We learned before that Reverse Osmosis uses what is called a semi-permeable membrane (SPM) to purify water. An SPM is a material designed with a pore size that lets specific materials pass through it, while blocking others. In water purification the membrane allows water to pass, but blocks salts, bacteria and other solids.
We also learned that when a SPM separates a highly concentrated solution from a less concentrated solution there is a net migration of the elements in the highly concentrated solution into the less concentrated solution. If we leave the solutions in place over time, eventually the solutions will come to have equal concentrations and they will be said to be at equilibrium with each other.
Dialysis uses this same principle to save and maintain the lives of those stricken with kidney failure. The renal patient cannot excrete contaminants, and suffers toxic levels in their blood. The good news is that by passing the blood in one direction through a SPM, against the flow of a lower concentration solution called dialysate, over a 3 – 4 hour treatment, by osmosis, the patient’s blood concentrations can be returned to those approaching normal.
The diagram provided at the end of the article is a simple illustration of just one hollow fiber SPM element. The hollow fiber is engineered to keep the blood from escaping, thereby allowing the process of contaminant osmosis to take place across the membrane. An actual artificial kidney is made up of thousands of these small hollow fiber SPMs. Over the treatment the blood is constantly recirculated through the artificial kidney, back into the patient and then back out through the artificial kidney over and over.
In the beginning of the treatment the transfer of contaminants across the membrane is the most rapid. This is because it is at this point that the patient’s blood has the highest concentrations. However, as the treatment proceeds, the blood toxin concentrations decrease and there is less osmotic force for removal of contaminants to take place.
The dialysate is not recirculated. It removes contaminants and goes directly to drain.
On a final note, in the real world of chemistry, the contaminants don’t just move one way. They actually move freely in-and-out, in both directions across the surface of the SPM. It is the net movement out of the blood into the dialysate that is the magic of osmosis and hemodialysis.