Hemodialyzers performance

Another index of hemodialyzer performance is the dialysance, (cm min ), defined as... 

Hemodialyzer performance is analyzed using a number of indices solute transfer efficiency i/ , clearance C, and dialysance Db. The solute transfer efficiency i/ (Michaels, 1966),... 

One widely used performance index of hemodialyzers is that of clearance, defined similarly to that of the human kidney. The clearance of a hemodialyzer is the conceptual volume of blood (cm inin ) from which a uremic substance is completely removed by hemodialysis. Let Qg (cm min ) be the blood flow rate through the dialyzer, Qjj fern min ) the dialysate flow rate, and Cg and Cj3 (mgem ) the concentrations of a uremic substance in the blood and the dialysate, respectively, with the subscripts i and o indicating values at the inlet and outlet, respectively. The rate of transfer of the substance in the dialyzer w (mgmin ) is then given as... 

This chapter will focus on three types of membrane extracorporeal devices, hemodialyzers, plasma filters for fractionating blood components, and artificial liver systems. These applications share the same physical principles of mass transfer by diffusion and convection across a microfiltration or ultrafiltration membrane (Figure 18.1). A considerable amount of research and development has been undertaken by membrane and modules manufacturers for producing more biocompatible and permeable membranes, while improving modules performance by optimizing their internal fluid mechanics and their geometry. 

Plasmapheresis typically employs a membrane module of similar configuration as a high-flux hemodialyzer. Alternatively, a rotating membrane separation element is used in which the tendency of the blood cells to deposit on the membrane surface is counteracted with hydrodynamic lift forces created by the rotation. The membrane element and the associated plasmapheresis circuitry are shown in Fig. 49. Worldwide, about 6 million plasmapheresis procedures are performed annually using this system, making this one of the largest biomedical membrane applications after hemodialysis. 

Perhaps the main reason for the tacit acceptance of the hemodialyzer stems from the fact that the human body contains two kidneys and can, usually, function satisfactorily with only one. This makes kidney donation and transplantation a more realistic operation than heart and other transplantations thus, kidney transplantations have been performed on a fairly common basis since the initial transplant in 1954. With the aid of careful tissue matching, preferably with a relative, and some immunosuppressant drugs, this transplantation operation has a high (85+%) success rate. In practice, however, the donor organs are not nearly as available as the demand, and 100,000+ people use the hemodialyzer routinely until a transplant becomes possible. In many cases, the hemodialyzer is used for decades, and secondary disorders, such as anemia, hemolysis, hypertension, and psychiatric problems, sometimes develop. 

Nevertheless to avoid the risk of serious damaging the chemical modification on hollow fibers hemodialyzers has to be performed in the absence of organic solvents and consequently the choice of the activation procedures is very limited. 

Derive the solution (Equation 8.13b) describing the performance of a hemodialyzer. (Hint Start with a differential balance for the dia-lyzer to obtain the effluent blood concentration, and use this value for the balance over the blood compartment.)... 

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