Notes on terms associated with solute particle concentration
(NOTE: This discussion replaces the last two paragraphs on page 36 of the
supplementary notes.)
We consider two sets of terms:
Isotonic, hypertonic, and hypotonic
and
Isosmotic (iso-osmotic), hyperosmotic, and hypoosmotic (hypo-osmotic). (There
appears to be no universal agreement on the meaning of these three terms.)
We shall use them as follows:
1) We will use the terms isosmotic, hyperosmotic, and hypo-osmotic in the
comparison of any two solutions. If the solutions have the same osmolarity they are
said to be isosmotic. If they have different osmolarities the solution with the greater
osmolarity is said to be hyperosmotic, and the solution with the lesser osmolarity is
said to be hypo-osmotic. (You will probably not encounter these terms very often.)
**2) We shall use the terms isotonic, hypertonic, and hypotonic when we are
comparing a given solution to a physiological solution, that is, a solution with a
milliosmolarity in the range 280-310 mOsmol/L. The osmolarity of blood and of the
fluid in normal cells falls within this range. (IV solutions are described using these
terms.)
We say that a solution with osmolarity less than that of a physiological solution is
hypotonic. A solution with osmolarity equal to that of a physiological solution is
said to be isotonic, and a solution with osmolarity greater than that of a
physiological solution is said to be hypertonic.
NOTE: The behavior of solutions of differing osmolarity depends on the nature of
the membrane separating them. A truly osmotic membrane allows only water to
pass through. The outer covering of a cell is not an osmotic membrane, but it
approximates one insofar as the passage of water is concerned.