Mechanisms
Haemodialysis
- Clearance of solutes via diffusion down a concentration gradient
- A counter-current flow of a solution containing various electrolytes on the opposite side of membrane to blood allows diffusion to occur
- No fluid is added to the filtrate after diffusion
- Rate of solute clearance determined by:
- Concentration gradient between plasma and dialysate
- Particle size, ionic charge and protein binding
- Membrane pores, thickness and surface area
Haemofiltration
- Clearance of solutes via convection driven by hydrostatic pressure
- Filtrate removal is balanced by the addition of a solution to maintain volume
- Does not significantly change the concentration of serum electrolytes and waste products unless a replacement fluid is infused into the blood, effectively diluting out those solutes the physician wishes to remove
- Rate of filtrate and solute removal determined by:
- Blood flow
- Transmembrane pressure gradient
- Membrane coefficient (pore size/permeability)
Haemodiafiltration (Combined)
- Clearance of solute via convection and diffusion
- Countercurrent dialysate is used in addition to hydrostatic pressure
- Fluid replacement is required to maintain plasma volumes