Hemodialysis works by circulating the blood through special filters. The blood flows across a semi-permeable membrane (the dialyzer or filter), along with solutions that help remove toxins. Hemodialysis requires a blood flow rate of >200 milliliters per minute (ml/mm). The usual IV line in an arm or leg will not support that volume of blood flow. Special form of access is therefore required.
The access can be temporary or permanent. Temporary access takes the form of dialysis catheters. These are large-bore catheters placed in large veins that can support acceptable blood flows. Most catheters are used in emergency situations, for short period of time. However, catheters called tunneled catheters can be used for prolonged periods of time, often weeks to months.
Permanent access is created by surgically joining an artery to a vein. This allows the vein to receive blood at high pressure, leading to thickening of the vein's wall. Now this "arterialized vein" can sustain repeated puncture and also provides excellent blood flow rates. The connection between an artery and a vein can be made using blood vessels (an arteriovenous fistula, or AVF) or a synthetic bridge (arteriovenous graft, or AVG).
The AVF is more desirable, because rates of infection are very low and it is quite durable. AVG is done only if it is not feasible. It may take a month for the AVF to mature or the AVG to be used, so careful planning is required.
Blood is diverted from the access to a dialysis machine. Here, the blood flows counter-current to a special solution called the dialysate. The chemical imbalances and impurities of the blood are corrected and the blood is then returned to the body. Typically, most patients undergo hemodialysis for three sessions every week. Each session lasts 3-4 hours.