Geometries of Dialyzers
There are two types of geometry of artificial kidneys. The geometry refers
to the cross sectional area which is in contact with the blood, semipermeable
membrane, and dialysate. The first geometry is rectangular. An example of
these dialyzers are the parallel plate design. The second geometry is
circular. An example of these dialyzers are hollow-fiber design.
Coil Kidneys
This artificial kidney was the first to be mass produced. Its
design was fairly simple and with that came lots of problems. It had a
cellulose coil wrapped around a wire mesh drum. The filtration rate was
unpredictable.
Parallel Plate Dialyzers
This type of artificial kidney is called the parallel plate
dialyzer for an obvious reason. Instead of the classic drum rotating system,
this dialyzer uses several parallel plates with ridges and grooves in them.
The dialysate flows along the grooves or ridges. A semipermeable membrane
rests between the grooves and the blood flow. With these dialyzers,
resistance to blood flow is low. The diagram below illustrates one type of parallel plate dialyzers, the disc hemofilter.
Some advantages to the use of this dialyzer are its low
resistance to blood flow. Because of this fact, there is not as much need
for the use of an anti-blood clotting solution. Another advantage of this
dialyzer is that its filtration rate is controllable and predictable. The
next advantage of this dialyzer is the amount of blood contained within the dialyzer
is relatively low. The less blood that is out of the body at one point in
time, the better the dialyzer. The final advantage of the parallel plate dialyzer
is that it is inexpensive.
Hollow-Fiber Artificial Kidneys
This type of artificial kidney is the most common type used. This
artificial kidney makes use of countercurrent flow (Fig
2). Countercurrent flow is
where the blood is flowing in one direction and the dialysate is flowing in
the opposite direction. Countercurrent flow is less efficient, but seems
to be more gentle. Because it is more gentle, it is used on pediatric patients and
some first time dialysis patients. The hollow-fiber dialyzer comes in many
different sizes. It looks like a cylinder filled with thousands of tiny
hollow-fibers(Fig 3). Blood flows into one end of the dialyzer and through these
thousands of tiny hollow-fibers. Dialysate, at the same time, is pumped
into the cylinder and across the tiny hollow-fibers. This method keeps
fresh dialysate circulating constantly.
