Last time we talked about convection, this time let’s also discuss diffusion. Diffusion is also one of the three main principles of solute removal in CRRT. This concept is relatively simple and easy to understand.
Diffusion refers to the process of spreading out. In a solution, due to the Brownian motion of solvent molecules, solute molecules are evenly dispersed throughout the solution.
In CRRT, the filter column is not made of solid fiber strands, but rather hollow fiber tubes, as seen in the left image below. Blood flows through the center of the hollow fiber tube (indicated by the blue arrow in the right image), while the dialysis fluid passes through the filter column outside the hollow fiber tube (indicated by the red arrow in the right image).
The hollow fiber tube has a semi-permeable membrane structure, where the liquid components of blood, namely plasma, contain metabolic waste, while the dialysis fluid mainly consists of electrolyte solution. The two liquids are on either side of the semi-permeable membrane, and molecules smaller than the pore size of the membrane can freely diffuse to achieve concentration equilibrium. This process effectively removes metabolic waste. The greater the concentration difference of solutes on either side of the semi-permeable membrane, the higher the removal efficiency. It primarily removes small molecules such as creatinine and urea nitrogen, as well as organic acids. The variation in blood potassium levels depends on the concentration difference of the liquids on either side of the membrane. Molecules larger than the pore size of the semi-permeable membrane are intercepted on one side of the membrane.
CRRT treatment that primarily uses diffusion is called dialysis, which mainly removes small molecular substances. To increase the concentration difference between the dialysis fluid and the blood, the flow rates of both can be appropriately increased, and the flow direction of the dialysis fluid should be opposite to that of the blood.
Unlike convection, where the movement of the solvent directly drives the movement of the solute, diffusion occurs independently of solvent movement; solute movement is due to Brownian motion and is relatively slow. Therefore, the dialysis fluid and blood need to have a certain contact time to facilitate concentration equilibrium of the solutes on both sides.
