Dialysis is a process that removes waste products and excess fluid that accumulate in the blood as a result of failure of the kidneys to function. Dialysis is not a cure for kidney failure. It only substitutes for some functions of the kidneys like removing some waste products and excess water from the body.

When the kidneys have ceased functioning permanently, they will not resume functioning again no matter how many dialysis treatments are given. The patient with permanently damaged kidneys (ESRD) will therefore need dialysis treatment for the rest of his life unless he has a successful kidney transplant.

Hemodialysis
Hemodialysis is a process in which blood is passed through a special filter, an 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.

Before hemodialysis can be done, a doctor must make an entrance, called an access, into the patient's blood vessels. This is done by minor surgery in the leg, arm or sometimes neck. The best access for most patients is called a fistula. Minor surgery is performed to join an artery to a vein under the skin to make a larger vessel.

If no vessels are suitable for a fistula, the doctor might use Dialysisa soft plastic tube called a vascular graft to join an artery and vein under the skin. For temporary dialysis in the hospital, a patient might need a catheter implanted into a large vein in the neck. Once the access is made and healed, two needles are inserted in the fistula or graft, one on the artery side and one on the vein side.

Blood drains into the dialysis machine to be cleaned. The machine has two parts, one side for blood and one for a fluid called Dialysate. A thin, semi–permeable membrane separates the two parts. As dialysate passes on one side of the membrane, and blood on the other, particles of waste from the blood pass through microscopic holes in the membrane and are washed away in the dialysate. Blood cells are too large to go through the membrane and are returned to the body.

During Hemodialysis, two processes occur
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 your blood (sodium, potassium, 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 (dialysate) 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

1. It is applied to the inside of the membrane. This forces excess water from the blood into the dialysate.
2. 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 blood from your body to the dialysis filter. Initially, temporary access is created by introducing a catheter in femoral or subclavian vein. However, a permanent access should soon be created for long term use : “shunt”, “fistula”, or “graft”.

The benefits of hemodialysis are that the patient requires no special training, and he or she is monitored regularly by someone trained in providing dialysis.