How Kidneys Work (Or Don’t)
Before we can describe how renal replacement therapy (dialysis) works, we need to understand how the kidney works. Dialysis replaces kidney function in a completely different way than our kidneys do their thing.
About 20% of each heart beat goes into the kidneys for filtration. Blood enters the glomerulus, a permeable capillary network with arterioles on either end. These arterioles each can enlarge or constrict to control flow and pressure within the glomerulus, based on signals from within the kidney, as well as neural and hormonal input from the rest of the body. In the glomerulus, the plasma (water-based portion) of the blood leaves the capillaries and enters the tubules of the nephron.
The arterioles that leave the glomeruli then intertwine with the tubules of the kidney to return “good stuff” in the filtrate to the rest of the body. For example, we filter ~10 grams of calcium each day. Since we only take in about 1 gram per day, our kidneys clearly need to retain most of that calcium. Normal people only lose 200 mg (0.2 grams) of calcium in their urine each day, so those tubules do a lot of work to keep us healthy.
As the filtrate travels along the nephron, the “good stuff” that the body needs is reabsorbed by the tubular cells. As the filtrate progresses, a number of specific segments of the tubule with different capabilities fine-tune the final composition of the urine based on signals from the entire body. Some “bad stuff” gets secreted by tubular cells, so the amount in the urine exceeds what was in the filtrate originally. Multiple systems exist to adjust the amount of virtually every chemical in the body- sodium, water, potassium, calcium, phosphorus, magnesium, and acid all can be compromised by problems with kidney function.
We have as yet to invent a manufactured tubule. Instead we perform dialysis to replace kidney function, although we only achieve a 10-15% level of function with standard protocols for treatment. Because our technology cannot remove everything and bring back the good stuff, we remove a portion of the plasma and attempt to minimize the “good stuff” we take off with it.
Next up: how we do just that with hemodialysis and peritoneal dialysis.