Question

what is the relationship of osmolality and tonicity as they relate to understanding aqueous environment in living system.

Answer 1

Osmolality is the concentration of osmoles (osmotically active
particles) per kilogram of solvent (water).

Take for example, 0.9% NaCl. The molar mass of NaCl is 58.5 g. A 0.9%
NaCl consists of 0.9 g of NaCl per liter of the solution.
Since 1 mol weighs 58.5 g, 0.9 g of NaCl is equivalent to 145
millimoles of NaCl per liter of the solution. However, in physiologic
solutions, NaCl is completely ionized (into Na and Cl ions) and the
total concentration of osmotically active particles will thus be 2 x
145 = 290 mOsm/liter of the solution. This solution has the same
osmolality as that of normal human plasma, which is normally between
285 and 295 mOsm/Kg H2O. In other words, a 0.9% NaCl is iso-osmotic
with normal human plasma (or simply an iso-osmotic solution).

Take for instance a 5% dextrose solution (D-glucose, molar mass 180 g,
5% = 5g/dL = 50 g/L approximately 280 mOsm/L of the solution); i.e.,
its osmolality is nearly the same as that of normal human plasma. This
is another iso-osmotic solution. However, if you administer 500 ml of
a 5% dextrose solution intravenously, rapid water shifts do not occur
between ECF and ICF. However, over a "period of time", almost all
dextrose (glucose) administered would enter cells and be metabolized.
The net result then is addition of 500 ml of water to ECF, and this
will dilute (or reduce the osmolality of) ECF in the steady state.
Some of this water can enter cells. Thus while 5% dextrose is an iso-
osmotic solution, it is a hypotonic solution. The implication is that
a 5% dextrose solution can be used to replenish ICF volume losses.

Thus, tonicity refers to the effect of a solution on the steady state
osmolality of ECF or plasma.