In Medicine, dialysis is a process for removing waste and excess water from the blood and is used primarily as an artificial replacement for lost kidney function in people with kidney failure. Dialysis may be used for those with an acute disturbance in kidney function ( acute kidney injury, previously acute renal failure) or progressive but chronically worsening kidney function- a state known as chronic kidney disease stage 5 ( previously chronic renal failure or end stage renal disease). The later form may develop over month or years, but in contrast to acute kidney injury is not usually reversible and dialysis is regarded as a “holding measure” until a kidney transplant can be performed or sometimes as the only supportive measure in those for whom a transplant would be inappropriate.

The kidneys have an important role in maintaining health. When healthy, the kidneys maintain the body’s internal equilibrium of water and minerals ( sodium, potassium, chloride, calcium, phosphorus, magnesium, sulfate). The acidic metabolism end products that the body cannot get ride of via respiration are also excreted through the kidneys. The kidneys also function as a part of the endocrine system, producing erythropoietin, calcitriol and renin. Erythropoietin is involved in the production of red blood cells and calcitriol plays a role in bone formation. Dialysis is an imperfect treatment to replace kidney function because it does not correct the compromised endocrine functions of the kidney. Dialysis treatments replace some of these functions through diffusion (waste removal) and ultrafiltration (fluid removal).


Dialysis works on the principles of the diffusion of solutes and ultrafiltration of fluid access a semi- permeable membrane. Diffusion is a property of substances in water; substances in water tend to move from an area of high concentration to an area of low concentration. Blood flows by one side of a semi- permeable membrane, and a dialysate, or special dialysis fluid, flows by the opposite side. A semi- permeable membrane is a thin layer of material that contains holes of various sizes, or pores. 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.

The two main types of dialysis, hemodialysis and peritoneal dialysis, remove wastes and excess water from the blood in different ways. Hemodialysis removes wastes and water by circulating blood outside the body through an external filter, called a dialyzer that contains a semi- permeable membrane. The blood flows in one direction and the dialysate flows in the opposite. The counter- current flow of the blood and dialysate maximizes the concentration gradient of solutes between the blood and dialysate, which helps to remove more urea and creatinine from the blood. The concentrations of solutes ( for example potassium, phosphorus, and urea ) are undesirably high in the blood, but low or absent in the dialysis solution, and constant replacement of the dialysate ensures that the concentration of undesired solutes is kept low on this side of the membrane. The dialysis solution has levels of minerals like potassium and calcium that are similar to their natural concentration in healthy blood. For another solute, bicarbonate, dialysis solution level is set at a slightly higher level than in normal blood, to encourage diffusion of bicarbonate into he blood, to act as a pH buffer to neutralize the metabolic acidosis that is often present in these patients. The levels of the components of dialysate are typically prescribed by a nephrologist according to the needs of the individual patient.

In peritoneal dialysis, wastes and water are removed from the blood inside the body using the peritoneum as 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 the abdominal cavity.


In 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 hollow synthetic fibers. The fibre 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. The pressure gradient causes water and dissolved solutes to move from blood to dialysate, and allows the removal of several liters of excess fluid during a typical  4-hour treatment. Hemodialysis treatments are typically given in a dialysis centre three times per week. Studies have demonstrated the clinical benefits of dialyzing 5 to 7 times a week, for 6 to 8 hours. This type of hemodialysis is usually called “nocturnal daily hemodialysis ” , which a study has shown a significant improvement in both small and large molecular weight clearance and decrease the requirement of taking phosphate binders. These frequent long treatments are often done at home while sleeping, but home dialysis is a flexible modality and schedules can be changed day to day, week to week. In general, studies have shown that both increased treatment length and frequency are clinically beneficial.

Pediatric Dialysis

Morbidity during dialysis sessions has decreased with seizures being exceptional and hypotensive episodes rare. Pain and discomfort have been reduced with the use of chronic internal jugular venous catheters and anaesthetic creams for fistula puncture.

A more biocompatible synthetic membranes and specific small size material dialyzers and new low extra- corporeal volume tubing have been developed for young infants, meaning that arterial and venous tubing length are made of minimum length and diameter, a <80 ml to < 110 ml volume tubing is designed for paeds patients and a > 130 to < 224 ml tubing are for adult patients, it’s regardless of blood pump segment size, which can be of 6.4 mm for normal dialysis or 8.0 mm for high flux dialysis in all patients, all dialysis machine manufacturers do design their machine to do the pediatric dialysis. In pediatric patient the pump speed should be kept at low side according to patient blood output capacity and the clotting with heparin dose should be carefully monitored, it is that the high flux dialysis is not recommended on pediatric patients.

In children hemodialysis has to be individualized and viewed as an “integrated therapy” considering their long- term exposure to chronic renal failure treatment. Dialysis is seen only as a temporary measure for children compared with renal transplantation because this enables the best chance of rehabilitation in terms of educational and psychosocial functioning. In long term chronic dialysis, however, the highest standards should be applied to these children to preserve their future ” cardiovascular life ” which might include more dialysis time and on- line hemodiafiltration online hdf with synthetic high flux membranes with surface area of 0.2 sq.m to 0.8 sq.m and blood tubing lines with low volume yet large blood pump segment of 6.4/8.0 mm, if we are able to improve on the rather restricted concept of small- solute urea dialysis clearance.

Peritoneal Dialysis

In peritoneal dialysis, a sterile solution containing glucose (called dialysate) is run through a tube into the peritoneal cavity, the abdominal body cavity around the intestine, where the peritoneal membrane acts as a partially permeable membrane. The peritoneal membrane or peritoneum is a layer of tissue containing blood vessels that lines and surrounds the peritoneal, or abdominal cavity and the internal abdominal organs (stomach, spleen, liver, and intestines). Diffusion and osmosis drive waste products and excess fluid through the peritoneum into the dialysate until the dialysate approaches equilibrium with the body’s fluids. Then the dialysate is drained, discarded, and replaced with fresh dialysate.

This exchange is repeated 4-5 times per day; automatic systems can run more frequent exchange cycles overnight. 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 at home by the patient, often without help. This frees the patients from the routine of having to go to a dialysis clinic on a fixed schedule multiple times per week. Peritoneal dialysis can be performed with little to no specialized equipment (other than bags of fresh dialysate).


Hemofiltration 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 the extra corporeal circuit during the treatment.


Hemodiafitration is a combination of hemodialysis and hemofiltration.

Intestinal dialysis

In intestinal dialysis, the diet is supplemented with soluble fibers such as acacia 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 is the ingestion of 1 to 1.5 liters of non- absorbable solutions of polyethylene glycol or mannitol every fourth hour.

Starting indications

The decision to initiate dialysis or hemofiltration in patients with kidney failure depends on several factors. These can be divided into acute or chronic indications.

* Indications for dialysis in the patient with acute kidney injury are summarized with the vowel acronym of “AEIOU”.

  1. Acidemia from metabolic acidosis in situations in which correction with sodium bicarbonate is impractical or may result in fluid overload.
  1. Electrolyte abnormality, such as severe hyperkalaemic, especially when combined with


  1. Intoxication, that is, acute poisoning with a dialysable substance. These substances can be represented by the mnemonic SLIME: salicylic acid, lithium, isopropanol, magnesium- containing laxatives, and ethylene glycol.
  2. Overload of fluid not expected to respond to treatment with diuretics.
  3. Uremic complications, such as pericarditis, encephalopathy, or gastrointestinal bleeding.

Indications for chronic dialysis:

Chronic dialysis may be indicated when a patient has symptomatic kidney failure and low glomerular filtration rate (GFR). The most recent published guidelines from Canada, for when to initiate dialysis, recommend an intent to deter dialysis until a patient has definite kidney failure symptoms, which may occur at an estimated GFR of 5-9 ml/ min/ 1.73 sq.m.

Some reason for dialysis initiation includes difficulty in medically controlling fluid overload or serum potassium. If a patient has intractable kidney failure symptoms or signs, start of dialysis may be recommended at eGFR levels above 10 ml/ min/ 1.73 sq.m.

Dialysable substances:

Dialysable substances have following properties:

  1. Low molecular mass
  2. High water solubility
  3. Low protein binding capacity
  4. Prolonged elimination ( long half life )
  5. Small volume of distribution.


  1. Ethylene glycol
  2. Procainamide
  3. Methanol
  4. Isopropyl alcohol
  5. Barbiturates
  6. Lithium
  7. Bromide
  8. Sotalol
  9. Choral hydrate
  10. Ethanol
  11. Acetone
  12. Atenolol
  13. Theophylline
  14. Salicylates

Medical applications:

* Apheresis, also known as plasmapheresis, is another extra corporeal technique that selectively removes specific constituents from blood.

* Hemodialysis

* Peritoneal dialysis

* Acute kidney failure

* chronic kidney disease

* Hepatorenal syndrome


Prof. Dr. A. K. M. Aminul Hoque

Formerly Prof. Of Medicine

Dhaka Medical College

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