Lesson 5 of 5 / Excretion in humans
How dialysis removes waste
How can dialysis remove urea without losing the blood cells?
I can explain dialysis using membrane permeability, gradients and flowing dialysis fluid.
I can explain dialysis when kidney function is inadequate.Syllabus
Syllabus K325, Topic 7(d). Explain dialysis when kidney function is inadequate.
About 5 min
Make a guess. You are not marked.
In kidney dialysis, blood flows past dialysis fluid. Should that fluid be pure water?
Show the answer
It holds glucose and salts at blood levels.
Matching useful solutes stops them diffusing out of the blood, while urea, with none in the fluid, still diffuses out.
The key idea
Small wastes diffuse from blood into dialysis fluid; the membrane retains blood cells and large proteins.
Dialysis places blood and a specially prepared fluid on opposite sides of a partially permeable membrane. They remain separate streams.
The arrows at the outer edges show opposite fluid-flow directions. The middle arrow shows net solute exchange, or equal opposing movements. Both sides represent equal sample volumes; symbol counts are illustrative. Water removal is controlled separately, including through a pressure difference.
Explanation
If kidneys cannot remove enough wastes and regulate fluid adequately, these substances can accumulate in the body. In haemodialysis, blood is circulated through a dialyser outside the body and then returned to the circulation.
A thin, partially permeable membrane separates blood from dialysis fluid. Urea and other small solutes can pass across it, but blood cells and most large proteins remain on the blood side. The membrane does not identify whether a small molecule is useful or harmful.
Fresh dialysis fluid contains little or no urea. Urea therefore diffuses down its concentration gradient from blood into that fluid. A large membrane area and a short diffusion distance allow more exchange in a given time.
The fluid has controlled concentrations of useful solutes such as ions and glucose, rather than being pure water. In the simple model, matching a useful solute's concentration to its desired blood concentration avoids an unnecessary net loss. Excess ions can be removed when a suitable gradient exists.
The dialysis fluid is continuously renewed and commonly flows in the opposite direction to blood, helping maintain concentration gradients. Removal of excess water is controlled, including by a pressure difference across the membrane; it is not explained by urea diffusion alone.
Dialysis replaces some kidney functions but does not turn the external fluid into urine inside a kidney. Blood returns to blood vessels. Dialysis supports waste removal and fluid balance; it does not by itself cure the cause of kidney failure.
Step by step
- 1
Check the membrane
Ask whether the selected substance is small enough to cross.
- 2
Compare its concentrations
For urea, a lower concentration in fresh dialysis fluid favours movement out of blood.
- 3
Maintain the gradient
Renewed fluid prevents wastes accumulating on the dialysis side until net diffusion stops.
Worked example
Why replace the fluid?
Predict what happens to net urea removal if the same dialysis fluid is left without renewal until its urea concentration approaches that in the blood.
One way to explain it
The urea concentration difference becomes smaller, so net diffusion of urea from blood slows. If concentrations become equal, urea molecules still cross both ways but there is no net diffusion. Replacing the fluid helps maintain the gradient.
Why this answer works
- Choose the urea gradient, not a generic claim that the membrane gets full.
- State the effect on net diffusion.
- Equal concentrations do not mean individual molecules stop moving.
Watch out for this
A student says: "The dialysis fluid should be pure water so that everything leaves the blood." What is wrong with this?
Show the answer
Useful small solutes can also cross. Carefully controlled fluid composition removes wastes while avoiding unnecessary losses or unsuitable water movement.
Check your understanding
Why do red blood cells normally stay on the blood side of a dialyser?
Show the answer
They are too large to cross the partially permeable membrane.
Small solutes such as urea can cross while cells and most large proteins remain in blood.
Exam-style questions
Check your understanding
Why does dialysis fluid contain glucose at the same concentration as normal blood plasma?
Show the answer
So that there is no net loss of glucose from the blood
With no concentration gradient, glucose does not diffuse out of the blood into the fluid.
Quick recall
How can dialysis remove urea without losing the blood cells?