Consider a dialysis cartridge in which blood flows through the inner tubes and b

Question

Consider a dialysis cartridge in which blood flows through the inner tubes and buffer flows through the outer “shell”. In order to be transported from the inner tube to the outer shell, the urea in the blood (a.k.a. Blood Urea Nitrogen or BUN) must overcome not only the diffusive resistance of the membrane, but also the convectional mass transfer resistance on both the tube and the shell sides of the membrane.

The membrane itself can be considered as flat surface due to it being thin compared to the radius of the tube. The diffusivity of the existing membrane material to urea is 1 × 10-10 m2/s, and the partition coefficient of urea for the membrane material with respect to aqueous solution is 2. Through materials research, there is the potential to develop a new membrane material in which the diffusivity of urea will be double that of the existing membrane. However, the partition coefficient and thickness is expected to remain unchanged. If the thickness of the existing membrane is 25 m, and the flux of BUN through it is 0.001 g of urea/m2s when its concentration in the blood is 25 mg/dL (25 mg/100 mL) and in the dialyzing fluid is zero,

(a) Calculate by what factor (ratio of new to old membrane) the flux would change if the diffusivity were doubled. Assume that the concentrations of BUN in the blood and in the dialyzing fluid stay the same, as do the convective mass transfer resistances.

(b) What could you do to further increase the flux of BUN through the membrane?

Explanation / Answer

b) The principle of hemodialysis involves the clearance of solutes across a semi-permeable membrane through diffusion and ultrafiltration mechanisms. The utilized membranes are classified into two main groups: low-flux, which is based on using dialyzers with low permeability for water; and high-flux, non-celluloses membrane with increased permeability, which is capable of removing moderate-sized molecules between 10000 to 15000 Dalton, including many of the inflammatory proteins, ß microglobulin and lipoproteins. Some studies have suggested that highflux membrane improves the removal of moderate-sized molecules such as lipid profiles or homocysteins while other studies have concluded it has no significant impact on these molecules such as homocysteine levels.

Because of incomplete removal of uremic toxins, 90% of hemodialysis patients reveal symptoms of pathologic amyloidosis caused by ß microglobulin after five years of dialysis. One of the most influential reasons to continue a certain treatment is the degree of its impact on the targeted disease; while, the inadequacy of dialysis has been recognized as a major reason for the mortality rate of the hemodialysis patients (11). If the efficiency of hemodialysis is not adequate, the level of blood toxins and the clinical symptoms of the patient are not controlled, which lead to either an increase in the duration of each dialysis session or the frequency of necessary dialysis per week. This will consequently increase the mortality and morbidity of the patients and the cost of dialysis (12, 13).

There are a number of factors, which influence the adequacy of the dialysis, such as the time of dialysis, the dialysate flow rate, the surface of dialyzer, and the blood flow rate. However, the employments of many of these factors are considered impossible, because they are neither beneficial, nor feasible. For example, increasing the duration of the dialysis over four hours is beyond the patient's tolerance and will increase the cost of dialysis to a large extent. Furthermore, increasing the dialysate flow rate do not have a significant effect on the adequacy of the dialysis (14).

With regard to the available capacity of the dialysis centers across the country and the increasing need for further facilities.

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