could teach SO mg%. The purpose of the dialysis is to bring the BUN level closer to the normal. In the artificial kidney, blood flows on one side of the dialyzer membrane and dialysate fluid flows on the other side. Mass transfer across the dialyzer membrane occurs by diffusion due to concentration difference across the membrane. Dilysate fluid is a makeup solution consisting of saline, ions, and the essential nutrients that maintains zero concentration difference for these essential materials across the membrane. However, during the dialysis, some hormones also diffuse out of the dialyzer membrane along with the urea molecule. Too-rapid dialysis often leads to depression in the individual because of the rapid loss of hormones. On the other hand, too-slow dialysis may lead to unreasonable time required at the hospital. 

In a dialysis process, separation of the different solutes relies on the difference between the rates at which they are transferred across a semipermeable membrane located between the two liquid phases and across which mass transfer takes place. The type of the membrane is the key difference between gas-diffusion and dialysis units both use identical modules. Dialyzers are normally placed in the transport-reaction zone, and they are also occasionally positioned in the loop of the injection valve for... 

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