Treating Kidney Failure: Dialysis and Transplants

Explain the effect of osmotic changes on cells.
Describe the loss of water, ions, and urea via lungs, skin, and kidneys.
Explain the formation of urea from excess amino acids (HT only).
Describe the function of kidneys in filtration and selective reabsorption.
Explain the control of water levels by ADH and negative feedback (HT only).
Describe kidney dialysis and organ transplants.

Why kidney failure requires treatment

Healthy kidneys continually remove urea from the blood and regulate its water and ion content. In kidney failure, these jobs are no longer carried out effectively. Urea accumulates, while water and ion concentrations can move away from safe levels. This disrupts the stable internal conditions needed for cells to function and can be fatal without treatment.

There are two main approaches. Dialysis is a mechanical treatment that repeatedly cleans the blood. A kidney transplant places a functioning donor kidney in the body so that it can work continuously. Both address the consequences of kidney failure, but in very different ways.

How dialysis separates substances

In haemodialysis, blood is taken from the patient and passed through a dialysis machine. Inside the machine, the blood and dialysis fluid flow on opposite sides of a partially permeable membrane. Blood cells and large plasma proteins remain in the blood, while small dissolved substances can cross the membrane. The treated blood is then returned to the body.

The composition of the dialysis fluid is crucial:

It contains no urea, so the urea concentration is higher in the blood. Urea therefore diffuses out of the blood, down its concentration gradient.
It contains the normal concentration of glucose found in healthy blood. There is therefore no concentration gradient and no net diffusion of glucose out of the blood.
It contains normal, controlled concentrations of mineral ions. Needed ions are retained, while an excess in the blood creates a gradient that allows net movement into the dialysis fluid.
Excess water is also removed, helping to restore the blood's water balance.

The dialysis fluid is continually replaced or circulated. This prevents urea from building up beside the membrane and maintains a steep concentration gradient, so removal can continue efficiently.

A not-to-scale cutaway shows blood flowing from the patient to the right through a membrane-walled tube, while dialysis fluid outside the tube flows left. Blood cells and large proteins remain inside the tube. Arrows crossing the membrane show urea, excess ions and excess water leaving the blood. Fresh fluid contains no urea and normal glucose and ion concentrations; used fluid carries removed substances away. A separate detail shows glucose crossing the membrane in both directions with no net movement because its normal concentration is equal on both sides. Continuous fluid replacement maintains the concentration gradients. — The composition and circulation of dialysis fluid allow urea and other excess substances to be removed while limiting the net loss of useful glucose and ions.

Dialysis is not a permanent cure because the patient's kidneys have not been repaired. Urea, excess ions and water begin to accumulate again after a session, so treatment must be repeated regularly. Patients may also need to control their diet and fluid intake between sessions. Repeated sessions take time, and access to the bloodstream carries an infection risk, but dialysis does not require a donor organ and involves no risk of organ rejection.

How a kidney transplant treats failure

A kidney transplant replaces kidney function with a healthy kidney from a donor. Only one functioning donor kidney is required. If it works successfully, it continuously filters the blood and regulates water and ion levels, rather than cleaning the blood only during treatment sessions.

A transplant can therefore allow fewer restrictions on diet, drinking and daily activities than regular dialysis. It can also cost less over the long term. However, a suitable donor kidney must become available, the patient must undergo major surgery, and a transplanted kidney may not continue working indefinitely.

Rejection and how its risk is reduced

Cells carry surface antigens. If the recipient's immune system recognises antigens on a donated kidney as foreign, lymphocytes may attack the organ. This is rejection, and it can stop the transplanted kidney from functioning.

Doctors reduce this risk in two connected ways. First, the donor and recipient are tissue matched, so their antigens are as similar as possible. A close match makes a strong immune response less likely. Secondly, the recipient takes immunosuppressant drugs, usually for as long as the transplanted kidney remains in place. These reduce immune-system activity, but this also makes the patient more vulnerable to infections.

Comparing the treatments

Neither treatment is best in every situation. Dialysis can begin without waiting for a donor and avoids rejection, but it is repetitive, time-consuming and less effective than a continuously working kidney. A successful transplant can offer greater freedom and more continuous control of the blood, but it depends on a scarce suitable organ and brings the risks of surgery, rejection and immunosuppression.

A sound evaluation therefore depends on the circumstances. A transplant may offer the better long-term outcome for a patient who is fit for surgery and has a suitable donor. Dialysis remains essential for patients awaiting a transplant, those for whom transplantation is unsuitable, or those whose transplant does not work.

The problem

Kidney failure causes urea to accumulate and disrupts the balance of water and ions in the blood. The two main treatments are dialysis and kidney transplantation.

Dialysis: recall the gradients

Blood and dialysis fluid are separated by a partially permeable membrane.
Dialysis fluid has no urea: urea diffuses from blood to fluid.
It has normal glucose and controlled ion concentrations: useful substances are not lost by net diffusion, while excess ions can leave.
Excess water is removed and cleaned blood returns to the patient.
Circulating fresh fluid maintains concentration gradients.
Treatment must be repeated because it does not cure the failed kidneys.

Transplants: recall rejection

One healthy donor kidney can restore continuous kidney function. A successful transplant usually permits a less restricted life than regular dialysis.

The recipient's immune system may recognise donor antigens as foreign and reject the kidney. Reduce the risk by:

matching donor and recipient tissue types;
giving immunosuppressant drugs.

Immunosuppression increases susceptibility to infection.

Evaluation snapshot

Dialysis: no donor or rejection; repeated sessions, diet and fluid restrictions, infection risk and high continuing cost.

Transplant: continuous function, fewer restrictions and often lower long-term cost; donor shortage, surgery, rejection and lifelong immunosuppression.

When explaining urea removal, link the facts in a chain: the dialysis fluid contains no urea, so a concentration gradient exists and urea diffuses across the partially permeable membrane from the blood into the fluid.

Do not say that dialysis fluid contains no glucose or ions. It contains normal concentrations of glucose and useful ions, preventing their net loss; excess ions can still leave the blood.

For a transplant evaluation, consider both benefits and costs: continuous kidney function and fewer lifestyle restrictions versus surgery, donor shortage, rejection and lifelong immunosuppressant treatment.

If asked for a judgement, relate it to the patient. A transplant may offer a better long-term quality of life if a suitable kidney is available, whereas dialysis can sustain someone while they wait or when transplantation is unsuitable.

Distinguish tissue matching from immunosuppression: matching makes recognition as foreign less likely, while immunosuppressant drugs reduce the immune response.

A condition in which the kidneys cannot remove enough urea or regulate water and ion levels effectively.

A repeated treatment in which a machine removes waste substances and excess water and ions from a patient's blood.

A fluid with carefully controlled concentrations that exchanges substances with the blood during dialysis.

A membrane that allows some small dissolved substances and water to pass through but prevents blood cells and large proteins from crossing.

The net movement of particles from a region of higher concentration to a region of lower concentration.

The surgical replacement of a failed or damaged organ with a functioning organ from a donor.

An immune response in which the recipient's body recognises a transplanted organ as foreign and attacks it.

The comparison of donor and recipient tissue types to reduce the chance that a transplanted organ will be rejected.

A drug that reduces immune-system activity, helping to prevent rejection of a transplanted organ.