Dialysis (hemodialysis, haemodialysis)
is a treatment to filter wastes and water from your blood, when your
kidneys do when they were healthy. Also, importantly, dialysis helps
control blood pressure and balance important mineral ion
concentrations in the blood e.g. potassium, sodium, and calcium, in
your blood.
People who suffer from
kidney failure can't filter their blood properly. to retain
requirements or remove waste substances.
Kidney patients may be treated either using a
kidney dialysis machine
or having a healthy kidney transplanted.
If the kidneys don't function
properly then waste products build up in the body's bloodstream and the
body's ability to control the water content and ion concentrations is lost,
a situation that can be fatal - kidney failure is a bad situation - a major
lack of this aspect of homeostasis control.
Though uncommon, kidney failure
can be caused by infection, severe poisoning, an injury with severe loss of
blood or very high blood pressure
Treatment by haemodialysis restores the
concentrations of dissolved substances in the blood to normal levels and has
to be carried out on a regular intervals to keep them at the right 'normal'
concentrations and remove waste that that the malfunctioning kidney cannot
deal with.
In particular, keeping the ion
concentrations at the right level and removing waste products is essential
and a dialysis machine can perform some of the functions of a real kidney.
In a
dialysis machine
a person’s blood flows between partially permeable membranes
(selectively permeable membranes) - shown in the dialysis system
diagram below.
Haemodialysis
Diagram of kidney dialysis
machine procedure - the dialyser connected to a patient's arm - blood is
pumped from an artery to the dialysis machine and the 'cleaned' blood pumped
back into a vein.
In a dialysis machine there is a
selectively permeable membrane (partially permeable) where the patient's blood flows on one side and
the dialysis fluid on the other.
The membrane allows substances like
excess ions
and urea waste to pass through the membrane from the blood to the dialysing fluid,
but, does not allow larger molecules like proteins through or white or red
blood cells.
This is a
similar action to the membranes in the kidney - the membranes are
indicated by the blue lines in the dialysis machine diagram
above.
The dialysis fluid contains the
same
concentration of useful substances e.g. of ions and glucose as healthy blood to ensure that glucose and useful mineral ions are
not lost from the blood
in the dialysis process - minimises the formation of diffusion gradients, so
minimises unwanted ion/molecule transfers.
Only waste substances such as urea
and excess ions pass
out from the blood through the membrane into the dialysis fluid.
This will
naturally happen by diffusion because the dialysis fluid contains no urea,
so, the diffusion gradient is from blood to dialysis solution.
However, large molecules
like proteins cannot pass through the dialysis membrane
and neither can any blood cells.
The levels of ions and glucose
in the dialysis fluid are set at approximately those required in the
patient's blood so that diffusion may occur in either direction to maintain
the correct concentrations in the blood.
For people with poorly
functioning kidneys, dialysis might be required several times a week with
each session lasting several hours (3-4 hours).
Regular dialysis will keep the
concentrations of dissolved substances in the blood at the normal levels
and remove potentially toxic waste substances.
BUT, dialysis is not without risks:
Dialysis may cause blood clots.
There is an increased risk of
blood infection.
It is also an expensive
unpleasant experience, even if its free on the NHS.
These risks and
unpleasantness must be balanced against giving the patient with
kidney failure some time, and therefore a chance, getting a
life-saving transplant of a donor organ.
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