Artificial Kidney Function and Dialysis Physiology: Mechanisms and Analysis of Hemodialysis and Peritoneal Dialysis

In this chapter we discuss artificial kidney function and physiological principles of dialysis. Dialysis is a mechanical or artificial way of replacing kidney function, and is widely used in patients with chronic kidney diseases (CKD) and acute kidney injury (AKI). Hemodialysis (HD) and peritoneal dialysis (PD) are the two main methods of dialysis. This chapter provide a complete coverage of the physicochemical principles of HD and PD. Clearance of solutes (like urea, creatinine, and other uremic toxins) and fluid removal are vital functions of dialysis. For that, three basic physiochemical principles of diffusion, osmosis, and ultrafiltration (UF) are used in dialysis. HD uses an external filter and dialysis machine, whereas PD uses the peritoneal membrane as a filter to perform the dialysis. Figure 11.1 illustrates the working of a HD machine. Therein, blood from the patient runs through the dialyzer filter made of a semi permeable membrane. The dialysis machine mixes fluid called dialysate in specific dilution to impart an electrolyte concentration of the dialysis fluid similar to the plasma electrolyte concentration. It heats the dialysate to the body temperature. Through an UF mechanism, it then removes the fluid from the patient's body. Through this diffusion mechanism, it balances and corrects the plasma electrolytes and bicarbonate from the patient's blood. The dialysed purified blood is then returned to the body. The dialyzer (Figure 11.2) is a filter that has a hollow-fiber compartment containing a bundle of approximately 10,000 hollow fibers. Each fiber has an inner diameter of about 200 µm when wet. The membrane thickness is about 20–45 µm, and the length is 160–250 mm. The dialyzer acts as an artificial kidney and allows the uremic blood to pass with the dialysate across the membrane, thereby removing uremic toxins by diffusion and convection. An example of HD prescription for a chronic maintenance HD patient is given in detail. This provides an understanding of replacing the excretory function of kidney by HD and its application. In PD, a catheter is placed in the abdomen. Through this catheter, a cleansing fluid is infused into the abdomen peritoneal cavity, as shown in Figure 11.10. The cleansing fluid is a solution of electrolytes, osmotic agent, and buffer. The peritoneum lining of the abdomen is a semipermeable membrane, which acts as a filter, and removes waste products from the blood. During treatment, a cleansing fluid called dialysate is cycled into the patient's abdomen through a small, flexible tube called a PD catheter, as shown in Figure 11.11. The peritoneum has the natural ability to act as a filter. Excess water and toxins are drawn through the peritoneum into the dialysis fluid. Uremic solutes and potassium diffuse from the peritoneal capillary blood into the PD solution, whereas glucose, lactate or bicarbonate and to a lesser extent of calcium diffuse in the opposite direction. Fluid is drained out of the abdomen in to a drain or waste bag and the excess waste and water are removed from the body. To increase the efficiency of PD and help the patient with these exchanges, automated PD can be employed to enable (1) higher clearance of solutes, due to higher volumes being used, (2) better fluid removal, due to shorter dwell time, and (3) and more freedom during the daytime with no exchanges needed to be made.