The kidneys and body balance
Back page Forward page
5 of 6
What is dialysis?
Why do people need dialysis?
If a person's kidneys fail, then they stop removing waste products (like urea) from the blood. These toxic waste products build up and will eventually kill the person.
Diagram of dyaliser
Picture 8. A simplified dialysis machine. The dialysis solution flows left to right and the blood flows in the opposite direction.
What is dialysis?
In dialysis, a patient's blood is passed through a dialysis machine. The dialysis machine takes over the job of their kidneys, removing waste products from the blood.
How does dialysis work?
Picture 8 shows a simplified diagram of a kidney dialysis machine.
  • a tube connects a person's vein to the dialysis machine
  • inside the machine, blood is pumped through tubes made from dialysing membranes
  • dialysis solution is on the other side of the dialysing membrane, kept fresh by a constant flow
  • blood returns to the person's arm
Concentration
Roll over diagram of osmosis
Picture 9. Diffusion makes the concentrations the same on both sides of the membrane.
The dialysing membrane allows small particles to pass through it. The result is that the concentration of these small particles will end up being the same on both sides of the membrane. This is called diffusion. Let's see how it happens.

The particles are moving randomly. They can pass both ways through the membrane. However, more particles will meet the membrane on the side with a higher concentration. Therefore more particles will pass through from this side. This means that there will be a net flow from the higher concentration to the lower concentration.

This continues until the concentrations are the same. Then there will be the same number of particles passing both ways.

Transferring substances

The dialysis solution contains water, glucose, salts and various substances at the correct concentration for the body. These substances diffuse through the membrane and into the blood. So the blood leaving the machine has all these substances in the same concentration as the dialysis solution.

The fresh dialysis solution does not contain urea - so urea (and other impurities) pass out of the blood. They are taken away by the flow of dialysis solution.

Proteins and blood cells are too big to pass through the membranes so stay in the blood.

Photo of donor organ bag
Picture 10. Human organs being transported for transplant surgery. They have to be kept cool and protected from shocks.
Dialysis on the move – CAPD
CAPD (continuous ambulatory peritoneal dialysis) is another sort of dialysis. CAPD uses the body’s own membrane for dialysis. Dialysis actually goes on inside the patient’s body. The good thing about it is that dialysis happens all the time, even when someone is sleeping or out shopping.

Back page Forward page
Question 4
a) Suggest what properties a material should have for making dialysis membrane.

b) Particles move from an area of greater concentration to an area of lower concentration.
i. What is the concentration of urea in the unused dialysis solution?

ii. Why must there be a difference between the concentration of urea in the blood and the dialysis solution?

c) The direction of flow of both the blood and the dialysis solution through the machine is important. Why does the arrangement shown in the picture work well?