When a patient's kidneys fail, toxic waste builds up in their blood rapidly. Dialysis is an artificial medical treatment that acts as a substitute kidney, clearing out this waste and balancing water and mineral ions.
The process follows a carefully controlled step-by-step mechanism:
The partially permeable membrane features microscopic pores. These are small enough to let water, urea, glucose, and ions pass through, but crucially do not allow large molecules (like blood proteins) or red and white blood cells to cross.
The dialysis machine does not use active transport; it relies entirely on precise fluid composition to control what stays in the blood and what leaves.
| Substance | Concentration in Blood | Concentration in Dialysis Fluid | Resulting Movement |
|---|---|---|---|
| Urea | High (toxic levels) |
| Diffuses out of the blood (High Low) |
| Glucose | Normal / Healthy level | Equal to normal blood level | No net movement |
| Mineral Ions | Excess | Normal / Ideal level | Excess diffuses out of the blood |
Because the glucose concentration is exactly the same on both sides of the membrane, there is no concentration gradient. This ensures the patient does not lose vital energy stores during treatment.
To maximise the rate of diffusion, the machine uses a counter-current system. The blood and the dialysis fluid flow in opposite directions, ensuring that a steep concentration gradient is maintained along the entire length of the dialyser. Fresh dialysis fluid is also constantly pumped in to replace used fluid, preventing the two liquids from ever reaching equilibrium.
Replacing a failing organ offers a more permanent biological solution. A donated kidney can come from either a living donor or a deceased donor.
During a transplant, the surgeon places the new organ in the patient's lower abdomen or groin. The patient's original, failing kidneys are usually left in place unless they are actively causing pain or high blood pressure.
To make the new kidney function, the surgeon must make three vital connections:
The human body's defence system is designed to destroy anything it does not recognise. When a new kidney is implanted, the patient's white blood cells may detect foreign proteins, known as antigens, on the surface of the donor organ.
If this happens, the recipient's immune system will produce antibodies to attack and destroy the transplanted tissue, resulting in organ rejection.
To minimise this risk, doctors use two main strategies:
Because their immune response is lowered, transplant patients are far more vulnerable to everyday infections, such as colds or the flu.
Patients and doctors must weigh the advantages and disadvantages of both approaches when managing long-term kidney failure.
| Feature | Kidney Dialysis | Kidney Transplant |
|---|---|---|
| Availability | Readily available with no waiting list. | Severe shortage of donors; average wait time is 2-3 years. |
| Lifestyle Impact | Highly restrictive. Requires 3-4 hospital visits per week (lasting 3-4 hours each) and strict limits on dietary protein and salt. | Restores a normal lifestyle with freedom to travel and eat normally. |
| Medical Risks | Risk of infection from repeated needle use and potential blood clots. | Major surgical risks and a lifelong risk of organ rejection. |
| Long-Term Cost | Highly expensive over time due to constant machinery, fluid, and nursing staff costs. | Cheaper in the long run (a one-off major surgery replaces years of weekly treatments). |
A hospital performed 350 successful living donor kidney transplants in 2021. Following a new awareness campaign, this number rose to 413 in 2022. Calculate the percentage increase in living donor transplants.
Step 1: State the formula for percentage change.
Step 2: Calculate the difference (change) and substitute the values.
Step 3: Calculate the final answer.
Do not write that urea is 'filtered' out of the blood in a dialysis machine. The required Edexcel terminology is that urea moves by 'diffusion down a concentration gradient'.
If asked why glucose is not lost during dialysis, explicitly state that there is 'no net movement' because the 'concentrations are the same' in both the blood and the fluid.
When explaining rejection, ensure you put the correct components in the correct places: the ANTIGENS are on the donor kidney, and the ANTIBODIES are produced by the patient's white blood cells.
For 6-mark 'Evaluate' questions comparing the two treatments, you must write a balanced argument. Make sure to mention at least two pros and two cons for BOTH dialysis and transplantation.
Dialysis
An artificial medical treatment that filters the blood to remove toxic waste like urea and balance excess water and ions when the kidneys fail.
Anti-coagulant
A substance added to the blood during dialysis to prevent it from clotting inside the machine.
Dialysis fluid
A specially formulated liquid used in a dialyser containing specific concentrations of glucose and mineral ions to ensure only waste and excess substances leave the blood.
Partially permeable membrane
A barrier with microscopic pores that allows small molecules like urea, water, and glucose to pass through by diffusion, but blocks larger molecules like proteins and blood cells.
Concentration gradient
The difference in the concentration of a substance between two connected areas (e.g., between the blood and the dialysis fluid).
Counter-current system
A mechanism in dialysis where blood and dialysis fluid flow in opposite directions to maintain a steep concentration gradient for maximum diffusion.
Renal artery
The major blood vessel that carries oxygenated, unfiltered blood into the kidney.
Renal vein
The major blood vessel that carries deoxygenated, filtered blood away from the kidney.
Ureter
The tube that transports urine from the kidney down to the bladder.
Antigen
A protein marker on the surface of a cell that can be recognised as foreign, triggering an immune response.
Antibody
A protein produced by the body's immune system to attack and destroy foreign cells or organs.
Organ rejection
When a recipient's immune system attacks and destroys a transplanted organ because it recognises its surface antigens as foreign.
Tissue typing
A medical process used to find a donor organ with antigens that closely match the recipient's cells, reducing the risk of rejection.
Immunosuppressants
Lifelong medications taken by transplant patients to weaken their immune system and prevent it from attacking the new organ.
Put your knowledge into practice — try past paper questions for Biology
Dialysis
An artificial medical treatment that filters the blood to remove toxic waste like urea and balance excess water and ions when the kidneys fail.
Anti-coagulant
A substance added to the blood during dialysis to prevent it from clotting inside the machine.
Dialysis fluid
A specially formulated liquid used in a dialyser containing specific concentrations of glucose and mineral ions to ensure only waste and excess substances leave the blood.
Partially permeable membrane
A barrier with microscopic pores that allows small molecules like urea, water, and glucose to pass through by diffusion, but blocks larger molecules like proteins and blood cells.
Concentration gradient
The difference in the concentration of a substance between two connected areas (e.g., between the blood and the dialysis fluid).
Counter-current system
A mechanism in dialysis where blood and dialysis fluid flow in opposite directions to maintain a steep concentration gradient for maximum diffusion.
Renal artery
The major blood vessel that carries oxygenated, unfiltered blood into the kidney.
Renal vein
The major blood vessel that carries deoxygenated, filtered blood away from the kidney.
Ureter
The tube that transports urine from the kidney down to the bladder.
Antigen
A protein marker on the surface of a cell that can be recognised as foreign, triggering an immune response.
Antibody
A protein produced by the body's immune system to attack and destroy foreign cells or organs.
Organ rejection
When a recipient's immune system attacks and destroys a transplanted organ because it recognises its surface antigens as foreign.
Tissue typing
A medical process used to find a donor organ with antigens that closely match the recipient's cells, reducing the risk of rejection.
Immunosuppressants
Lifelong medications taken by transplant patients to weaken their immune system and prevent it from attacking the new organ.