When the kidneys fail, two treatments can replace their function: haemodialysis filters blood through an artificial membrane every two to three days, while a kidney transplant provides a permanent working kidney. Both treatments manage waste removal and ion balance but carry different demands on the patient's quality of life.
Why do the kidneys fail?
Healthy kidneys filter about 180 litres of blood per day, removing urea (produced from excess amino acids in the liver), excess ions, and excess water. They return glucose, useful ions, and most water to the blood.
Kidney failure occurs when both kidneys lose so much function that waste products build up to toxic levels in the blood. Common causes include:
- Type 2 diabetes (high blood glucose damages the filtration units, the glomeruli)
- High blood pressure (damages the delicate capillaries in the kidneys)
- Infections (e.g. glomerulonephritis)
- Inherited conditions (e.g. polycystic kidney disease)
Without treatment, kidney failure is fatal within days to weeks as urea and potassium accumulate.
How does haemodialysis work?
In haemodialysis, the patient is connected to a dialysis machine (an artificial kidney) through a tube inserted into a blood vessel. The machine pumps blood past a partially permeable membrane (made of cellulose acetate) suspended in dialysate fluid.
The key principles are diffusion and osmosis:
- Urea is absent from the dialysate, so urea diffuses from the blood (high concentration) across the membrane into the dialysate (low concentration) and is removed.
- Ions (sodium, potassium, chloride, bicarbonate) are present in the dialysate at the correct body concentrations, so only excess ions diffuse out — useful ions are not lost.
- Glucose is included in the dialysate at normal blood concentration, so no glucose is lost by diffusion.
- Excess water is removed by adjusting the osmotic concentration of the dialysate.
A typical haemodialysis session takes 3–4 hours and must be repeated three times per week because the machine cannot work continuously like a real kidney.
What happens during a kidney transplant?
In a kidney transplant, a functioning donor kidney is surgically placed — usually in the pelvis — and connected to the patient's blood vessels and bladder. The patient's own failed kidneys are often left in place unless they are causing problems.
Donor sources:
- Living donors (a family member or partner can donate one of their two kidneys)
- Deceased donors (kidneys retrieved shortly after death)
The rejection problem: The immune system recognises the donor kidney as foreign tissue and launches an attack against it, a process called rejection. To prevent this:
- Tissue typing — donors and recipients are matched as closely as possible for cell surface antigens (HLA matching)
- Immunosuppressant drugs — the recipient takes medication for the rest of their life to dampen the immune response
Immunosuppressants carry a trade-off: they prevent rejection but also reduce the patient's ability to fight infections and cancer.
What are the advantages and disadvantages of each treatment?
| Feature | Haemodialysis | Kidney transplant |
|---|---|---|
| Frequency | 3 sessions per week, ~4 hours each | One-off surgery |
| Diet | Very restricted (low K⁺, low fluid) | Near-normal after recovery |
| Freedom | Tied to machine; travel is difficult | Much greater freedom |
| Long-term medication | Anti-hypertensives; dietary control | Immunosuppressants for life |
| Infection risk | IV access carries some risk | Higher (immunosuppressants) |
| Cost | High ongoing cost (machine, staff) | High surgical cost but cheaper long-term |
| Waiting | Available immediately | Average wait 2–3 years in UK |
| Success | Maintains life indefinitely if continued | 85–90% of kidneys functioning at 5 years |
Can the patient do anything to help before receiving a transplant?
Patients awaiting transplant can improve outcomes by:
- Staying as healthy as possible (blood pressure control, healthy diet)
- Avoiding blood transfusions where possible (can sensitise the immune system to foreign antigens)
- Attending all dialysis sessions to keep waste levels manageable
- Being contactable 24/7 because a donor kidney must be transplanted within hours of becoming available
Frequently asked questions
Why must dialysis be done so frequently?
Healthy kidneys filter blood continuously — around the clock. A dialysis machine can only clean the blood during the session, so waste products accumulate between sessions. Three sessions per week is the minimum to keep urea and potassium at safe levels. Some patients use overnight home dialysis (peritoneal or nocturnal haemodialysis) to reduce dietary restrictions and improve quality of life.
Why do kidney transplant patients take immunosuppressant drugs for life?
The donor kidney has cell surface antigens (HLA proteins) that differ from the recipient's own. The recipient's immune system recognises these as "non-self" and would attack the kidney cells. Immunosuppressant drugs (such as ciclosporin and tacrolimus) reduce the activity of the immune system, preventing rejection. These drugs must be taken every day for as long as the transplant kidney is functioning, because stopping them would rapidly trigger rejection.
What is tissue typing and why does it matter for transplants?
Tissue typing identifies the human leukocyte antigen (HLA) proteins on the surface of a donor's cells. The closer the HLA match between donor and recipient, the less likely the recipient's immune system is to reject the kidney. A perfect 6-antigen match (very rare) dramatically reduces the need for immunosuppressants. In the UK, the NHS Blood and Transplant service maintains a national database and allocates donor kidneys to the best-matched patient.
What is the difference between haemodialysis and peritoneal dialysis?
In haemodialysis, blood is pumped through an external machine. In peritoneal dialysis, the patient's own peritoneal membrane (lining of the abdominal cavity) acts as the filtration membrane: dialysate fluid is pumped into the abdomen, waste products and excess water move from the blood vessels lining the peritoneum into the fluid, and the fluid is drained out after a few hours. Peritoneal dialysis can be done at home overnight and gives patients more independence, but carries a risk of peritoneal infection.
For Socratic GCSE biology with Professor Darwin — tracing kidney function from nephron to whole-body homeostasis — visit aitutors.me.