Osmotic pressure corresponds to the pressure present on the higher concentration side of a semipermeable or selective permeable membrane in the solvent. The pressure drives the flow of solvent through the membrane and dictates its direction. Diseases related to osmotic pressure include decreased pressure resistance of blood cells.
What is osmotic pressure?
Diseases related to osmotic pressure are, for example, decreased pressure resistances of blood cells. Medicine uses the term osmotic pressure to refer to the physiological pressure that allows osmosis to occur. Osmosis corresponds to the directional flow of molecular particles through semipermeable or selectively permeable interfaces. Thus, osmosis is an essential transport of substances in the human body. Osmotic pressure is the main prerequisite for this mass transfer process. The dissolved molecules in a solvent cause the osmotic pressure on that parting layer side with the higher concentration. The resulting pressure conditions drive the flow of the solvent through the respective membrane. In this way, the solvent moves through the membrane from the side with the lower particle concentration and thus flows to the side with the higher concentration in each case, where the osmotic pressure exists. The molecular particles themselves cannot pass through the semipermeable or selectively permeable membrane.
Function and task
Osmotic pressure depends on the concentration ratios of two solutions located on different sides of a semipermeable or selective permeable membrane. Although osmotic pressure also exists on the lower concentration side, the pressure on the higher solute concentration side is always higher. In the human body, there is an influx of water into individual cells from the interstitium. This inflow occurs from a side with lower concentration to a side with higher concentration. Cells have a certain internal pressure. This pressure is also called turgor. The inflow progresses until the turgor inside the cells reaches the same level as the osmotic pressure. The pressure existing inside and the pressure acting from the outside are thus equal to each other at the end of the inflow. The osmotic pressure can be measured and calculated. In principle, the same laws of physics apply in dilute liquid solutions as in ideal gases. For this reason, the osmotic pressure is always proportional to the respective absolute temperature. In addition, there is a proportionality between the molar concentration of the respective dissolved substance and the level of the osmotic pressure. The pressure thus depends primarily on the number of molecular particles of the solute. In a solution of one mole of substance in 22.4 liters of solvent, the osmotic pressure at temperatures of 0 degrees Celsius or 273.15 Kelvin is 101.325 kPa. Van ‘t Hoff’s law gives these relationships. However, the law applies exclusively to dilute solutions below a value of 0.1 M.
The analogy to the laws of ideal gases can be understood as follows: the osmotic pressure counteracts the inflow of solvents in each case. For this reason, the inflow of solvent stops as soon as equilibrium is reached. Osmometers can be used to determine the osmotic pressure of a solution. Either the pressure is measured statically, after equilibrium has been reached, or dynamically. In dynamic measurement, external pressure must be applied to the riser manometer to interrupt the osmotic flow. By measuring the pressure, the average molecular mass of the macromolecules can also be determined.
Diseases and ailments
Diseases related to osmotic pressure can affect the blood cells, for example. Red blood cells have osmotic resistance. In various diseases, this osmotic resistance of red blood cells is decreased. Just as many diseases are accompanied by an increase in osmotic resistance. To detect such diseases, red blood cell osmotic resistance is measured. The measurement primarily enables the diagnosis of resistance-reducing diseases. These diseases include, for example, spherocyte anemia. However, other hemolytic anemias can also reduce the osmotic resistance of red blood cells.Hemolytic anemias are a group of diseases associated with anemia due to increased or premature decay of erythrocytes. This circumstance is referred to in medicine as hemolysis. Hemolyses are often accompanied by underlying diseases. They can be caused by mechanical processes or genetic disposition. In addition to physiological hemolysis due to red cell age, mechanical overstress such as a heart valve replacement, thermal damage due to heating, and osmotic damage may determine decay. In case of osmotic damage, hyper- or hypoosmolar solutions are the actual cause of decay. To measure osmotic resistance, a patient’s red blood cells are placed in tubes of increasing salt concentration. One of the tubes contains approximately pure water. One contains a salt concentration that is optimal for red blood cells. After 24 hours, the blood cells in the pure water burst. In tubes with higher salt concentration, usually only a few of the blood cells burst. If the patient has a disease with reduced osmotic resistance of the blood cells, the corpuscles will burst even in higher salt concentrations and will not be able to resist the osmotic pressure. Osmotic resistance may also be increased. An increase in resistance is nonspecific and may be the result of various diseases. Examples of diseases with osmotic resistance increase in red blood cells include thalassemia, iron deficiency anemia, and sickle cell anemia. In addition, jaundice and liver damage can increase resistance.
