A dialysis machine is a medical device that performs part of the job of healthy kidneys when they can no longer filter the blood well enough. In hemodialysis, blood is pumped from the patient through tubing into a dialyzer, often called an artificial kidney. The machine helps remove urea, extra salts, and excess water while returning cleaned blood to the body.
This technology matters because it can keep a person with kidney failure alive while waiting for recovery, transplant, or long-term treatment.
Key Facts
- Hemodialysis removes wastes by diffusion across a semipermeable membrane.
- Water removal is controlled by ultrafiltration, driven by a pressure difference across the membrane.
- Diffusion moves solutes from higher concentration to lower concentration.
- Typical blood flow rate in hemodialysis is about 200 to 500 mL/min.
- Mass removed = concentration difference x dialysance x time, in simplified treatment models.
- Fluid removed = ultrafiltration rate x treatment time.
Vocabulary
- Dialyzer
- A cartridge containing hollow fibers with semipermeable membranes that separate blood from dialysate.
- Dialysate
- A carefully mixed fluid that flows past the membrane to draw wastes and excess electrolytes out of the blood.
- Semipermeable membrane
- A barrier that allows small molecules and water to pass through while keeping blood cells and most proteins in the blood.
- Diffusion
- The movement of particles from an area of higher concentration to an area of lower concentration.
- Ultrafiltration
- The removal of water from blood by applying a pressure difference across the dialyzer membrane.
Common Mistakes to Avoid
- Thinking dialysis adds healthy kidney cells to the blood. Dialysis does not replace kidney tissue, it filters blood using physical processes such as diffusion and pressure-driven water movement.
- Drawing blood and dialysate flowing on the same side of the membrane. They must be separated by the semipermeable membrane so wastes can cross while blood cells stay in the blood circuit.
- Assuming all substances leave the blood during dialysis. Useful components such as blood cells and most proteins are retained, and dialysate composition is chosen to help keep needed ions near safe levels.
- Confusing diffusion with ultrafiltration. Diffusion mainly removes dissolved wastes down a concentration gradient, while ultrafiltration removes excess water through a pressure difference.
Practice Questions
- 1 A dialysis machine runs with a blood flow rate of 300 mL/min for 4 hours. What total volume of blood passes through the dialyzer in liters?
- 2 A patient needs 2.4 L of excess fluid removed during a 3 hour treatment. What ultrafiltration rate is needed in mL/hour?
- 3 Explain why urea can pass from blood into dialysate through the dialyzer membrane, but red blood cells normally remain in the blood circuit.