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In a clinical context:

Dialysis is a method for removing waste such as urea from the blood when the kidneys can no longer do the job. The two types of dialysis are: hemodialysis and peritoneal dialysis.

In hemodialysis, the patient's blood is passed through a tube into a machine that filters out waste products. The cleansed blood is then returned to the body.

In peritoneal dialysis, a special solution is run through a tube into the peritoneum, a thin tissue that lines the cavity of the abdomen. The body's waste products are removed through the tube.

There are three types of peritoneal dialysis. Continuous ambulatory peritoneal dialysis (CAPD), the most common type, needs no machine and can be done at home. Continuous cyclic peritoneal dialysis (CCPD) uses a machine and is usually performed at night when the person is sleeping. Intermittent peritoneal dialysis (IPD) uses the same type of machine as CCPD, but is usually done in the hospital because treatment takes longer. Hemodialysis and peritoneal dialysis may be used to treat people with diabetes who have kidney failure.

It works by having the blood flow along one side of a semi-permeable membrane, with the dialysis solution (usually a highly concentrated saline) flowing along the other side. Due to the difference in osmolarity between the two liquids, water traverses the membrane in order to dilute the dialysis liquid, carrying along the unwanted blood contents.

Original version from the Diabetes dictionary

How Hemodialysis is typically done

1) Dialysis is conducted in a dedicated facility, either a special room in a hosipital or clinic that specializes in hemodialysis dialysis.

2) Nurses and technicians working in the facility have special training specific to dialysis.

3) A dialysis patient will be given a prescription by a nephrologist (a doctor specializing in kidney issues. All dialysis treatement issues are ultimately referred back to this doctor or alternate, though the attending nurse will often make minor care decisions without refering to the doctor.

4) The dialysis prescription will specify various parameters for setting up dialysis machines. It will also specify times and durations of dialysis sessions. In the US, 3-4 hour sessions, 3 times a week are typical.

5) The dialysis center to be used by the patient is contacted and schedules the patient for a specific time period.

6) Before or around the time the patient arrives for his/her scheduled session, a dialysis machine will be prepared. There are many models of dialysis machines, but typically in modern machines there will be a computer, CRT, a pump, and facility for disposable tubing and filters. The filters (the actual artificial kidneys) are cylindrical, clear plastic outside with the filter materiel visible inside (looks like thick paper). They are perhaps 15-18" long, and 2-3" thick. They have connectors at both ends. The technician or nurse will setup plumbing on the machine in a moderately complex pattern that has been worked out to move blood through the filter, allow for saline drip (or not), allow for various other medications/chemicals to be administered. How the plumbing is setup may vary between models of machine and they types of filters. For some filters, it is necessary to clear sterlizing fluid (Renaline, or others) from the filter before connecting the patient. This done by altering the plumbing to push saline through the filter, and carefully checked with a type of litmus test. The pump does not directly contact the blood or fluid in the plumbing - it works by applying pressure to the tubing, then moving that pressure point around. Think of a disk with a protrusion in it. Put this into a close fitting 270 degree enclosure. Put plastic tubing between the enclosure and the disk, entering and exiting in the 90 open degrees. Now imagine the disk turning. It will put pressure on the tubing, and the pressure point will roll around through the 270 degrees, forcing the fluid to move. It is characteristic of dialysis machines that most of the blood out of the patients body at any given time is visible. This facilitates troubleshooting, particularly detection of clotting.

7) The patient arrives and is carefully weighed. Standing and sitting blood pressures are taken. Temperature is taken.

8) Access is setup. For patients with a fistula (a surgical modification to an arm or leg vein to make it more robust, and therefore usable for high capacity blood movement required by dialysis) this means inserting to large gauge needles into the fistula. (Yes, it hurts.) Fistulas are widely considered the desirable way to get access for hemo-dialysis, but they take time to setup and mature. For other patients, access may be via a catheter installed to connect to large veins in the chest. (This means no needles, but there are other severe downsides to a catheter). There are some other arrangements that can be made as well.

9) When access has been setup, the patient is then connected to the preconfigured plumbing - creating a complete loop through the pump and filter. The pump and a timer are started. Hemodialysis is underway.

10) Periodically (every half hour, nominally) blood pressure is taken. As a practial matter, fluid is also removed during dialysis. Most dialysis patients are on moderate to severe fluid restrictive diets (in addition to other dietary restrictions). This is because kidney failure usually includes an inability to properly regulate fluid levels in the body. A session of hemodialysis may typically remove 2-5 kilograms (5-10 pounds) of fluid from the patient. The removal of fluid done to achieve a predetermine "dry weight" of the patient. This is a weight that the care staff believes represents what the patient should weigh without fluid built up because of kidney failure. Removing this much fluid can cause or exacerbate low blood pressure. Monitoring is intended to detect this before it becomes too severe. Low blood pressure can cause cramping, naseua, shakes, dizzyness, lightheadedness, and unconciousness.

11) At the end of the prescribed time, the patient is disconnected from the plumbing (which is removed and discarded, except perhaps for the filter, which may be sterilized and reused with the same patient at a later date). Needle wounds (in case of fistula) are bandaged with gauze, held for 5-10 minutes with direct pressure to stop bleeding, then the taped in place. It's just like getting blood draw, only it takes a lot longer, and more fluid is lost.

12) Temperature, standing and sitting blood pressure, and weight are all measured again. Temperature changes may indicate infection. BP discussed in point 10 above. Weighing is to confirm the removal of the desired amount of fluid.

13) Care staff verifies that the patient is in condition suitable for leaving. The patient must be able to stand (if previously able), maintain a reasonable blood pressure, and be conherent (if normally coherent). Different rules apply for in-patient treatment - in those cases the patient isn't leaving the facility...

In a biochemical laboratory context:

Dialysis also refers to a common laboratory technique which operates on the same principle as clinical dialysis described above. Typically a solution of several types of molecules is placed in dialysis tubing, and the tubing is sealed. The sealed tube is placed jar of a different solution, or pure water. Molecules small enough to pass through the tubing (often water, salts and other small molecules) tend to move into or out of the tubing, in the direction of decreasing concentration. Larger molecules (often proteins, DNA, or polysaccharides) cannot pass through the tubing. One common reason for using this technique would be to remove the salt from a protein solution.