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This is a process in which blood is passed through
a special filter (artificial kidney) which contains a special
membrane . This membrane is made of a type of cellophane. The
membrane contains millions of tiny holes or pores through which
waste products and excess fluid are removed.
How it works : During hemodialysis, two processes occur
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Diffusion :
In many types of dialysis filters (artificial kidneys), the membrane is
constructed in the form of many long tubes or capillaries. The blood passes
through the capillary tubes and dialysis fluid from dialysis machine (dialysate)
runs outside the capillary tube. The dialysis fluid contains the same salts as
are in the blood (sodium, potassium, chloride, calcium, magnesium) but does not
contain the waste products. As the blood passes through the capillary tubes and
comes in contact with the inside of the membrane, the waste products urea,
creatinine, phosphate etc, pass from the blood through the tiny pores in the
membrane into the dialysate that flows between the capillary tubes. These waste
products are diffusing from an area of very high concentration in the blood to a
fluid (diaiysate) that contains no waste product. The dialysate empties into a
drain. Fresh dialysate always bathes the outside of capillary tubes through
which blood is flowing. Therefore, the process of removing waste products
continues as long as blood flows through dialysis filter.
Ultrafiltration :
This involves the use of pressure by the machine
to force excess fluid out of the blood. Pressure can be applied in
two ways :
-
It is applied to the inside of the membrane. This forces
excess water from the blood into the dialysate.
-
Pressure on the outside of the membrane (dialysate side) is
lowered, causing a sucking of excessive water from the blood.
Access for Hemodialysis :
In order to perform hemodialysis, it is necessary to create a
method of getting the blood from your body to the dialysis filter. Initially, a
temporary access is created by introducing a catheter in femoral or jugular
vein. However, a permanent access should soon be created for long term use :
shunt, fistula, or graft.
 Femoral
and jugular vein catheterisation is performed using a double lumen catheter.
This catheter although appearing to be a single tube, has inside it two tubes.
Through one tube called the arterial line, impure blood from the body is
obtained and carried to artificial kidney for purification and through other
tube called venous line the purified blood is returned back. The femoral vein is
a large vein in the groin while a jugular vein is a large vein in the neck. A
large vein is required because blood has to be obtained for dialysis at a high
flow rate (250 ml./minute). This vascular access should not be maintained for
long as it increases chances of infection. Therefore, a permanent device should
soon be created.
A Shunt is made of two special plastic tubes that
carry blood directly from an artery to a vein. One of the tubes is connected to
the artery in the arm or leg through teflon vessel tip and the other is
connected to a vein close to the artery. The two tubings are connected by a
small connector. During dialysis, the tube in the artery is attached to a tube
which carries blood to the artificial kidney. The tube in the vein is attached
to a piece of tubing that carries the blood back to your body from the
artificial kidney. The shunt permits the patient to have many treatments without
the use of needles. However, with this method there is a risk of infection with
repeated usage. This form of access is rarely used now-a-days.
The fistula is an internal (under the skin) connection between an artery
and a vein in the arm. It is done by a minor surgical procedure using the local
anesthetic. Arteries carry blood at a faster rate under high pressure. Veins
normally carry blood at a slow rate under low pressure. When a fistula is
created, artery is connected to a vein so that blood flows directly from the
artery into the vein. This makes the veins larger. It normally takes 4-6 weeks
for the veins to become large and usable for dialysis. It is therefore advisable
to get a fistula made well before dialysis has to be initiated. If this planning
is done, which is possible in patients with slowly progressive renal disease,
one can avoid creating temporarily vascular access. During dialysis, two needles
are placed in the enlarged veins; one is attached to the tubing that carries
blood to the dialysis machine and other to the tubing that carries blood back to
the body.
A graft is an internal connection between an artery and a vein using
synthetic material. This is required when the veins are not good for fistula
construction.
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