Haemodialysis

Abstract

Hemodialysis is a treatment in which blood is passed through an artificial kidney (dialyzer) and then returned to the body. Within the dialyzer, the solute constituents of the blood are altered by exposing it to dialysis fluid across a semipermeable membrane. Most dialyzers are around 20–30-cm long and contain hundreds of tiny long tubes arranged side by side, made from semipermeable membrane (Fig. 77.1). The blood flows through the tubes providing a large surface area to allow passage of fluid and diffusion of electrolytes. The tubes are bathed by dialysis fluid running in the opposite direction providing a countercurrent system to maximize the concentration gradient across the membrane. Conceptually, the membrane can be thought of as a sheet containing holes or pores. Water molecules and low molecular weight solutes such as urea, creatinine, and electrolytes can pass through the pores, but high molecular weight solutes such as plasma proteins and some drugs cannot. Blood flows through the dialyzer at 300–400 mL/min and is anticoagulated with unfractionated or low molecular weight heparin to prevent clotting within the dialysis circuit. The dialysis treatment can be altered by changing the electrolyte composition of the dialysis fluid, the rate of fluid removal (ultrafiltration) and the duration of treatment.