The principle of Dialysis
The principle of Dialysis
principle of haemodialysis
- Blood flows on one side of a semipermeable membrane, and dialysis fluid, an osmotically balanced solution of electrolytes, buffer, and glucose in water, flows on the other.
- The pores of the semipermeable membrane allow water molecules and small molecular weight solutes to pass through into the dialysate, but larger solutes such as proteins and blood cells are retained in the blood.
Solute transport
Diffusion
ultrafiltration-based convection
The net rate of passage of a given solute across the membrane depends on the magnitude and direction of its concentration gradient between blood and dialysate.
To maintain a maximal concentration gradient during haemodialysis, blood and dialysis fluid flow through the compartments in opposite directions
:Solute Clearance Depends upon
- Concentration gradient
- Blood flow rate
- Dialysate flow rate
- Properties of dialysis membrane
- Size and physicochemical properties of solutes
Small solute clearance depends on blood flow rate
The rate of diffusion
molecular weight
the membrane resistance to diffusion, decreasing with increasing molecular weight and increasing membrane resistance.
The resistance of the membrane
high if the membrane is thick and if there are few pores which are small in size.
Additional factors
size, shape, and charge of the molecules.
Ultrafiltration
- remove excess water from the patient. If, for instance, the hydrostatic pressure is greater in the blood compartment than in the dialysate compartment, the small water molecules are forced through the membrane from blood to dialysate. In a process called solvent drag,
- solutes with low molecular weights that can pass through the membrane pores are swept along with the water
Peritoneal Dialysis
: The transport of solutes and water across a membrane that separates 2 fluid containing compartments
- Blood in the peritoneal capillaries
- Dialysis solution in the peritoneal cavity
PD : How It Works
Transport processes
-
Diffusion
-
Convection
-
Absorption
The three - pore model
-
Large pores ( 20 – ۴۰ nm )
- Transport of macromolecules
- Vary in size
-
Small pores ( 4.0 – ۶.۰ nm )
- Transport of small solutes
- Constant radius
-
Ultra small pores ( <0.8 nm)
- Transport of water
Peritoneal diffusion (1 )
- Concentration gradient
- Maximal at the start
- More frequent exchange
- Increasing dwell volume
Peritoneal diffusion (2 )
- Effective peritoneal surface area
- Total surface area
- Degree of vascularity
- Larger fill volumes
Peritoneal diffusion (3 )
- Intrinsic membrane resistance
- Difference in the number of pores
- Distance of capillaries and mesothelium
- Molecular weight of the solute
- Low versus high
Peritoneal convection
- Solute transport with ultrafiltration
- Water transport induced by osmotic gradients
PD : How It Works
PD : How It Works
