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 - we have seen other diagrams laid
out in completely different ways yet summarising the
As shown in the diagram above, there are two separate circuits along which liquids flow.
- The blood-flow from the patient, into the dialyser,
through the dialyser, then back to the patient, and
- 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
- 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
Parts of a Kidney Dialysis Machine
- 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
The dialysate (solution) has the same solute concentrations as
those in ordinary plasma. Therefore if the patient's blood
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
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.
Heparin is the usual anticoagulant that is added to the patient's
blood as it enters the dialysis machine (in order to prevent
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/she leaves.
Move to read the next page: Kidney
Note: This topic is not strictly "Basic Anatomy
& Physiology" but is simple information about
a procedure. It may be of interest to students, and/or in some cases
also part of course requirements.