AQA GCSE Biology (8461) — Homeostasis
When a person's kidneys fail, toxic urea and excess ions rapidly build up in their blood. If left untreated, this loss of filtration becomes fatal. There are two main medical treatments used to keep patients alive and restore blood concentrations: dialysis and organ transplants.
Dialysis is a clinical procedure that uses an artificial machine to act as a substitute for the normal function of the kidney. Blood is typically taken from a vein in the patient’s arm and pumped into the dialysis machine.
Inside the machine, the blood flows through tubes made of a partially permeable membrane (often called a selectively permeable membrane in exams). This membrane separates the patient's blood from the specially formulated dialysis fluid.
The membrane contains small pores that allow waste products to pass through while keeping essential components in the blood. Blood cells and large proteins do not pass through into the fluid because they are too large to fit through the pores.
Urea moves out of the blood and into the dialysis fluid by diffusion, moving down a steep concentration gradient. Excess water moves out by osmosis, and excess mineral ions diffuse out to restore healthy levels in the plasma.
To ensure essential substances are not lost, the dialysis fluid is formulated to contain the same concentration of glucose and mineral ions as healthy blood. Because the concentrations match, there is no net movement of glucose out of the blood, preserving it for cellular respiration.
The machine maximizes the rate of diffusion by constantly refreshing the dialysis fluid so it always contains zero urea. It also uses a counter-current flow, where blood and fluid flow in opposite directions to maintain a steep concentration gradient along the entire length of the membrane.
While dialysis keeps patients alive, a kidney transplant offers a long-term solution. A healthy kidney from a living or deceased donor is surgically implanted into the patient's lower abdomen (groin area).
The patient's diseased kidneys are typically not removed unless they are causing severe infections or high blood pressure. The donor kidney's artery and vein are connected directly to the recipient's femoral artery and vein, and its ureter is joined to the patient's bladder.
Before a transplant, tissue typing is performed to match the donor and recipient's blood types and cellular antigens as closely as possible. An antigen is a unique protein marker on a cell's surface used by the immune system to identify "self" from "non-self".
If the match isn't identical, the recipient's lymphocytes (white blood cells) will recognize the donor kidney's antigens as "foreign". These white blood cells will produce antibodies that attack and destroy the transplanted cells, leading to organ rejection.
To prevent this, transplant patients must take immunosuppressant drugs every day for the rest of their lives. While these drugs stop the body from attacking the new kidney, they lower the general immune response, making the patient highly vulnerable to common infectious diseases.
Doctors and patients must weigh the surgical risks, lifestyle impacts, and economic costs of both treatments. A transplanted kidney is not permanent; it lasts an average of 9 years before the patient may need a new transplant or must return to dialysis.
| Treatment | Advantages | Disadvantages |
|---|---|---|
| Dialysis | Available immediately; no major surgery required; no immunosuppressants needed. | Time-consuming (3 sessions a week lasting 3–8 hours); strict diet and fluid limits; risk of blood clots and infection; more expensive long-term for the NHS. |
| Transplant | Long-term solution offering a normal diet and greater lifestyle freedom; cheaper long-term. | High risk of organ rejection; requires lifelong medication; risks of major surgery (bleeding, infection); long waiting lists (2–3 years) due to a donor shortage. |
Students often use the term "selective reabsorption" when describing dialysis. This is incorrect; dialysis strictly relies on diffusion and osmosis, while selective reabsorption only occurs in natural kidney tubules via active transport.
When answering 6-mark "Evaluate" questions, always include a "quality of life" argument, such as how transplants free patients from spending hours tied to a dialysis machine every week.
If asked to explain organ rejection, examiners specifically look for three key words to award marks: 'antigens', 'lymphocytes' (or white blood cells), and 'antibodies'.
Always use the exact phrase "no net movement" when explaining why glucose and essential ions are not lost from the blood into the dialysis fluid.
Dialysis
A clinical purification of blood by an artificial machine acting as a substitute for the normal function of the kidney.
Partially permeable membrane
A membrane with small pores that allows only certain small molecules to pass through while blocking larger ones.
Selectively permeable membrane
Another acceptable term for a partially permeable membrane used in dialysis and cell biology.
Dialysis fluid
A specially formulated liquid used in dialysis machines that contains ideal concentrations of glucose and ions, but no urea.
Concentration gradient
The difference in the concentration of a substance between two areas, driving the rate of diffusion.
Diffusion
The net movement of particles from an area of higher concentration to an area of lower concentration.
Osmosis
The net movement of water molecules across a partially permeable membrane from a region of higher water concentration to a region of lower water concentration.
Kidney transplant
A surgical procedure to replace a failed kidney with a healthy one from a living or deceased donor.
Tissue typing
A blood test used to match donor and recipient antigens to reduce the risk of organ rejection.
Antigen
A unique protein marker on the surface of a cell used by the immune system to identify "self" vs "non-self".
Lymphocytes
A type of white blood cell that recognises foreign antigens and produces antibodies.
Antibodies
Proteins produced by lymphocytes that bind to specific antigens to trigger their destruction.
Organ rejection
The process where a recipient's immune system attacks and destroys a transplanted organ.
Immunosuppressant drugs
Medication that reduces the activity of the immune system to prevent it from attacking a transplanted organ.
Put your knowledge into practice — try past paper questions for Biology
Dialysis
A clinical purification of blood by an artificial machine acting as a substitute for the normal function of the kidney.
Partially permeable membrane
A membrane with small pores that allows only certain small molecules to pass through while blocking larger ones.
Selectively permeable membrane
Another acceptable term for a partially permeable membrane used in dialysis and cell biology.
Dialysis fluid
A specially formulated liquid used in dialysis machines that contains ideal concentrations of glucose and ions, but no urea.
Concentration gradient
The difference in the concentration of a substance between two areas, driving the rate of diffusion.
Diffusion
The net movement of particles from an area of higher concentration to an area of lower concentration.
Osmosis
The net movement of water molecules across a partially permeable membrane from a region of higher water concentration to a region of lower water concentration.
Kidney transplant
A surgical procedure to replace a failed kidney with a healthy one from a living or deceased donor.
Tissue typing
A blood test used to match donor and recipient antigens to reduce the risk of organ rejection.
Antigen
A unique protein marker on the surface of a cell used by the immune system to identify "self" vs "non-self".
Lymphocytes
A type of white blood cell that recognises foreign antigens and produces antibodies.
Antibodies
Proteins produced by lymphocytes that bind to specific antigens to trigger their destruction.
Organ rejection
The process where a recipient's immune system attacks and destroys a transplanted organ.
Immunosuppressant drugs
Medication that reduces the activity of the immune system to prevent it from attacking a transplanted organ.
