They make up the largest quantity of blood cells. In practice, the other cells are "neglected" when reporting the number of cells after centrifuging a blood tube. The components separated, the haematocrit (the percentage of red cells in the total blood) is said to be 45%, even though the other cells are also between these two phases as a very thin white layer.


An erythrocyte is a biconcave disc about 7µm in diameter. Due to the haemoglobin, erythrocytes (and therefore blood) are red. Haemoglobin is the blood molecule that transports oxygen to the organs and removes carbon dioxide. It consists of four subunits - two alpha and two beta chains, haem, globin and iron-containing protoporphyrin. One haemoglobin molecule can bind four oxygen molecules. There are, on average, 200,000 haemoglobin molecules in an erythrocyte. It has the critical ability to take up oxygen first but then release it into the tissue. How much oxygen is released depends on the oxygen saturation of the respective tissue. But it is also influenced by temperature, pH, 2,3-biphosphoglycerate concentration and CO2 concentration. A small example: during physical work, the muscle consumes much more oxygen than at rest. Muscle contraction, from a chemical and biological point of view generates heat and lactic acid. This raises the temperature, and the pH drops. And the consumption of oxygen produces CO2, so the CO2 saturation increases. All this leads to oxygen being released more easily. A self-regulating system. How this is regulated and influenced is far beyond the scope of this website (and probably the interest of most readers...). But I always find numbers particularly interesting, so here's one fun-fact: 1 litre of blood transports about 200ml of oxygen :-).

Red blood cells lack a nucleus and are eliminated from the bloodstream after 120 days. They undergo breakdown into their parts, particularly the iron from haemoglobin, in the spleen and liver, which is then recycled.


Except for stem cells and granulocyte concentrates, all other blood products intended for transfusion can be prepared in two ways:

  • One is by separating a whole blood donation into its component parts - this production method requires faithful, fearless blood donors who take about 45-60 min to give 500 ml of blood at blood drives, with the actual donation taking only about 10 minutes.
  • The other variant is apheresis. Here, an equally fearless donor is connected directly to a machine where the blood is collected and centrifuged. Different centrifugation speeds create layers that always contain a specific type of cell. This can then be selectively filtered out, and the rest is returned to the donor. This takes much longer - up to 1.5 hours - but is less stressful for circulation. Stem cells and granulocytes can only be collected in this way.

There are approx. 250 ml of blood suspension in an erythrocyte concentrate.

The required haematocrit is 50-70%.

The liquid (SAG-M) in which the cells are located is a nutrient solution that enables the red blood cells to be suitable for transfusion for up to 42 days. 

Since about 2000, all cellular blood products have been filtered to remove leukocytes. The maximum residual leukocyte content in a blood unit must not exceed 10^6. 

The storage temperature of blood products should be between 2 - 6°C. However, a drop in temperature is much more problematic because haemolysis of the cells can then occur. However, if the temperature rises, such ECs may be stored at room temperature for another 6 hours - then they must be transfused. 

A blood product that has been opened (erythrocytes, platelets, plasma) MUST be transfused within 4 h.


These are healthy erythrocytes - they are about 7 µm in diameter and have a lifespan of 120 days in circulation. They get their red pigmentation from haemoglobin, which binds iron.


A healthy person has between 3.5 and 5.5 x 10^3 erythrocytes per µL (the different concentrations in men and women were not considered here).

And in case anyone wonders what all those short tubes are for - they are also filled with the blood from the blood bag and sealed off into segments approx. 2 -3 cm long. A compatibility test must be done every time a red blood unit is ordered for a patient. For this, the blood from the bag is needed. However, since the bag must not be opened, exactly these segments are taken for this purpose.

Last update on 13.08.2023.