Diffusion is a process by which substances (solutes) move down a concentration gradient. In the case of RRT, some of these substances can pass across a semi-permeable membrane. The rate of diffusion is directly proportional to the concentration gradient, temperature, diffusivity constant (a measure of how easily a particular particle can pass through the membrane), and the membrane surface area.
Convection is a process in which a solvent travels through a semi-permeable membrane and carries various solutes with it. Convection is dependent on the membrane permeability coefficient (Km) and the transmembrane pressure (which is dependent on the hydrostatic and osmotic pressures on both sides of the membrane.
The principle of dialysis is based on the concept of diffusion. In the case of dialysis, blood and a dialytic fluid move in countercurrent directions on opposite sides of a filter.
Chronic Renal Failure
It is important to understand the distinction between RRT for chronic renal failure and RRT in the ICU setting (usually due to acute renal failure). In the case of the former, RRT is provided intermittently in order to achieve relatively arbitrary goals (e.g. urea < 30 mg/dL, creatinine < 2.5 mg/dL).
Acute Renal Failure
RRT for ARF is different from RRT in the setting of chronic disease in that it is generally utilized to correct a specific abnormality (e.g. hyperkalemia, hypervolemia, etc.). There are no evidence-based guidelines for initiation of RRT (nor are such guidelines forthcoming).