Kidney Dialysis

If one or both of a patient's kidneys fail (or fail to operate to a sufficient level) then dialysis may be used to regulate the concentration of urea and solutes in the blood. Although this process may be clinically effective it is often inconvenient for the patient so may be used as only a short-term measure - until a kidney transplant becomes possible.

The following diagram is a much simplified representation of kidney dialysis. (There are many different ways of illustrating this such as diagrams laid out in completely different ways, yet summarising the same processes.)

As shown in the diagram above, there are two separate circuits along which liquids flow.

They are:

1. The blood-flow from the patient, into the dialyser, through the dialyser, then back to the patient, and
2. The flow of dialysate around its own circuit which includes the dialyser plus some other stages.

Notes about Kidney Dialysis:

The main stages that blood passes through during the dialysis process include:

• Blood enters machine from body (under pressure from radial artery).
• Pump (some diagrams show a roller pump) controls pressure and flow rate.
• Anticogulant added to prevent clotting.
• Blood passes through dialysis membrane (equivalent to kidney nephrons).
• Bubble Trap removes any gas bubbles from blood.
• Blood is filtered then returned to the patient's radial vein.

Parts of a Kidney Dialysis Machine

1. Dialysis Membrane (sometimes referred to as simply a 'dialyser')
   Note that there are two types of artificial kidney dialysis in clinical use: Hemodialysis uses a cellulose-membrane tube immersed in fluid, whereas peritoneal dialysis uses the lining of the patient's abdominal cavity (peritoneum), as a dialysis membrane. This section flows the diagram above and so only describes the case of hemodialysis.
   The "dialyser" part of a kidney dialysis machine consists of a large surface area of cellulose acetate membrane mechanically supported by a plastic structure. Blood is pumped past one side of this membrane while the dialysate fluid passes on the other side. The membrane may be folded-over many times so that the large area of the membrane occupies a practical volume of space.
2. Dialysate
   The dialysate (solution) has the same solute concentrations as those in ordinary plasma. Therefore if the patient's blood plasma contains excess concentrations of any solutes, these will move into the dialysate, and if the blood plasma lacks the ideal concentration of any solutes, these will move into the patient's blood. Conversely, the dialysate fluid does not contain any waste products such as urea - so these substances in the patient's blood move down the concentration gradient into the dialysate.
3. Anticoagulant
   Heparin is the usual anticoagulant that is added to the patient's blood as it enters the dialysis machine (in order to prevent the blood from clotting as it passes through the machine). Preventing the blood from clotting should, in turn, prevent any blood clots from blocking the filtration surface of the system. However, heparin is not added during the final hour of dialysis in order to enable the patient's blood clotting activity to return to normal before he or she leaves.

Move to read the next page: Kidney Transplantation.

Note: This topic is not strictly 'Basic Anatomy & Physiology'. It is simple information about a procedure that might be of interest to students or part of course requirements.