The process of haemodialysis involves the pumping of blood around an extracorporeal (outside the body) circuit which includes a haemodialysis filter. The filter allows the blood to equilibrate with dialysis fluid from which it is separated by a membrane. In early filters, the membrane was made of cellulose but modern filters are made of synthetic materials, which are more bio-compatible. The membrane acts as a semi-permeable barrier across which solutes and water can pass.
The dialysis fluid contains purified (not sterile) water with added sodium, chloride, bicarbonate, calcium and magnesium. The concentrations of these solutes in the dialysis fluid are such as to allow the fluid to equilibrate with the blood, maintaining serum concentrations in the normal range. Because of the metabolic acidosis associated with renal failure there is movement of bicarbonate from the dialysis fluid into the blood, while there is little net movement of the other solutes. The absence of solutes which need to be excreted, such as potassium, urea, phosphate, in the dialysis fluid, means that they can diffuse out of the blood and be cleared in the dialysate.
In addition to dialysing out unwanted solutes from the blood, it is also necessary to remove water from the body. This process is known as ultrafiltration. To achieve this, the haemodialysis machine generates a pressure gradient across the dialysis membrane and this is able to suck water from the blood. The amount of water removed can be carefully controlled by modern haemodialysis machines.
One of the main problems with carrying out haemodialysis in small children is vascular access. A large vein needs to be cannulated to achieve the high blood flows required for efficient haemodialysis. Access is achieved either through a double lumen catheter inserted into a central vein, usually the internal jugular vein, or by creating an arteriovenous fistula, preferably in the forearm. The arteriolised vein which is produced is then accessed using two needles. The smaller size of blood vessels in children, compared with adults, makes achieving adequate vascular access more difficult.
This web site is aimed primarily at patients on haemodialysis but gives some simple information about haemodialysis:
http://kidney.niddk.nih.gov/kudiseases/pubs/hemodialysis/index.htm
This website describes vascular access for haemodialysis:
http://kidney.niddk.nih.gov/kudiseases/pubs/vascularaccess/index.htm