Does hemodialysis take so many hours because the dialyser is inefficient (in other words, every time blood passes through the dialyser only a small fraction of the waste is removed) or because there are about 5 Liters of blood in the body and it takes a while for it to go through the machine?
During dialysis only about 10 percent of the blood is outside your body at any given time, while the rest continues to circulate. If the blood flows at 600mL/min, then it doesn't take very long to clean 5 liters (say 8 min). So why aren't you done that quickly?
There are three reasons:
- Each time cleaned blood is returned, it mixes with the waste-filled blood
- The proportion of wastes extracted isn't high to start out, and
- How much is extracted each time depends on how "dirty" the blood is
So imagine that initially 10% of the waste is removed from blood that goes through the dialyzer. That means that if you stopped after that pint is returned to you, your blood would be 1% cleaner (10% of 10% = 1%). Not much difference. The next cycle would clean another pint (1% cleaner than the first one) and so on and so on.
Thus each time a pint of blood flows past the filters in the dialyzer, there's a smaller gradient of wastes between it and the clean fluids on the other side, so less of the waste gets extracted from that pint. That's a good thing, but it's frustrating, because each hour is important, but gets less and less wastes out.
If that's hard to imagine, imagine this:
Say you're trying to clean red paint out of water in a glass, but all you can do is to dipping in a wet sponge-brush, then rinsing the sponge-water by pushing it against the side of a gallon-size tub full of water. Some of the water will be squeezed into the tub, and the pressure is released, part of the sponge is filled with clean water, so the next time you dip and squeeze the sponge inside the glass, you squeeze out a partially-cleaned sponge. That's the equivalent of partly-cleaned blood returning from the dialyzer. The first many cycles will look like they squeeze out lots of paint, yet the water in the glass will still be dark red.
After many, many cycles, the water in the glass will finally look translucent red. Now, whenever you push the sponge against the side of the tub, the squeezed out water will look less reddish, so it's not as satisfying, but it's still helpful; the water you squeeze back into the glass to start each new cycle is still cleaner.
Caveat: in dialysis the blood stays on your side of the filters in the dialyzer; only the wastes are pulled across, so the analogy isn't quite right.