Origin of the Term “Isotonic”
Authors: Kuang Serena Y. , Walter Stefan , Yang Xiaoqi , Li Xiaonan 
Published: May 2022

IntroductionIn our 1st and 2nd abstracts to EB2022, we addressed the inconsistency in the definitions of tonicity1 and the problems of introducing both osmolarity and effective osmolarity2 and resolved these issues. In this abstract, we introduce the origin of the term “isotonic”, which further clarifies the confusion about tonicity and leads to an understanding of the membrane (m)‐dependency of tonicity, one of the 3 properties of tonicity3.MethodLogical reasoning.Results1) The term “isotonic” was coined by the botanist Hugo de Vries in the 19th century4. In his experimental setup, if S1 caused a certain degree of shrinkage of a cell and S2 caused the same degree of shrinkage of the same cell, then S1 and S2 were considered “isotonic solutions” because they have an equal “water attracting force”. We now know that it is the impermeant solute particles (imp‐SP) in S1 and S2 that pull water to their compartments in the osmosis systems S1‐m‐S3 and S2‐m‐S3, respectively, where S3 refers to the intracellular fluid (ICF) of the cell. That time, De Vries seemed not to be aware that when his S1 is isotonic to S2, S1 and S2 are both hypertonic to S3, or vice versa, S3 is hypotonic to S1 and S2.2) The term “x‐tonic (i.e., hypertonic or isotonic or hypotonic)” compares the 2 water attracting forces in an osmosis system, such as S1‐m‐S3 or S2‐m‐S3. Obviously, it is the imp‐SP that exert this “water attracting force” (more appropriately this “pressure”). If we call this pressure “the osmotic tone” (i.e., the osmotic energy or strength), then the meaning of the term “x‐tonic” is clear: It compares the 2 osmotic tones in an osmosis system such as S1‐m‐S2. Lack of an understanding of this point leads to the following illogical descriptions about tonicity: a common saying “S1 is isotonic to the cell” compares an apple with an orange and another common saying “S1 is isotonic” is not a complete sentence. These illogical statements result from the inconsistency in the definitions of tonicitywe addressed previously1 and increase the level of confusion about what tonicity is, which we have addressed previously1, 3.3) The logic that if a cell swells/shrinks in a solution, the solution is hypotonic/hypertonic to ICF is commonly reflected in the literature, but this reverses the cause and effect: It is because the ICF is hypertonic/hypotonic to the solution, so that the ICF wins/loses the water competing game (i.e., osmosis) across the cell membrane, or in other words, the winner (ICF) gains water and volume (cell swelling)/the loser (ICF) loses water and volume (cell shrinkage).ConclusionsThis abstract and our previous works listed in the References section together eliminate all problems related to the concept of tonicity.References1. Kuang et al. “A Resolution for the Inconsistency in the Definitions of Tonicity” [Abstract], submitted to EB20222. Kuang et al. “Resolutions to the Problems Caused by Introducing both Osmolarity and Effective Osmolarity” [Abstract], submitted to EB20223. Kuang et al., The Concept of Tonicity: Problems and Resolutions. The FASEB Journal, Vol 34(S1)4. Hamburger HJ. Twenty‐five years of osmotic pressure in the medical sciences. Science 34: 583‐589, 1911.