Your kidneys continuously adjust the composition of your blood. They remove urea, regulate the amount of water and mineral ions in the body, and produce urine containing substances that need to be excreted. When both kidneys fail severely, these jobs can no longer be carried out adequately.
This creates a serious problem because the substances normally removed by the kidneys do not simply disappear. Urea accumulates, the balance of mineral ions may be disturbed, and excess water may remain in the body. Dialysis provides an artificial way of removing some of these substances from the blood.
The key to understanding dialysis is not memorising the parts of a machine. Instead, keep asking one question: what concentration gradient is being created across the dialysis membrane, and which substances are able to cross it?
Try these five questions before reading the chapter. They check whether you already understand the membrane, concentration gradients and dialysis fluid.
- 5 diagnostic questions
- Diffusion and membrane reasoning
- Instant explanation for every answer option
Why is dialysis needed?
Before understanding the machine, first remember what has gone wrong. The kidneys normally help remove urea and regulate the amounts of water and mineral ions in the blood. These functions help keep the internal environment suitable for cells.
Urea is particularly important because it is a nitrogenous waste product. It is produced when excess amino acids are broken down in the liver and is transported in the blood to the kidneys for excretion.
If kidney function becomes severely reduced, urea and other substances that should be removed can accumulate. Water and ion balance can also be disturbed. Without treatment, these changes can become life-threatening.
Core idea: dialysis does not repair the damaged kidneys. It temporarily performs part of their excretory and regulatory work by altering the composition of the patient's blood.
What happens during haemodialysis?
In haemodialysis, blood is removed from the patient's circulation and pumped through a device called a dialyser. Inside the dialyser, the blood is separated from specially prepared dialysis fluid by a partially permeable membrane.
The blood and dialysis fluid never need to mix directly. Instead, selected substances move across the membrane according to concentration gradients.
Step 1: Blood is taken into the dialysis circuit
Blood is withdrawn from the patient's circulation and pumped towards the dialyser. In simplified O-Level diagrams, you may see blood shown travelling from the patient's arterial side into the machine and then returning to the venous side after treatment.
Step 2: Blood passes through the dialyser
Inside the dialyser are many narrow hollow fibres or lengths of tubing. Their walls act as a partially permeable membrane.
The blood flows along one side of this membrane while dialysis fluid flows along the other. The two liquids are therefore close enough for small dissolved substances to cross the membrane, but they remain physically separated.
Step 3: Small molecules cross the membrane
Molecules such as urea are small enough to cross the dialysis membrane. If their concentration is higher in the blood than in the dialysis fluid, they show net diffusion from the blood into the dialysis fluid.
Excess mineral ions can also be removed when a concentration gradient favours their movement out of the blood. The composition of the dialysis fluid is adjusted so that the desired concentration gradients are produced.
Step 4: Blood cells and plasma proteins remain in the blood
Red blood cells, white blood cells, platelets and large plasma proteins are much larger than molecules such as urea and mineral ions. They cannot normally pass through the dialysis membrane and therefore remain within the blood circuit.
Small enough to cross: substances such as urea, mineral ions, glucose and water, depending on the membrane and the concentration or pressure gradients present.
Retained in the blood: blood cells and large plasma proteins.
Step 5: The treated blood returns to the patient
After passing through the dialyser, the blood has a more suitable concentration of urea and other small solutes. It is then returned to the patient's circulation.
This process must be repeated regularly because the patient's cells continue carrying out metabolism and therefore continue producing waste between dialysis sessions.
The dialysis membrane is the key boundary
A frequent mistake is to think that the dialysis membrane somehow recognises which molecules are “good” and which are “bad”. It does not.
Instead, whether a substance crosses depends partly on properties such as its size and partly on the concentration gradient across the membrane. The dialysis fluid is therefore just as important as the membrane itself.
Think membrane + gradient: the membrane determines which particles can cross, while the composition of the dialysis fluid determines the direction and extent of their net movement.
How the dialyser creates useful concentration gradients
The following simplified diagram shows the central idea. Blood and dialysis fluid move on opposite sides of a partially permeable membrane. The opposite directions of flow help maintain a difference in urea concentration along the dialyser.
Why do the blood and dialysis fluid flow in opposite directions?
As dialysis fluid receives urea, its urea concentration gradually increases. If the same fluid simply remained beside the blood for too long, the difference in concentration would become smaller and diffusion would slow.
In a dialyser, blood and dialysis fluid can flow in opposite directions. Fresh dialysis fluid therefore continually encounters blood that still contains more urea. This helps maintain a concentration gradient over a greater length of the membrane.
In an exam, avoid saying that counter-current flow “makes diffusion happen”. Diffusion occurs because of a concentration gradient. Counter-current flow helps maintain that gradient.
What is in dialysis fluid?
Dialysis fluid is not simply distilled water. Its composition is carefully controlled so that unwanted substances leave the blood without causing large unwanted losses of useful substances.
| Component | How its concentration is arranged | Why this matters |
|---|---|---|
| Urea | Very low or absent in fresh dialysis fluid | Produces a concentration gradient so that urea diffuses from the blood into the dialysis fluid. |
| Glucose | Kept at a suitable concentration close to the normal concentration in blood | Prevents a large concentration gradient that would cause unnecessary net loss of glucose from the blood. |
| Mineral ions | Carefully controlled | Allows excess ions to be removed while avoiding an excessive loss of ions that are already at suitable concentrations. |
| Water | Water balance is controlled during treatment | Helps remove excess fluid and restore a more suitable blood volume and composition. |
Why not use pure water?
Imagine placing blood beside pure water across a membrane permeable to small dissolved molecules. There would be large concentration gradients for many useful solutes as well as for unwanted ones. Useful substances could therefore be lost from the blood.
Dialysis fluid avoids this by containing appropriate concentrations of useful small solutes. This allows the machine to control which substances show substantial net movement.
Exam reasoning: do not say that glucose remains in the blood because it is too large to cross the dialysis membrane. Glucose is a small molecule. What prevents substantial net glucose loss is the small or absent concentration gradient for glucose across the membrane.
How does urea leave the blood?
Suppose the patient's blood entering the dialyser contains a high concentration of urea while the fresh dialysis fluid contains little or none.
Urea molecules are moving randomly on both sides of the membrane. However, because there are more urea molecules per unit volume on the blood side, more urea molecules cross from blood to dialysis fluid than cross in the opposite direction.
There is therefore net movement of urea down its concentration gradient. This is diffusion.
Higher urea concentration: patient's blood entering the dialyser
Lower urea concentration: fresh dialysis fluid
Net movement: blood → dialysis fluid by diffusion
Why are there many long, narrow dialysis fibres?
Modern dialysers contain many narrow hollow fibres rather than one short, wide tube. This creates a large surface area across which exchange can occur.
A larger membrane area allows more molecules to cross at the same time, increasing the overall rate of exchange between the blood and dialysis fluid.
The membrane is also thin, keeping the diffusion distance short.
Link the feature to its advantage: many narrow fibres → large membrane surface area → more area available for diffusion → faster exchange of dissolved substances.
What happens to excess water?
Dialysis must also help control the patient's fluid balance because failing kidneys may be unable to remove enough water.
In haemodialysis, excess water can be removed across the dialysis membrane under a controlled pressure difference. This pressure-driven removal of fluid is called ultrafiltration.
For O-Level questions about the basic dialysis mechanism, the most important molecular idea remains the diffusion of small waste solutes such as urea across a partially permeable membrane. Do not confuse this with selective reabsorption in a functioning nephron.
Dialysis and a functioning kidney are not exactly the same
A dialysis machine can replace important parts of the kidneys' filtering and regulatory functions, but it does not recreate every process occurring in a healthy kidney.
| Functioning kidney | Haemodialysis |
|---|---|
| Continuously receives blood throughout the day | Treatment occurs during dialysis sessions |
| Forms filtrate by ultrafiltration in the nephron | Blood is exposed to dialysis fluid across an artificial membrane |
| Selectively reabsorbs useful substances from kidney filtrate | Uses controlled dialysis-fluid concentrations to limit unwanted net loss of useful substances |
| Excretes urea in urine | Allows urea to diffuse into dialysis fluid, which is then discarded |
| Continuously adjusts water and ion balance | Water and ion balance are adjusted during treatment |
Why must dialysis be repeated?
Dialysis removes accumulated wastes, but the patient's cells do not stop respiring, synthesising proteins or carrying out other metabolic reactions after the treatment ends.
Urea therefore continues to be produced and transported in the blood. Water and ions also continue entering and leaving the body through food, drink, sweat and other processes.
If kidney function remains severely impaired, dialysis must therefore be repeated to prevent dangerous imbalances from developing again.
Dialysis is a treatment, not a cure for kidney failure
Dialysis can keep a person alive when kidney function is severely reduced, but it does not restore the damaged kidneys themselves. Patients may need repeated treatment for long periods, unless kidney function recovers or another treatment such as a successful kidney transplant becomes possible.
Dialysis can also place restrictions on daily life. Treatment takes time, fluid intake and diet may need to be controlled, and vascular access must be protected from problems such as infection or clotting.
For an O-Level explanation of dialysis, concentrate first on the mechanism: blood + partially permeable membrane + dialysis fluid + concentration gradient + diffusion. These five ideas explain most examination questions on the topic.
Common Mistakes
| Common mistake | Why it is wrong | Better approach |
|---|---|---|
| Saying urea leaves the blood by active transport | Urea moves down its concentration gradient across the dialysis membrane. | State that urea leaves the blood by diffusion. |
| Saying fresh dialysis fluid contains the same urea concentration as the blood | This would greatly reduce the concentration gradient and therefore reduce net removal of urea. | State that fresh dialysis fluid contains little or no urea. |
| Saying glucose cannot cross the dialysis membrane because it is too large | Glucose is a small molecule and can cross many dialysis membranes. | Explain that a suitable glucose concentration in the dialysis fluid prevents substantial net loss of glucose. |
| Saying dialysis fluid is pure water | Pure water would create inappropriate concentration gradients for many substances in the blood. | State that dialysis fluid contains carefully controlled concentrations of useful solutes such as glucose and mineral ions. |
| Saying red blood cells diffuse into the dialysis fluid | Blood cells are much too large to pass through the dialysis membrane. | State that blood cells and large plasma proteins remain in the blood. |
| Saying counter-current flow causes diffusion | Diffusion is caused by random molecular movement resulting in net movement down a concentration gradient. | Say that counter-current flow helps maintain the concentration gradient. |
| Confusing dialysis with selective reabsorption | Selective reabsorption is a process occurring in functioning nephrons, whereas haemodialysis uses an artificial membrane and dialysis fluid. | For dialysis, focus on diffusion across the partially permeable membrane and controlled dialysis-fluid composition. |
| Saying dialysis permanently cures kidney failure | Dialysis removes wastes and helps regulate blood composition but does not repair the damaged kidney tissue. | Describe dialysis as a treatment that replaces some kidney functions and must usually be repeated. |
What you should be able to do
Explain why dialysis may be necessary
Link kidney failure to the accumulation of urea, excess ions and excess water in the blood.
Describe the basic dialysis setup
State that blood and dialysis fluid are separated by a partially permeable membrane inside a dialyser.
Explain how urea is removed
Describe net diffusion of urea from the blood into dialysis fluid down a concentration gradient.
Explain why blood cells remain in the blood
Recognise that cells and large plasma proteins cannot pass through the pores of the dialysis membrane.
Explain the composition of dialysis fluid
Relate little or no urea and suitable concentrations of useful solutes to the concentration gradients required.
Explain counter-current flow
State that opposite blood and dialysis-fluid flow helps maintain concentration gradients along the dialyser.
Link dialyser structure to efficiency
Explain how many narrow fibres provide a large membrane surface area and a short diffusion distance.
Distinguish dialysis from kidney function
Separate diffusion in a dialyser from ultrafiltration and selective reabsorption in a functioning nephron.
These ten questions test whether you can apply concentration-gradient and membrane ideas to unfamiliar dialysis situations rather than simply recalling the steps.
- 10 mastery questions
- Application and concentration-gradient reasoning
- Instant explanation for every answer option
Frequently asked questions
Kidney Dialysis: questions students often ask
Once the basic mechanism is clear, most remaining questions come down to membrane permeability, concentration gradients and exactly what the dialysis fluid contains.
Does dialysis fluid contain urea?
Fresh dialysis fluid contains little or no urea. This keeps its urea concentration lower than that of the patient's blood, creating a concentration gradient that causes net diffusion of urea from the blood into the dialysis fluid.
Why does glucose not simply diffuse out of the blood during dialysis?
Glucose is small enough to cross the dialysis membrane, so it cannot simply be trapped because of its size. Instead, the glucose concentration of the dialysis fluid is kept at a suitable level close to that of normal blood. This produces little or no concentration gradient for unnecessary net glucose loss.
Why can urea cross the dialysis membrane but plasma proteins cannot?
Urea molecules are very small compared with plasma proteins. The pores of the dialysis membrane allow small dissolved molecules to cross while retaining much larger molecules such as plasma proteins.
Do red blood cells enter the dialysis fluid?
No. Red blood cells are much too large to pass through a correctly functioning dialysis membrane. They remain within the patient's blood throughout treatment.
Why is dialysis fluid continuously replaced?
As dialysis fluid receives urea and other wastes, their concentrations in the fluid rise. Replacing the fluid keeps waste concentrations low on the dialysis-fluid side and therefore helps maintain concentration gradients for continued removal from the blood.
Why do blood and dialysis fluid flow in opposite directions?
Opposite, or counter-current, flow means that blood containing more waste continues to encounter dialysis fluid containing less waste along the length of the dialyser. This helps maintain concentration gradients and improves exchange.
Does dialysis use active transport?
The important O-Level mechanism for removing urea is diffusion, not active transport. Urea moves down its concentration gradient from the blood across the partially permeable membrane into the dialysis fluid.
Is dialysis the same as ultrafiltration in the kidney?
No. In a functioning kidney, blood is ultrafiltered at the glomerulus and useful substances are later selectively reabsorbed along the nephron. Haemodialysis instead brings blood close to dialysis fluid across an artificial partially permeable membrane so that substances such as urea can diffuse according to concentration gradients.
How is excess water removed during haemodialysis?
Clinical haemodialysis can remove excess water across the dialysis membrane by a controlled pressure difference, a process called ultrafiltration. For O-Level explanations of waste removal, however, remember that urea and other small solutes are mainly discussed in terms of diffusion down concentration gradients.
Does a person need dialysis only once?
Usually not if severe kidney failure is permanent. Dialysis removes wastes present at the time of treatment, but metabolism continues afterwards and new waste products accumulate. Treatment therefore has to be repeated unless kidney function recovers or another effective treatment, such as kidney transplantation, replaces the need for dialysis.
Do I need to memorise that blood comes from an artery and returns through a vein?
Follow the labels given in an examination diagram if artery and vein are shown. More importantly, understand the mechanism: blood is withdrawn from the patient's circulation, passes through the dialyser and is then returned after treatment. Real haemodialysis commonly uses specialised vascular access such as an arteriovenous fistula.
What is the shortest complete exam explanation for removal of urea by dialysis?
State that blood is separated from dialysis fluid by a partially permeable membrane. Fresh dialysis fluid has a lower urea concentration than the blood, so urea diffuses from the blood across the membrane into the dialysis fluid down its concentration gradient. Blood cells and large plasma proteins remain in the blood because they cannot pass through the membrane.
Kidney dialysis becomes much easier once you stop viewing the dialyser as a mysterious machine. At its heart, it is an exchange surface. A partially permeable membrane decides which substances can cross, while carefully prepared dialysis fluid creates the concentration gradients needed to remove unwanted solutes. If you can explain those two ideas clearly, you can reason through most O-Level dialysis questions instead of memorising the machine step by step.