Hemodialysis: Diffusion and Ultrafiltration

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Introduction

Normal kidneys perform at least two major functions; first they remove a variety of toxins and second they remove excess fluid from the body. In addition, the kidneys are important metabolic organs involved in glucose metabolism and production of erythropoietin, renin, vitamin D, etc. The kidneys accomplish its toxin-excreting function by first filtering large amounts of plasma to form a filtrate with a composition resembling that of the plasma except for the absence of proteins and protein-bound substances. Much of the filtered fluid is reabsorbed leaving behind a volume of urine that contains the waste products that require removal from the body every day. All of this is a continuous process that occurs twenty four hours a day. The goals of hemodialysis are mainly also two fold. First hemodialysis removes kidney failure-related toxins and second it is capable of removing excess water and salt. Hemodialysis accomplishes these goals in a manner different from what a normal kidney does. Plasma is passed outside of the body into a dialyzer (i.e., a filter) containing a large number of hollow fibers. These fibers separate the plasma from the dialysate and provide a large surface area for diffusion to take place. The dialysate is formed by mixing purified water with proper amounts of electrolytes and other essential constituents (such as glucose). As opposed to our own kidneys, there is some barrier to movement of molecules, even for those of relatively small sizes such as vitamin B12 (molecular weight 1,355). Also there is no reabsorption with dialysis, making adding needed small molecule to the dialysate, the only way of not removing these molecules from the body.

Hemodialysis as currently practiced is not a continuous process, unlike our kidneys. Even though the removal, during the time of dialysis, of small molecules such as urea is not dissimilar to the removal provided by the normal kidney, the overall clearance of urea is only about one tenth of that of the normal kidneys. This is because people commonly only receive dialysis for 12 hours or less a week whereas the normal kidney labors every second of the day. The dialysate composition is now standardized in most dialysis units with room allowed for small variations. However theoretically there is unlimited possibility to vary the dialysate composition based on the needs of the patient. During hemodialysis treatments, water and sodium are not ordinarily removed by diffusion but rather through the process of ultrafiltration. Ultrafiltration is commonly accomplished by lowering the hydrostatic pressure of the dialysate compartment of a dialyzer, thus allowing water containing electrolytes and other permeable substances to move from the plasma to the dialysate. The sodium level of an ultrafiltrate is not too distant from that of plasma. Finally, noteworthy is the fact that the dialyzability of a substance depends not only on the size of the substance but also on the permeability of the dialyzer membrane and the degree of protein-binding of that substance.

Contact between Blood and Dialysate

In order for hemodialysis to take place blood and dialysate have to meet inside the dialyzer even though the two fluids are separated by a semi-permeable dialyzer membrane (Figure 1). Dialyzers come in different sizes but are often cylindrical and about 20-30 cm (8-12 inches) long. The main job of the dialysis machine (Figure 2) is to make the blood and dialysate go through the dialyzer.

 

 

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