Contents

DialysisA NOVEL SEPARATION TECHNIQUEContents What is dialysis?

History of Dialysis

Factors affecting rate of Dialysis

Principle involved

Types of dialysis

Details of purification of blood

What is Dialysis?Dialysis

FromGreek

dialusis Dissolutiondia -throughlysis - loosening or splitting

is a process for removing waste and excess waterfrom theblood, and is used primarily to provideanartificial replacementfor lostkidney functioninpeople withrenal failureWhat is Dialysis?Dialysis is the movement of molecules by diffusion from high concentration to low concentration through a semi-permeable membrane.

Only those molecules that are small enough to fit through the membrane pores are able move through the membrane and reach equilibrium with the entire volume of solution in the system.

Once equilibrium is reached, there is no further net movement of the substance because molecules will be moving through the pores into and out of the dialysis unit at the same rate.

By contrast, large molecules that cannot pass through the membrane pores will remain on the same side of the membrane as they were when dialysis is initiated.What is Dialysis?To remove additional unwanted substance, it is necessary to replace the dialysis buffer so that a new concentration gradient can be established.

Once the buffer is changed, movement of particles from high (inside the membrane) to low (outside the membrane) concentration will resume until equilibrium is once again reached.

With each change of dialysis buffer, substances inside the membrane are further purified by a factor equal to the volume difference of the two compartments.

What is Dialysis?Example

If one is dialyzing 1 ml of sample against 200 ml of dialysis buffer, the concentration of the dialyzable substance at equilibrium will be diluted 200 less than at the start.

Each new exchange against 200 ml of new dialysis buffer will dilute the sample 200 times more.

For three exchanges of 200 ml, the sample will be diluted 200 x 200 x 200 or 8,000,000 times, assuming complete equilibrium was reached each time before the dialysis buffer was changed.

History of DialysisDr.Willem Kolff, a Dutch physician, constructed the first working dialyzer in 1943 during theNazi occupation of the Netherlands.

Due to the scarcity of available resources, Kolff had to improvise and build the initial machine usingsausage casings,beverage cans, awashing machine, and various other items that were available at the time.

Over the following two years, Kolff used his machine to treat 16 patients suffering fromacute kidney failure, but the results were unsuccessful.

Then, in1945, a 67-year-old comatose woman regained consciousness following 11 hours of hemodialysis with the dialyzer, and lived for another seven years before dying of an unrelated condition. She was the first-ever patient successfully treated with dialysis.

History of DialysisKolffs original rotating drum (1943)

Factors Affecting Rate of DialysisFactors that affect the completeness of dialysis

(1) Dialysis buffer volume

(2) Buffer composition

(3) The number of buffer changes

(4) Time

(5) Temperature

(6) Particle size vs. pore size.

Principle Dialysis works on the principles of thediffusionof solutes andultra - filtrationof fluid across asemi-permeable membrane.

A semipermeable membrane is a thin layer of material that contains holes of various sizes, or pores.

Diffusion describes a property of substances in water.

Substances in water tend to move from an area of high concentration to an area of low concentration.

Principle The main fluid (eg blood) flows by one side of a semi-permeable membrane, and a dialysate, or special dialysis fluid, flows by the opposite side.

Smaller solutes and fluid pass through the membrane, but the membrane blocks the passage of larger substances (for example, red blood cells, large proteins).

This replicates the filtering process that takes place in the kidneys, when the blood enters the kidneys and the larger substances are separated from the smaller ones in the glomerulus.

Types of DialysisThere are three primary and two secondary types of dialysis:

Hemodialysis (primary),

Peritoneal dialysis(primary)

Hemofiltration(primary)

Hemodiafiltration(secondary)

Intestinal dialysis (secondary)

Types of DialysisHemodialysis

This process removes wastes and water by circulating blood outside the body through an external filter, called adialyzer, that contains asemipermeable membrane.

The blood flows in one direction and thedialysateflows in the opposite.

The counter-current flow of thebloodand dialysate maximizes the concentration gradient of solutes between the blood and dialysate, which helps to remove more urea andcreatininefrom the blood.

Types of DialysisIn hemodialysis, the patient's blood is pumped through the blood compartment of a dialyzer, exposing it to a partially permeable membrane.

The dialyzer is composed of thousands of tiny synthetic hollow fibers. The fiber wall acts as the semipermeable membrane.

Blood flows through the fibers, dialysis solution flows around the outside of the fibers, and water and wastes move between these two solutions.

The cleansed blood is then returned via the circuit back to the body.

Ultrafiltration occurs by increasing the hydrostatic pressure across the dialyzer membrane. This usually is done by applying a negative pressure to the dialysate compartment of the dialyzer. This pressure gradient causes water and dissolved solutes to move from blood to dialysate, and allows the removal of several litres of excess fluid during a typical 3- to 5-hour treatmentSchematic Diagram for Hemodialysis

Types of DialysisPeritoneal Dialysis

Inperitoneal dialysis, wastes and water are removed from the blood inside the body using theperitoneal membrane of theperitoneumas a natural semipermeable membrane.

Wastes and excess water move from the blood, across the peritoneal membrane, and into a special dialysis solution, called dialysate, in theabdominal cavitywhich has a composition similar to the fluid portion of blood.

Types of DialysisIn peritoneal dialysis, a sterile solution containing glucose is run through a tube into theperitoneal cavity, theabdominalbody cavity around theintestine, where the peritoneal membrane acts as a partially permeable membrane.

The dialysate is left there for a period of time to absorb waste products, and then it is drained out through the tube and discarded.

This cycle or "exchange" is normally repeated 4-5 times during the day, (sometimes more often overnight with an automated system). Each time the dialysate fills and empties from the abdomen is called one exchange.

A drain process is the process that follows in which the dialysate full with waste products and extra fluid is drained out of patient's blood.

Ultrafiltration occurs viaosmosis; the dialysis solution used contains a high concentration of glucose, and the resulting osmotic pressure causes fluid to move from the blood into the dialysate.

Schematic Diagram for Peritonial Dialysis

Types of DialysisComparison

Peritoneal dialysis is less efficient than hemodialysis, but because it is carried out for a longer period of time

The net effect in terms of removal of waste products and of salt and water are similar to hemodialysis.

Peritoneal dialysis is carried out at home by the patient. Although support is helpful, it is not essential.

It does free patients from the routine of having to go to a dialysis clinic on a fixed schedule multiple times per week, and it can be done while travelling with a minimum of specialized equipment.

Types of DialysisHemofiltration

It is a similar treatment to hemodialysis, but it makes use of a different principle.

The blood is pumped through a dialyzer or "hemofilter" as in dialysis, but no dialysate is used.

A pressure gradient is applied; as a result, water moves across the very permeable membrane rapidly, "dragging" along with it many dissolved substances, including ones with large molecular weights, which are not cleared as well by hemodialysis.

Salts and water lost from the blood during this process are replaced with a "substitution fluid" that is infused into theextracorporealcircuit during the treatment.

Types of DialysisHemodialfiltration

It is a combination of hemodialysis and hemofiltration. In theory, this technique offers the advantages of both hemodialysis and hemofiltration.

Intestinal dialysis

In this process, the diet is supplemented with soluble fibres such asacacia fibre, which is digested by bacteria in the colon. This bacterial growth increases the amount of nitrogen that is eliminated in fecal waste. An alternative approach utilizes the ingestion of 1 to 1.5 liters of non-absorbable solutions ofpolyethylene glycolormannitolevery fourth hour.

Purification of Blood by DialysisAs we know, dialysis is the transfer of solute molecules across a membrane by diffusion from a concentrated solution to a dilute solution.

This phenomenon is used to remove solutes from liquid mixtures in the biomedical, biotechnology, and chemical industries.

The largest commercial market is for hemodialysis membranes. These membranes are used to manufacture artificial kidneys that can replace the function of human kidneys.

The current market for hemodialyzers is approximately one billion dollars per year. Purification of Blood by DialysisHemodialyzers consist of a bundle of 10,000 15,000 hollow fibre membranes.

The ends of the bundle are encased in tubesheets and the tubesheets seal to an external plastic housing that encloses the fibre bundle.

The case possesses two external ports on either end of the fibre bundle and two ports along the periphery of the case.

This design allows introduction of a fluid stream through one of the end ports into the fibre lumens (i.e., interior) and withdrawal from the opposing end.

Similarly, one may introduce a fluid stream into the shell (i.e., space external to the fibres) through a peripheral port and withdraw it from the other.

Such hollow fibre modules are the mass transfer equivalent of a shell and tube heat exchanger two fluid streams flow through the hollow fibre or tube bundle, without direct mixing, while mass or heat transfer occurs across the fibre or tube wall.

Purification of Blood by DialysisSchematic diagram of a typical hollow fibre membrane module and the case that holds it.

Purification of Blood by DialysisThe blood and dialysate flow counter-currently to each other as in heat exchangers.

In operation, blood flows through the fibre lumens while dialysate flows through the shell.

The wastes produced by cell metabolism diffuse from the blood to the diaysate, due to a concentration difference, which cleans the blood.

The fibre wall is porous with pores large enough to permit passage of low molecular wastes but small enough to prevent loss of proteins, blood cells, and other large plasma components. The dialysate serves as a reservoir into which the wastes accumulate and are removed ultimately.

A Hemodialyzer Machine