140 Chapter 6
One mole of solute per liter depresses the freezing point
of water by 2 1.86 8 C. Accordingly, a 1.0 m glucose solution
freezes at a temperature of 2 1.86 8 C, and a 1.0 m NaCl solu-
tion freezes at a temperature of 2 3 ( 2 1.86) 5 2 3.72 8 C
because of ionization. Thus, the freezing-point depression is
a measure of the osmolality. Since plasma freezes at about
2 0.56 8 C, its osmolality is equal to 0.56 4 1.86 5 0.3 Osm,
which is more commonly indicated as 300 milliosmolal (or
300 mOsm).To n i c i t y
A 0.3 m glucose solution, which is 0.3 Osm, or 300 milli-
osmolal (300 mOsm), has the same osmolality and osmotic
pressure as plasma. The same is true of a 0.15 m NaCl
solution, which ionizes to produce a total concentration of
300 mOsm. Both of these solutions are used clinically as
intravenous infusions, labeled 5% dextrose (5 g of glucose
per 100 ml, which is 0.3 m ) and normal saline (0.9 g of NaCl
per 100 ml, which is 0.15 m ). Since 5% dextrose and normal
saline have the same osmolality as plasma, they are said to be
isosmotic to plasma.
The term tonicity is used to describe the effect of a solu-
tion on the osmotic movement of water. For example, if an
isosmotic glucose or saline solution is separated from plasma
by a membrane that is permeable to water, but not to glucose or
NaCl, osmosis will not occur. In this case, the solution is said
to be isotonic (from the Greek isos 5 equal; tonos 5 tension)
to plasma.
Red blood cells placed in an isotonic solution will neither
gain nor lose water. It should be noted that a solution may
be isosmotic but not isotonic; such is the case whenever the
solute in the isosmotic solution can freely penetrate the mem-
brane. A 0.3 m urea solution, for example, is isosmotic but
not isotonic because the cell membrane is permeable to urea.
When red blood cells are placed in a 0.3 m urea solution,
the urea diffuses into the cells until its concentration on both
sides of the cell membranes becomes equal. Meanwhile, the
solutes within the cells that cannot exit—and which are
therefore osmotically active—cause osmosis of water into
the cells. Red blood cells placed in a 0.3 m urea solution will
thus eventually burst.
Solutions that have a lower total concentration of sol-
utes than that of plasma, and therefore a lower osmotic
pressure, are hypo-osmotic to plasma. If the solute is
osmotically active, such solutions are also hypotonic to
plasma. Red blood cells placed in hypotonic solutions gain
water and may burst—a process called hemolysis. When
red blood cells are placed in a hypertonic solution (such
as seawater), which contains osmotically active solutes at
a higher osmolality and osmotic pressure than plasma, they
shrink because of the osmosis of water out of the cells.
This process is called crenation (from the Medieval Latin
crena 5 notch) because the cell surface takes on a scalloped
appearance ( fig. 6.13 ).Figure 6.12 The effect of ionization on the osmotic
pressure. ( a ) If a selectively permeable membrane (permeable to
water but not to glucose, Na^1 , or Cl^2 ) separates a 1.0 m glucose
solution from a 1.0 m NaCl solution, water will move by osmosis
into the NaCl solution. This is because a 1.0 m NaCl solution has
a total solute concentration of 2.0 Osm, since NaCl can ionize to
yield one-molal Na^1 plus one-molal Cl^2. ( b ) After osmosis, the total
concentration, or osmolality, of the two solutions is equal.
H 2 O
H 2 O- 0 m NaCl
2.0 Osm
H 2 OH 2 O- 0 m glucose
1.0 Osm
1.5 Osm1.5 Osm(a)(b)