Hormonal Coordination in Humans » Kidney Failure: Dialysis and Transplants

When the kidneys fail

Healthy kidneys filter the blood and remove excess water, ions and urea in the urine. You met how they do this in the lesson called Water Balance and the Kidneys. Some people suffer from kidney failure. Their kidneys can no longer do this job properly.

Without treatment, urea, extra ions and extra water build up in the blood. The concentration of the body fluids goes out of balance and body cells stop working efficiently. Kidney failure is very serious. There are two main treatments: kidney dialysis and a kidney transplant.

Key terms:

• Kidney failure: the kidneys no longer remove waste substances and extra water from the blood properly.
• Dialysis: a treatment that uses a machine to clean the blood of a person whose kidneys have failed.
• Dialysis fluid: the liquid in a dialysis machine that waste substances move into.
• Donor: a person who gives an organ to someone else.

How kidney dialysis works

In kidney dialysis, a machine does the job of the kidneys from outside the body. Blood flows out of the patient, through the machine, and back into the patient.

Inside the machine, the blood flows next to a partially permeable membrane. On the other side of the membrane is the dialysis fluid. Small molecules can pass through the membrane. Large molecules, such as proteins, and blood cells cannot.

The dialysis fluid is made with the right concentrations of useful substances. It has the same concentration of glucose and ions as normal healthy blood. This is important because:

• Glucose and ions that the body needs do not diffuse out of the blood, because there is no concentration gradient for them.
• If the patient has too many ions in the blood, the extra ions can diffuse out into the fluid.
• The waste product, urea, is removed.

The cleaned blood then flows back into the patient. The dialysis fluid, now carrying the urea, is thrown away. Fresh fluid keeps being supplied so the concentration gradient for urea stays high.

Dialysis is not a one-off cure. A patient has to be connected to the machine again and again, usually for several hours each time and several times a week, for as long as the kidneys are failing.

Kidney transplants

The other treatment is an organ transplant. A healthy kidney is taken from a donor and placed in the patient's body. The new kidney then filters the blood and makes urine just like a healthy kidney. A person only needs one working kidney, so a living person can sometimes donate one. Kidneys can also come from someone who has died.

A transplant has a problem. The patient's immune system can see the new kidney as foreign and attack it. This is called rejection, as you saw for heart transplants. To reduce the risk, the donor kidney is matched as closely as possible to the patient's tissue, and the patient takes drugs that lower the activity of the immune system.

Dialysis or transplant?

When you are asked to evaluate, give points for both treatments and then decide which is best for the person in the question. A patient who is fit enough for an operation may be better with a transplant. A patient who is too ill for an operation, or who is waiting for a donor, will need dialysis.

Exam-style question

A patient has kidney failure and is treated with kidney dialysis.

(a) Explain why urea moves out of the patient's blood in the dialysis machine. [2 marks]

(b) The dialysis fluid contains the same concentration of glucose as normal blood. Explain why. [2 marks]

(c) Give one advantage and one disadvantage of a kidney transplant compared with dialysis. [2 marks]