The Haemodialysis is a technique which replaces the main functions of the kidney, bypassing the blood through a filter (acting as an artificial kidney) where debugging is performed, returning the patient free of impurities again.


It depends on the dialysis needs of each patient, but as the blood has to pass several times through the filter, the average is 4 hours, three times a week. Keep in mind that the healthy kidney performs this work 24 hours a day and every day of the week.

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The Dialysis Machine

Blood is drawn through the haemodialysis to the patient, which is taken to a machine specially designed to purify it, eliminating what the kidney cannot and returning it to the patient in suitable conditions.

A dialysis machine works in the following way

Extract blood > Clean blood > Return clean blood
To be able to be dialyzed, a vascular access (catheter or arterio-venous fistula) with two directions of blood circulation is required. Through one direction, blood is sent to the machine to be cleaned, and through the other, it returns already purified.

What happens inside the dialyzer?

Basically, two different processes occur:

  • The dialyzer removes elements from the blood that should be filtered by the kidney through diffusion processes. Such elements as urea and potassium, if they were not purified, would cause serious damage to the organism. The membrane of the dialyzer makes the filter of these particles passing from the high concentration of them in the blood, to be discarded on the other side of the membrane where the concentration is lower so that this exchange can exist.
  • Ultrafiltration also occurs through an artificial osmotic pressure exerted by the dialysis machine. Through this technique, excess fluid in the patientís body is eliminated because the kidney progressively loses the ability to excrete urine. For this reason, a patient loses weight every time he attends a haemodialysis session. But it should be noted that the patient is not losing fat, but fluid accumulated in blood. To know how much weight (i.e., excess fluid) a patient has to lose each time they go on dialysis, a dry weight is established.

What is dry weight?

Each patient has their own dry weight, and is defined as the ideal weight that they should have as long as they do not have accumulated fluid. So, for example, an individual with a dry weight of 70 kg, if before connecting to the machine weighing 72 kg, should lose 2 kg in it. Each time a patient is dialyzed, it is intended, as much as possible, that once the session ends, they will be able to stay in their dry weight, since this would indicate that the excess of fluid in their body has been eliminated.
Perhaps in the following scheme, you can see more clearly what is described here:

  • Dry weight: It is the ideal weight of an individual, which is when there is no excess liquid in blood.
  • Pre-Dialysis Weight: It is the weight of the patient before being connected to the machine.
  • Post-Dialysis Weight: It is the weight of the patient once disconnected from the machine.

Always try that the post-dialysis weight is the same as the dry weight, although it is not always possible, since a very abundant weight loss in a single session is not advised. For example, if a patient with a dry weight of 70 kg, weighs 76 kg before being connected, will not lose 6 kg in a single day, because it could be very harmful to it.

The dry weight of a person can vary whenever there are changes in the food. If the patient has more appetite, the amount of fat in his body will increase (he will gain weight). In this case, the patient will have to say it in the dialysis centre to adjust the dry weight there. It can also be detected if the patient is dizzy, or has muscle cramps, when in dialysis more weight is lost than the one that has been adjusted as dry at that moment.

The most common side effect of losing fluid during a haemodialysis session is low blood pressure (hypotension), and the more frequent the loss is. Lower blood pressure can cause dizziness, cramps, nausea, dizziness and unconsciousness.

On the other side, excess fluid can accumulate in the legs and around the lungs, making things as common as walking difficult and others as important as breathing. So important in dialysis is to eliminate fluid as to purify the blood.

The amount of liquid that a patient acquires between two dialysis will depend directly on the diet that he / she takes (that is to say, of the food, and especially of the drink). The patient in haemodialysis treatment has restrictions with the diet, because the water of beverages and food is not eliminated, but accumulates, and it is very important that it meets them, as a matter of health, and to avoid complications when coming to dialysis.