James T. answered • 07/21/20
Patient and Knowledgeable RN, BSN
Hi Daniel!
Osmosis is when water molecules cross a semi permeable membrane, from an area of higher concentration of water to a lower concentration of water. Lets break the definition down.
A semi permeable membrane is a boundary, such as a cell membrane, that allows some molecules to pass through it, but not all of them.
In a given solution with water, there are a certain number of molecules of water, and whatever other molecules are dissolved in the water.
On either side of the semi-permeable membrane, the concentration of water molecules wants to be the same, and this is achieved through osmosis. For example, imagine two compartments with a semi-permeable membrane between them. One compartment contains a solution that is 1% salt, and the other contains a solution that is 5% salt. In order to achieve equilibrium (the same concentration of water on either side of the membrane), water will permeate across the membrane from the 1% concentration side to the 5% concentration side, thus diluting the 5% concentration side until it achieves equilibrium at 1% concentration. The salt will not move, because it cannot permeate the membrane. Thus, dilution of the 5% compartment through osmosis will be the only option to achieve equilibrium.
This is a critical concept for fluid and electrolyte balance in the human body. For example, if someone needs IV fluids to replace blood volume loss from a traumatic accident, we cannot simply give plain old water into an IV. This is because water contains a different concentration of salt (and other molecules) than fluid inside the cells. If, for some reason, we did give plain water, osmosis would occur across the semi-permeable membrane of the body cells. Since body cells contain a higher concentration of salt and other molecules than water does, the water we administered would travel across the cell membrane into the cells to achieve equilibrium. But, since there is only so much space inside the cell to hold incoming water, in this example, water would travel into the cells and they would burst like a water balloon...that would be a bad day for that patient! Thus, we must administer IV fluids at the same concentration of salt as the fluid inside the cells, so no water shifting will occur.
A simple way to remember the concept of osmosis is "water moves towards the salty pretzel"; in other words, water will transfer from one side of a semi-permeable membrane with less salt (or other molecules) to the side with more salt (or other molecules), thus the water moves "towards the salty pretzel".
Hope this helps!
