Dialysis:

How does dialysis work?

Dialysis takes over the role of the kidney. It is used to remove waste products, such as urea, from the blood and to balance fluid, electrolytes and pH levels. Both types of dialysis involve dialysis fluid which the blood is passed alongside with a partially permeable membrane separating the two. The system works on diffusion and osmosis. The composition of the dialysis fluid is carefully designed. Substances that need to be retained in the blood such as glucose and amino acids need to be in the same concentration in the dialysis fluid so no net diffusion takes place. However, substances that need to be removed from the blood need to be absent from the dialysis fluid so these substances, like urea, will diffuse out of the blood and into the dialysis fluid.

Haemodialysis

In haemodialysis blood is removed from the patient and passed through a haemodialysis machine where it will be passed close to the dialysis fluid. In order to achieve this a fistula needs to be made in the patient’s arm several weeks before dialysis starts. This connects a vein to an artery so that blood can be taken and returned to the body in the same place. The increased blood pressure from the artery also strengthens the vein of the fistula. A patient will need to be attached to the machine for several hours and several times a week. The procedure will need to be carried out in a hospital.

Continuous ambulatory peritoneal dialysis

(CAPD) This type of dialysis makes use of a natural membrane in the body (the peritoneal membrane) to carry out the filtration. Dialysis fluid is pumped into the peritoneal cavity. Waste will diffuse from the body fluids into the dialysis fluid. The fluid is then drained into a bag. This type of dialysis needs to be carried out more frequently than haemodialysis but can be done in the patient’s own home, can be done over night while the patient sleeps and the patient is able to move about at the same time (which is where the term ambulatory comes from).

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