Dialysis tubing is a type of tubing used in medicine to remove toxins from a patient's bloodstream. It is effective for this purpose because it is a semipermeable membrane, allowing some particles to pass through while blocking others, and so can be used as a filter. This kind of tubing is used primarily in kidney dialysis to treat people who suffer from diminished or lost kidney function.
The tubing itself consists primarily of cellulose, an organic polymer from which the cell walls of plants are made. Cell walls are semipermeable structures that regulate the entry and exit of materials in the plant cells, so cellulose is naturally suited for use as a filter. Unmodified cellulose by itself has problems with biocompatibility and can trigger parts of the immune system, and so modern dialysis tubing uses modified materials such as cellulose acetate or the cellulose-derived bioplastic cellophane. Synthetic polymers such as polyvinylpyrrolidone can also be used to make dialysis tubing.
The kidneys play an important role in purifying the bloodstream of toxins, and in the absence of treatment, total loss of kidney function is quickly fatal. Dialysis is used to supplement or replace this function in people suffering from kidney injury or disease. The most common method of kidney dialysis, called hemodialysis, makes use of dialysis tubing.
In hemodialysis, blood from the patient is pumped out of him or her through a needle or catheter to a dialysis machine. The dialysis tubing is inside the machine, surrounded by a sterilized solution called a dialysate or diffusate. The dialysate is most commonly water with electrolytes, sugars, or other substances as solutes, with the precise composition varying according to the needs of the patient.
Particles naturally spread themselves out from areas of higher concentration into areas of lower concentration if allowed to do so, a process called diffusion, and dialysis exploits this process. As blood flows through the dialysis tubing, waste product molecules concentrated in the blood diffuse through the semipermeable tubing into the solution in the dialysis machine until their levels in the blood and dialysate are in equilibrium. Desirable substances can also be prevented from diffusing out of the blood by including the same substances in the dialysate, reducing or eliminating the difference in concentration that causes diffusion. This process cleans the blood of the toxins that would normally be filtered out by the kidneys, and the blood is then pumped back into the patient's own circulatory system.