How Dialysis Works
“Football and Dialysis”
(With apologies to the Geelong Football Club)
Imagine an Australian Rules Football Oval
Out in the middle… the Geelong Cats are playing like a well-oiled machine!
There are two equal sized enclosures for the spectators
The two enclosures are separated by a chain-mesh fence (…)
Each enclosure has a locked gate (…) awaiting the gate-keepers
One gate-keeper opens his gate to let the fans – a mixture of both adults and kids – into the enclosures.
The second gatekeeper sleeps in.
His gate stays locked.
All the supporters therefore crowd into the one enclosure!
This causes problems
The kids can’t see the game … the adults are too big and block their view
Kids, being kids, see some holes in the chain-mesh fence that divides the two enclosures
What would you do if you were a small kid?
The smaller children begin to crawl through the holes so they can see the game more easily.
Meanwhile, the adults remain in the other enclosure … they cannot fit through the holes in the chain-mail fence.
Over time, the numbers in each enclosure would tend to equalize, giving the best view to all
But not everyone is happy
‘Security’ is furious ... ‘kids can’t do that’!
They round up the kids and take them away, clearing out the second enclosure
Suddenly … one enclosure is empty again
As soon as ‘Security’ turn their backs, the cycle continues
The remaining kids in the adults’ enclosure can’t resist the holes in the fence, the empty enclosure, the chance to see the game … and the thrill!
What’s more, the Cats are ‘on fire’ and they want a better view!
As Security leaves with the first load of ‘offenders’ … the remaining kids crawl through the fence for a better view of the game
As they do, the number staying in the adults’ enclosure falls even further
What about the younger teenagers?
Let’s suppose that not all holes are really small … that some are just a little bigger.
Some of the middle-sized kids – the teens – will also try to get through, eve though it may be more difficult to squeeze through and it may take them longer as they get caught up in the wire
Still … give them a bit more time to work on the holes and bend the wire back and … yes … some will eventually make it through too
In the end, the two enclosures will be depleted (cleared) of all the smaller children … and a few of the middle sized kids if they have enough time to make it through … leaving only the larger ‘adults’ behind.
So … let us think about the principle of what is happening here?
During the game, the smallest people easily cross through the fence for a better view … though they are constantly being removed by security.
Some middling-sized teens will make it through too – though they will take longer (need more time) and fewer will manage the journey
By the end of the game, both enclosures will be empty of all the ‘small people’ and some of the medium-sized teens as a result of …
1. a constant escape through the fence down a ‘concentration gradient’ - demonstrating the principle of “diffusion"
2. a constant removal and clearing away of the escapees by Security
In dialysis …
Just as we have just seen occurring in this example of a football crowd, a crowd in any crowded space will tend to disperse to a less crowded space if the barrier separating the two spaces can be crossed
So... if blood, containing high levels of waste (as it does in kidney failure) could be brought close to a fluid which contains no waste, and if the blood and fluid compartments could be separated by a ‘leaky’ membrane containing tiny holes of different sizes through which some of the wastes can freely pass (the small with ease, the bigger with some effort … and more TIME … the wastes would then move through the membrane from the blood and into the fluid.
Small wastes would ‘ooze’ easily through the holes in the membrane and into the waste-free fluid on the other side ... just like the kids would crawl through the wire-mesh fence in our football game example.
Medium sized wastes may – to an extent – make it through too – if given additional time to do so.
If the fluid in the fluid compartment can be consistently removed and renewed with fresh, waste-free fluid, a continual ‘gradient’ (difference) will be maintained down which more small wastes will flow.
Bigger wastes … and, importantly, the cells and proteins that the blood needs … can’t make it through at all. It is important to design membranes so they will not leak these things. The design of a membrane to prevent the loss of important cellular components is just as important as making it leaky to wastes but … and here is the catch … no membrane will only remove wastes and yet remove all of them while, at the same time, not allowing the removal of any of the ‘good stuff’ – the cells and proteins that the blood needs. There will always be some wastes (usually the bigger sized wastes) that can’t make it through … or only do so slowly and if given extra time.
To make a membrane that is so leaky that all wastes (small and large) can escape will, unfortunately, allow too many of ‘the goodies’ to get away too. This is the sad, but true, fact about the limitations of dialysis membranes. Some compromise is needed. Extra dialysis time helps – there is no doubt of this – and one day, membrane technology may solve this problem … but for now … …
So ... slowly, the blood will be cleansed of most of its high waste levels.
This football crowd example illustrates ‘how dialysis works’.
In summary …
In dialysis, small wastes pass across a ‘semi-permeable’ membrane
Over time, the blood is slowly cleared of its waste material.
Bigger wastes remain a problem as some of these will take much, much longer to ‘pass through’ … while some won’t get through at all!
But, you can also see that time also actually matters!
Not all wastes are small in size
Some wastes are middle-sized and some are big … and the bigger they are, the harder it is for them to pass through the membrane and the longer they will take to negotiate the holes in the membrane
The bigger-sized wastes therefore need either…
Considerably more time
A leakier membrane
Or … BOTH!
Authored by Prof John Agar. Copyright © 2012