Hemodialysis
Normal Kidney:
• Normal kidneys are two bean-shaped organs,
each about the size of human fist. They are
located in the middle of human back.
• Human kidneys weigh about 0.5 % of human
total body weight.
• They receive a huge amount (20 %) of the
blood pumped by the heart.
Normal Kidneys do the following tasks:
1- Regulate the composition of human blood
A- keep the concentrations of various ions and
other important substances constant
B- keep the volume of water in human body
constant
C- remove wastes from human body (urea,
ammonia, drugs, toxic substances)
D- keep the acid/base concentration of human
blood constant
2- Help regulate human blood pressure.
3- Stimulate the making of red blood cells.
4- Maintain human body's calcium levels.
Kidney Failure:
• When the kidneys stop working, the condition
is called kidney failure or renal failure.
• A patient can not get rid of excess fluid/ waste
products and they stay in the body.
• The body may not be able to produce enough
of red blood cells.
• Blood chemistry may become abnormal. This
means that the patient no longer has the right
balance of solutes such as potassium, calcium
and phosphorus in the body
Definition of Hemodialysis:
• Hemodialysis means cleaning the blood.
• Blood is circulated through a machine which
contains a dialyzer .The dialyzer has two
spaces separated by a thin membrane. Blood
passes on one side of the membrane and
dialysis fluid passes on the other. The wastes
and excess water pass from the blood through
the membrane into the dialysis fluid which is
then discarded. The cleaned blood is returned
to human bloodstream.
Hemodialysis machine:
• It is used for preparing a solution which is
called dialysate.
• Dialysate is composed of substances the same
as in blood but in lower concentration.
• Dialysate is diluted by purified water which is
prepared in the water treatment unit.
• Dialysate tends to absorb from the blood all
that should be removed and nothing else.
• Wastes and electrolytes move from the blood
into the dialysate because of their
concentration in the blood is higher.
Historic dialyzers:
• Rotating Drum Kidney
• Parallel Plate Dialysers
Modern dialysis with Hollow Fiber:
• Cylinder filled with up to 11,000 cellulose
hollow fibers forming membranes
• Internal diameter of fibers approx 200-300
micron. Blood flows in fibres, dialysate flows
in countercurrent direction outside fibers.
Advantages of modern dialysis with Hollow Fiber:
• Small priming volume - no requirement for donor
blood.
• Low blood resistance.
• Gentle on blood.
• Single use, disposable, inexpensive, pre-sterilized
dialysis compartment &emdash; reduces infection
risks
• Good efficiency with reliable & predictable
ultrafiltration
• Strong cellulose membranes give very low frequency
of membrane ruptures
Theory of hollow fiber dialyzer:
• Wastes removed by solute diffusion through
membrane pores down specially maintained
concentration gradient from blood to
dialysate.
• Dialysate contains blood ionic substances in
required concentrations, excess ions diffuse
down concentration gradient until equilibrium
reached.
• Larger proteins & blood cells can’t cross
membrane while some do.
• Pores don’t exactly have the correct size.
• Membrane can’t selectively remove particles.
Extracorporeal blood circuit:
1- Blood tubing:
• Made from PVC (polyvinylchloride) which is
made moldable & flexible by addition of
plasticizers.
• Tubing molded to achieve smooth inner
surface and treated with silicon to minimize
blood clotting.
2- Arterial pressure monitoring:
• Pressure monitoring indicates tubing
disconnections or obstructions & aids in
controlling fluid removal from blood by
pressure gradient .
• Pressure transducer signal amplified &
converted to electrical signal .
• Alarms register pressure outside limits, blood
pump can be shut off.
3- Blood pump:
• It is a Roller pump - blood tubing placed in
curved track of the pump, rotation of roller
compresses tubing and forces blood along
tube.
• Pump adaptable to different sized tubing for
different sized patients, smaller patients
require smaller tubing.
4- Heparin pump:
• Usually use syringe pump, may use roller
pump.
• Infusion site located downstream of blood
pump.
• If located upstream of pump where
subatmospheric blood pressure exists, air
embolism risk is increased.
5- Venous pressure monitor:
• Same method as arterial pressure monitoring,
also important in maintaining correct
transmembrane pressure for fluid removal.
6- Venous line bubble trap/drip chamber:
• As blood flows through chamber micro
bubbles float to surface & are trapped .
• Mesh screen acts as mechanical filter to help
trap bubbles. It does not protect against
macro bubbles.
7- Air leak monitoring.
• Uses ultrasound transmission. Voltage source
vibrates ceramic crystal, which produces
ultrasonic waves that pass through bubble
trap to ceramic crystal detector .
• Detector vibrates & electrical output signal
produced .
• Fluids transmit sound more efficiently than air
• Decrease in intensity of ultrasonic waves when
detector indicates presence of air bubbles.
8- Optional automatic clamps:
• Shuts off blood supply in venous line.
• Bloodline clamps occlude against maximum
pressure in blood circuit using solenoid valve.