Cell membranes are selectively permeable. This means that they allow the movement of some molecules freely across them, but do not allow the free passage of others. In broad terms, there are three ways in which molecules move across membranes. These processes are diffusion, osmosis and active transport.
In this article, we will discuss osmosis, and consider the clinical relevance of this process.
Process of Osmosis
Osmosis is the process by which water molecules pass through a semipermeable membrane, from a less concentrated solution into a more concentrated one.
This may sound counter-intuitive, but the water molecules are still moving down a concentration gradient; from an area of high water concentration (a dilute solution) to an area of lower water concentration (a concentrated solution). Water molecules, like urea, are small and uncharged, thus travel via simple diffusion.
Red blood cells are a key example of the importance of osmosis in the body. In a hypotonic environment, where there are lots of water molecules outside the cells relative to the concentration of solute, water moves into red blood cells. This causes cell swelling and in severe cases, the cell membrane can rupture.
In contrast, in a hypertonic environment, water exits the red cells which causes them to shrivel. Thus, the maintenance of an isotonic environment in the blood is vital for preserving healthy red blood cells.
Fig 1 – Osmosis
Clinical Relevance – Central Pontine Myelinolysis
In chronic hyponatraemia, the level of sodium in the blood is low relative to normal. As sodium is present in a greater concentration extracellularly than intracellularly, when sodium levels are reduced the extracellular space becomes relatively hypotonic.
To create balance between the intracellular and extracellular space, over time the cells of the central nervous system respond by reducing the number of osmotically active molecules they produce inside the cell. Examples of these include glutamine and inositol. By reducing the number of osmotically active molecules intracellularly, the effect of hyponatraemia on osmolarity is accounted for.
If this hyponatraemia is corrected too quickly, the extracellular space becomes rapidly hypertonic compared to the inside of these brain cells, which have deliberately reduced their intracellular osmolarity.
Therefore, water rapidly exits the cells of the central nervous system, causing severe damage to myelin. This can lead to paralysis, difficulty swallowing, seizures and even death.