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Principle of Hemodialysis and Peritoneal Dialysis


Principle of hemodialysis : Osmosis, diffusion and ultrafiltration

Osmosis: It is a process in which fluid moves from area of higher water / fluid concentration to the area of lower fluid concentration.

Dialysis machine use process of osmosis to take over the filtering function of the kidney. Dialysis machine use a semipermeable membrane through which small particles such as water, electrolytes and other toxins can pass but large particles such as proteins, blood cells etc can't pass. During hemodialysis excess fluid moves from blood to dialysate through semipermeable membrane due to osmosis until the level becomes same between blood and dialysate.

Diffusion : It is a property of solute in solvent. Solute in solvent tend to move from area of higher concentration of solute to area of lower concentration of solute. During hemodialysis blood moves from one side of semipermeable membrane and dialysate moves from another side of semipermeable membrane, where counter current mechanism takes place.Concentration of waste products and other solutes is high in blood so they move from blood to dialysate through semipermeable membrane and electrolytes also move from their higher concentration to lower concentration due to which electrolyte balance takes place.

Ultrafiltration :It is property of dialysis machine. It is also called as forced osmosis. In hemodialysis excess fluid is removed by ultrafiltration by using dialysis machine. The pressure on the dialysate side is lower so water moves from blood (place of high pressure) to dialysate (place of low pressure) . This is how hemodialysis machine removes excess fluid from blood.

The ultrafiltration rate as well as length of dialysis treatment time control the amount of fluid to be removed .The ultrafiltration rate is to set on the basis of fluid weight gain by patient from his last dialysis treatment. So, the goal is to achieve dry weight (dry weight is weight of patient without extra fluid ) & in this way excess fluid from the blood is taken out.

*Principle of Peritoneal dialysis: Diffusion, Ultrafiltration & Fluid absorption

Diffusion : The dialysis fluid called as dialysate which is used during peritoneal dialysis is introduced into the peritoneal cavity of dialysis patient, thus molecules such as urea, creatinine etc diffuse from blood to dialysate (where the initial concentration of these substances is zero). Glucose & lactate are present in higher concentration in the dialysate and thus diffuse into the blood .

° Factors affecting the diffusion are :-

1) Concentration gradient between the blood and dialysate.

2) Effective surface area and permeability of membrane.

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3) Molecular weight of solute.

4)Sum of all the resistances to the path of diffusing molecule.

Ultrafiltration :- Movement of solvent across a peritoneal membrane controlled by the pressure gradient is called as ultrafiltration.It occurs due to the presence of osmotic agent like glucose in the dialysate. The concentration of osmotic agent is added based on how much fluid is to be removed from the blood. The pressure gradient is typically maximal at the start of peritoneal dialysis & decrease with time due to dilution of dialysate glucose by Ultrafilitrate from the plasma & to diffusion of glucose from dialysate solution into the blood.

°Factors affecting ultrafiltration:-

1) concentration gradient for osmotic agent.

2) Effective surface area and permeability of peritoneal membrane.

3) Hydrostatic pressure gradient.

4) oncotic pressure gradient.

5) Seiving

• Fluid absorption :- fluid absorption from the peritoneal cavity occurs via lymphatics at a relatively constant rate. There is a little or no seiving. Fluid absorption reduces the efficiency of both solute and fluid removal by peritoneal dialysis.

°Factors affecting the fluid absorption:-

1) Intraperitoneal hydrostatic pressure
2) Effectiveness of lymphatics

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