Principles of extracorporeal therapy: Haemodialysis, haemofiltration and haemodiafiltration

Dialysis allows separation of the components of a complex solution by diffusive solute exchange across a semipermeable membrane. Convective movement of water from blood across the membrane is driven by the transmembrane pressure and also involves the bulk movement of dissolved solute across the membrane (solute drag). Diffusion is the underlying physicochemical principle of haemodialysis, whilst solute transport in haemofiltration is purely convective. The two principles are combined in haemodia-filtration. Early dialysers were manufactured from cellulose membranes. Modern dialysers are made of synthetic materials such as polysulfone, polycarbonate, polyamide, polyacrylonitrile or polymethylmethacrylate and are constructed in a hollow fibre configuration. Blood traverses through each individual capillary, whilst dialysis fluid flows in between the capillaries in a countercurrent fashion, thus maximising concentration gradients. Modern dialysis machines are designed to control and monitor the production of dialysis fluid at the prescribed flow rate, temperature and chemical composition, to control ultrafiltration, and to monitor and control the extracorporeal circuit.