Is dark blood normal during dialysis?

Dialysis is a process that cleanses a person's blood of toxic substances when their kidneys are no longer able to do so. It usually has to be repeated throughout your life, as the body is constantly producing toxins.

Dialysis is performed if a patient has typical symptoms of excessive accumulation of toxins (vomiting, nausea, lack of appetite, itching, loss of strength) or if the doctor examines other symptoms such as: B. an inflammation of the pericardium or inflamed skin. Very high levels of urea and creatinine in the blood can also make dialysis necessary. Dialysis can also save lives if a person is poisoned and their organs cannot remove the toxic substances from the body in time.

With hemodialysis (blood washing with a machine, "artificial kidney"), the patient's blood circulation is connected to an "artificial kidney". This device is called Hemodialysis machine and purifies the patient's blood. The actual blood purification takes place in the dialyzer, the filter of the dialysis machine. In order to direct the blood through the hemodialyzer, the doctor inserts a catheter into a vein, similar to a blood sample, through which the blood to be purified is pumped through the artificial kidney.

Many dialysis patients also have to take medication to alleviate the consequences of kidney weakness and to support blood washing. These include:

  • Phosphate binders: Since phosphates are difficult to remove from the blood by dialysis, they prevent these salt components from accumulating in the body.
  • Erythropoietin: Erythropoietin deficiency leads to anemia in patients with kidney disease. Injections of erythropoietin into the vein counteract anemia. Iron supplements can help the hormone work.
  • Vitamin D: Vitamin D slows down the formation of parathyroid hormone in the parathyroid gland and can thus prevent the bones from losing calcium and breaking easily.
  • Heparin: Heparin inhibits blood clotting and prevents the blood from clotting in the tubes of the dialysis machine.


In the hemodialyzer, the patient's blood flows along a semi-permeable membrane. On the other side of the membrane there is rinsing liquid, the so-called Dialysate. The patient's blood usually flows past the membrane in one direction, the rinsing liquid in the other direction (countercurrent principle). Toxic substances enter the rinsing liquid through tiny pores in the membrane. Smaller molecules are better removed from the blood. Nutrients and salts such as sodium, glucose or bicarbonate enter the patient's blood from the dialysate. These substances are important, among other things, so that z. B. the acid-base balance of the patient is balanced and the blood sugar balance does not get mixed up. Other non-toxic substances, such as B. vitamins are also removed from the blood during dialysis and have to be artificially fed back into the body.

How quickly and how strongly the blood is cleaned depends on the type of membrane, the composition of the rinsing liquid and the pressure values ​​set. Since the blood cannot be completely cleaned in one run, the blood is pumped through the artificial kidney around 15 times per dialysis session. Heparin keeps it liquid to prevent it from coagulating.

Dialysis shunt

Since the patient's blood has to be cleaned regularly, the recurring punctures damage the blood vessels. In addition, the blood vessels of people with chronic kidney failure often have a thin wall and are therefore very vulnerable and easy to burst. That is why dialysis patients receive an easily accessible, stable blood vessel in an operation: the dialysis shunt (pronounced: Schant) or Cimino shunt.

The surgeon connects an arterial blood vessel to a vein, e.g. B. on the forearm. Since the blood flows in arteries with a higher pressure than in the veins, the blood now flows with a high pressure from the artery into the vein. This dilates this vein and creates a thick wall. This means that the vein can be punctured repeatedly for dialysis. Since the blood vessels run directly under the skin, the bulging vessels are easily visible in many dialysis patients. Until the wall of the vein has thickened sufficiently, dialysis is carried out through a catheter, which is usually placed on the patient's neck (central venous catheter (CVC) or Shaldon catheter).

If the shunt is blocked by a blood clot, the clot can be removed with a small operation. A constriction can also be widened surgically or with a balloon catheter.


The dialysis process itself is completely painless for the patient, except for the need to insert the dialysis needles into the shunt. Just a few days to weeks after the start of dialysis, the patient's condition improves significantly: appetite improves, feelings of nausea decrease and patients feel more rested. In addition, the blood pressure normalizes and any previously existing itching disappears.

Dialysis problems are rare. Possible side effects can be:

  • Muscle spasms on dialysis
  • Dizziness and fatigue from falling blood pressure too much
  • Allergic symptoms with itching or other intolerances with e.g. headache or nausea

Dialysis cannot completely replace the functioning of the kidneys and cannot remove all toxins from the blood. Therefore, substances accumulate over the years, which can lead to various complications. These include:

  • Bone pain, broken bones
  • Itching, vulnerable skin
  • Impotence, infertility
  • Easy fatigue
  • Heart failure
  • Muscle weakness
  • Depressive mood
  • Nerve disorders (e.g. tingling sensations)

These symptoms are likely caused by toxins that are not adequately captured by dialysis. They are also caused by inadequately controlled side effects of kidney weakness, such as insufficiently well controlled blood pressure. They cannot be completely avoided even with well-performed dialysis. The only way out in such cases is a kidney transplant.

Different dialysis procedures

The blood can then be purified in different ways:

  • Hemolysis
  • Peritoneal dialysis
  • Hemofiltration
  • Hemodiafiltration
  • Hemoperfusion


Hemodialysis is the most commonly used dialysis procedure. It works on the principle of diffusion: from the blood with a high concentration of waste substances these substances flow into the liquid with the lower concentration, in this case the rinsing liquid (Dialysate). Conversely, sodium and bicarbonate, for example, move from the dialysate into the blood when the concentration in the rinsing liquid is higher than that in the blood. The particles move depending on the difference in concentration in the two liquids.

How long it takes to clean the blood and how often dialysis has to be carried out varies from patient to patient and depends on their height and how well the patient's kidneys are still functioning.

In the chronic intermittent hemodialysis the patient is connected to the dialysis machine 3 times a week for 4-5 hours. Some patients feel more comfortable and efficient with a daily blood purification. When and how often patients dialyze should be carefully discussed with their doctor.

Dialysis can be performed in certain dialysis centers or on a machine at home (home dialysis). Holiday dialysis is now also possible at a large number of classic holiday locations with an appropriate lead time

Peritoneal dialysis

The patient's own peritoneum serves as a membrane. The abdominal cavity is the "container" for the irrigation fluid. The patient feeds it to himself via a catheter inserted in a small operation. There are two different types of peritoneal dialysis:

In the continuous ambulatory peritoneal dialysis (CAPD) there is always flushing fluid in the abdomen. The patient or a helper changes the rinsing liquid (2-2.5 liters) 4 to 5 times a day. The fluid stays in the body for 4-6 hours, overnight up to 9 hours. In the continuous cyclic peritoneal dialysis (CCPD), the rinsing liquid is regularly exchanged by a device, a so-called cycler. This is conveniently done at night, as the catheter has to be connected to the device from the patient's abdomen. The device performs approx. 6 cycles per night. In total, around 12-15 liters of irrigation fluid are exchanged in the abdominal cavity.

What many patients appreciate about peritoneal dialysis is that they are not connected to a device for several hours several days a week. It also has the advantage that the toxic substances are removed more evenly than with hemodialysis and that no blood is lost. In many patients, the residual kidney function can be maintained longer than with hemodialysis. Continuous cyclic peritoneal dialysis, in turn, has the advantage over ambulatory peritoneal dialysis that patients are hardly affected by their kidney disease during the day. It is therefore well suited for working people. The suitability for one of the two procedures is individually different. It depends largely on the transport properties of the peritoneum of the individual patient and must be carefully discussed with the doctor.

However, some patients find internal dialysis uncomfortable or cumbersome. The bags with the rinsing liquid must be stored and preheated each time. In addition, with peritoneal dialysis there is a risk that germs will migrate from the outside into the abdomen via the catheter and that the catheter or the peritoneum will become inflamed (peritonitis). Since dialysis via the peritoneum is somewhat less effective than dialysis with a dialyzer, it does not make sense for patients who require a high rate of detoxification.


In most dialysis patients, urine production drops significantly after a few months. In order to avoid water retention, water can be removed from the blood by means of filtration. The blood to be cleaned is under a higher pressure than the rinsing liquid on the other side of the membrane. In this way, liquid is pressed from the blood into the dialysate and toxic substances in the blood are removed from the blood. By squeezing out liquid, this method can also detoxify the blood. However, liquid must be added to replace the liquid squeezed out with the toxins.

In terms of detoxification performance, this method is significantly more ineffective than dialysis - even if it is more gentle on the circulation. It must therefore be used as a continuous process over 24 hours, which makes its use only in the intensive care unit and in special special cases. However, filtration to remove water is part of almost every dialysis.


Hemodiafiltration is a combination of hemodialysis and hemofiltration. Patients with chronic kidney weakness are dialyzed about 3 times a week for 4-5 hours.


In hemoperfusion, the blood is pumped through substances that can absorb very well, e.g. B. activated carbon. Toxins stick to the charcoal and are thus removed from the blood. This procedure is mainly carried out when a patient has poisoned himself, e.g. B. from an overdose of sleeping pills or sedatives.