Dialysis a physical process of diffusing blood across a semi-permeable membrane to remove toxic materials, and to maintain fluid, electrolyte and acid-base balance in cases of impaired kidney functions or absence of the kidney. The entire process is somewhat analogous to filtration.

In medical sciences, dialysis refers to kidney dialysis. The human kidney is a filtration organ that receives blood through arteries. Since arterial blood circulates through the body, it is deoxygenated and carries many impurities. The kidney filters this blood to remove impurities that are then released into the urine and passed out of the body. Patients with impaired kidney function are unable to purify blood resulting in accumulation of excessive amounts of toxic substances including nitrogenous metabolic wastes in their blood. Such patients thus have to be subjected to kidney dialysis through artificial means using a dialysis instrument or dialyzer.

The dialysis process may be of several types. Kidney patients mostly use either peritoneal dialysis or haemodialysis. In peritoneal dialysis, the lining of the peritoneal cavity is used as the dialysing membrane. Dialysing fluid introduced into the peritoneal cavity is circulated for several hours and is then removed. The procedure is repeated as required. In haemodialysis, which is more efficient than peritoneal dialysis, arterial blood is passed through the dialyzer through which the blood flows past a semi-permeable membrane. Across the other side of the membrane a dialysis fluid is passed which is low in dissolved substances and does not contain any of the blood chemicals that are to be filtered out. The dialysis fluid is so made with respect to its chemical composition that small low molecular weight impurities from the blood can easily diffuse into the fluid, leaving the blood substantially free of these impurities.

The dialyzer is a relatively simple instrument that saves many lives, both young and old. Patients with chronic kidney malfunctions have to be given haemodialysis from time to time to keep accumulation of toxic substances in blood within tolerable limits. Although a simple procedure, heamodialysis is not widely accessible to patients in Bangladesh. The former Institute of Post-Graduate Medicine and Research (now, bangabandhu sheikh mujib medical university) first established facilities for haemodialysis in the late 1980s. The service is now offered in Bangladesh by several government, autonomous and private hospitals. [Zia Uddin Ahmed]