How the dialysis machine works
The patient's blood is taken from an artery and flows through a dialysis machine. Inside the machine, blood passes through tubes made of a partially permeable membrane. Dialysis fluid flows on the other side of the membrane in the opposite direction (countercurrent flow).
Urea is at a high concentration in the blood and absent from the dialysis fluid. It diffuses out of the blood across the membrane into the fluid, down its concentration gradient. The fluid is continuously refreshed so the concentration gradient remains steep throughout the treatment.
Why the fluid composition matters
The dialysis fluid is carefully formulated to contain the same concentration of glucose and useful mineral ions as normal blood. Because there is no concentration gradient for these substances, they are not lost from the blood by diffusion.
If the fluid contained no glucose, the patient would lose blood glucose during treatment. If it contained no mineral ions, essential ions would diffuse out. The precise composition ensures that only waste products (mainly urea) and excess water are removed.
| Substance | In blood | In dialysis fluid | Movement |
|---|---|---|---|
| Urea | High | None | Out of blood (diffusion) |
| Glucose | Normal | Normal (same) | No net movement |
| Mineral ions | Normal | Normal (same) | No net movement |
Dialysis versus kidney transplant
Dialysis must be performed regularly (typically three times per week, several hours per session). It replaces kidney function temporarily but restricts the patient's diet and lifestyle. There is a risk of infection at the access point.
A kidney transplant replaces the failed kidney with a healthy donor kidney. If successful, the patient no longer needs dialysis. However, the donor kidney must be tissue-matched to reduce rejection, and the patient must take immunosuppressant drugs for life to prevent the immune system from attacking the foreign tissue. There is also a shortage of donor kidneys.