Red cell osmotic resistance is a measure of how strongly the membranes surrounding red cells resist an osmotic pressure gradient. A partial osmotic pressure develops at the semipermeable membranes of erythrocytes when they are surrounded by a saline solution that is below their own (physiological) salt concentration of 0.9 percent. Red blood cells absorb water via osmosis, swell, and those that are most likely to burst exhibit the least red blood cell osmotic resistance.
What is red cell osmotic resistance?
Red cell osmotic resistance is a measure of how strongly the membranes surrounding red cells resist an osmotic pressure gradient. Aqueous solutions with different solute concentrations, develop an osmotic pressure gradient when separated by a semipermeable membrane. Substances from the solution with the higher concentration have the tendency to migrate to the solution with the lower concentration to compensate for the concentration gradient. If the permeable membrane is difficult to pass for the usually larger substance molecules, for example NaCl (common salt), the small water molecules (H2O) enter from the weak to the stronger solution instead. In the case of erythrocytes, which are also surrounded by a semipermeable membrane, the same effect occurs via osmosis. If erythrocytes, the red blood cells, are surrounded by a saline solution whose concentration is below that of their own cytoplasm of about 9 percent (hypotonic solution), an osmotic partial pressure gradient occurs. This causes water from the surrounding solution to enter the erythrocytes via osmosis, since the salt molecules have great difficulty passing through the semipermeable membrane to the outside. The erythrocytes swell to the point of bursting due to water entry, a process known as hemolysis. The rate at which erythrocytes swell and burst when surrounded by a saline solution of defined concentration is a measure of their red cell osmotic resistance. The shorter the time to burst, the lower their osmotic resistance.
Function and task
Osmotically regulated mass transfer between erythrocytes and the surrounding blood plasma plays one of the main roles in the exchange of carbon dioxide for oxygen or oxygen for carbon dioxide in capillaries. The composition of the semipermeable membrane surrounding the erythrocytes is of particular importance. A change in the composition of the membrane affects the osmotic mass transfer and the functionality of the red blood cells. A change in the composition of the cell membrane can lead to a decrease or an increase in the permeability of the membrane. Both phenomena can have a detrimental effect on red cell functionality. Indirect evidence for the nature of the membranes and the osmotic capacity of erythrocytes is provided by their osmotic resistance, which can be measured in special procedures. For example, about twenty test tubes are prepared with saline in ascending concentration up to isotonic concentration of 0.9 percent. A few drops of blood are trickled into each test tube and left to stand. After 24 hours, a slight red coloration of the solution shows within which concentration the first dissolution of red platelets has taken place. In the test tubes with the weaker concentrated salt solutions, the red coloration becomes stronger because a larger proportion of the erythrocytes have burst and the escaping hemoglobin has mixed with the salt solution. The test tube in which no sediment of erythrocytes has formed corresponds to the one with the concentration below which all erythrocytes are lysed. The reference values for incipient lysis of erythrocytes within 24 hours are at a saline concentration of 0.46 to 0.42 percent. The values for complete lysis of erythrocytes after 24 hours are in the range of 0.34 to 0.30 percent in healthy individuals. In hemolytic anemias and in so-called spherocytic anemia, the determination of pathologically reduced red cell osmotic resistance plays an important role as a diagnostic tool.For the diagnosis of other hemolytic diseases, such as the inherited diseases thalassemia, sickle cell anemia, and others in which red cell osmotic resistance is increased, the determination of resistance plays a less important role because better diagnostic tools are available for these specific clinical pictures.
Diseases and medical conditions
One of the best-known diseases associated with an increase in red cell osmotic resistance is thalassemia. It is a hereditary disease that occurs in many variants with mild and severe courses and is due to gene alterations. The most common variant is beta-thalassemia. Interestingly, the causative gene defects are particularly common in southern Europe, Arab countries and sub-Saharan Africa, the classic malaria regions. Presumably because thalassemia gives affected individuals advantages in overcoming malaria. Thalassemia shortens the lifespan of red blood cells, so the body has an increased production rate to compensate, which can be lifesaving in malaria cases by accelerating the supply of newly produced red cells. From a population genetics perspective, the slight survival advantage that people suffering from thalassemia have over certain forms of malaria has favored the gene defects in malarial regions and led to a slight gene drift. Sickle cell anemia is another inherited disease associated with increased red cell osmotic resistance. It is caused by genetic defects that result in defective hemoglobin, called sickle cell hemoglobin, which leads to clumping and blockages in the veins because of the fibers it contains. Anemias caused by iron deficiency also lead to an increase in red cell osmotic resistance. They can be caused by high blood loss due to injury, by a disorder of hematopoiesis, or by excessive breakdown of erythrocytes. So-called spherical cell anemia is also hereditary and is manifested by a decrease in red cell osmotic resistance, as the normally oblate and concave red cells take on a spherical shape due to a defectively formed cytoskeleton and become predisposed to hemolysis while still in the spleen.
