What is Dialysis?

Dialysis treatment tries, as the terms kidney replacement therapy or artificial kidney suggest, to replace the lost function of the body’s own kidneys as far as possible.

I.e. The substances that are excreted by the kidneys with the urine in a healthy person (metabolic products, drugs, components of food, etc.) are removed by artificial “filters” in patients with no or limited kidney function.
In addition, dehydration takes place during dialysis, as this usually also occurs. in kidney patients the natural way (urinating) is restricted or not done at all.

There are different types of dialysis.

The different types of dialysis
A basic distinction is made between two different types of dialysis treatment.

With hemodialysis, the patient’s blood is “cleaned” outside the body in the so-called dialyzer and excess water is withdrawn from him before it is returned to the bloodstream (for details, see the physico-chemical principles of hemodialysis).

Peritoneal or peritoneum dialysis has been an alternative to hemodialysis since 1976. One also speaks of CAPD (continious ambulatory peritoneal dialysis), which means something like: continuous ambulatory peritoneal dialysis.

The exchange processes for detoxification and dehydration take place on the peritoneum instead of outside the body.

One speaks of continuous because with this form of treatment a dialysate fluid is given into the abdominal cavity several times a day via a catheter, which remains there for several hours, i.e. dialysis takes place continuously.
This procedure is outpatient because, after a short learning phase, the patient carries out the treatment independently at home.

Vascular access (shunt / fistula)
Since the blood is “cleaned” outside the body during hemodialysis, it is necessary to provide access to the patient’s bloodstream.
During dialysis, 200-300 ml of blood per minute is withdrawn from the patient via this access, cleaned and then returned.

Such a vascular access is created by creating an operative connection from a forearm artery, which carries blood from the heart to the hand, to a forearm vein, which carries the blood back from the hand.
The resulting higher pressure in the vein widens it and can be punctured with a dialysis cannula.

This procedure was invented in 1966 by vascular surgeons Brescia and Cimino. This type of vascular access is called a Brescia-Cimino fistula or shunt, English for short circuit (between artery and vein).

Physico-chemical principles
During dialysis, the patient’s blood and the “cleaning fluid” (dialysate) flow past each other in the artificial kidney (dialyzer), separated by a semipermeable (semi-permeable) membrane.

Substances that are present in higher concentrations in the blood (metabolic products, drugs, etc.) pass into the dialysate and are transported away. This happens due to the physical phenomenon of diffusion:

If two liquids are separated by a semipermeable (semi-permeable) membrane, the particles dissolved in the liquids tend to move from the side of higher concentration to the side of lower concentration.
Diffusion scheme

Of course, only substances pass over whose molecular size is smaller than the pore size of the membrane.

In order to avoid that substances are withdrawn from the blood that should not be withdrawn from it (sodium, calcium, potassium, etc.), these substances are added to the dialysate in sufficient form.

If the dissolved particles are too large to diffuse through the membrane, the phenomenon of osmosis occurs:

The solvent (water) moves from the side of the lower particle concentration to the side of the higher particle concentration due to the osmotic force.
Osmosis scheme

A negative pressure on the side of the dialysate increases the transfer of water from the blood into the dialysate. Water can thus be specifically withdrawn from the blood by controlling the negative pressure. One then speaks of ultrafiltration.

Through the process of ultrafiltration (see above), dissolved particles are carried away by the overflowing water (convection), which intensifies the cleaning process of the blood.

Convection scheme

Readers of the chapter on the physical principles of peritoneal dialysis get back to their topic here.

The dialyzer:
In practice one usually finds Capillary dialyzers. A plastic tube contains 10,000 to 15,000 individual capillaries, which form the semipermeable membrane and through which the patient’s blood f