Page 389 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

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Concentrations, Fractions, Activity tive electrodes (e.g., for H , Na , K , Cl , or Ca ). The
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The word concentration is used to describe activity and molality of a solution are identical when
many different relationships in physiology and the total ionic strength (µ) of the solution is very
small, e.g., when the solution is an ideal solution. The
medicine. Concentration of a substance X is ionic strength is dependent on the charge and con-
often abbreviated as [X]. Some concentrations centration of all ions in the solution:
are listed below: 2 2 2
—Mass concentration, or the mass of a sub- µ " 0.5 (z 1 ! c 1 + z 2 ! c 2 + ... + z i ! c i) [13.1]
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stance per unit volume (e.g., g/L = kg/m ) where z i is the valency and c i the molal concentration
—Molar concentration, or the amount of a sub- of a given ion “i”, and 1, 2, etc. represent the different
stance per unit volume (e.g., mol/L) types of ions in the solution. Owing to the high ionic
—Molal concentration, or the amount of sub- strength of biological fluids, the solute particles in-
stance per unit mass of solvent (e.g., mol/ fluence each other. Consequently, the activity (a) of a
solution is always significantly lower than its molar
kg H 2O). concentration (c). Activity is calculated as a = f · c,
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The SI unit of mass concentration is g/L (kg/m , where f is the activity coefficient.
Example: At an ionic strength of 0.1 (as it is the
mg/L, etc.). The conversion factors for older case for a solution containing 100 mmol NaCl/
units are listed below: kg H 2O), f = 0.76 for Na . The activity important in
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1 g/100 mL = 10 g/L biophysical processes is therefore roughly 25% lower
Appendix 1 g% = 10 g/L than the molality of the solution.
1 % (w/v) = 10 g/L
In solutions that contain weak electrolytes
1 g‰ = 1 g/L
ity and activity of free ions also depend on the
13 1 mg% = 10 mg/L which do not completely dissociate, the molal-
1 mg/100 mL = 10 mg/L
1 µg% = 10 µg/L degree of electrolytic dissociation.
1 γ% = 10 µg/L. Fractions (“fractional concentrations”) are
Molarity is the molar concentration, which relative units:
is expressed in mol/L (or mol/m , mmol/L, — Mass ratio, i.e., mass fraction relative to total
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etc.). Conversion factors are listed below: mass
1 M (molar) = 1 mol/L — Molar ratio
1 N (normal) = (1/valency) · mol/L — Volume ratio, i.e. volume fraction relative to
1 mM (mmolar) = 1 mmol/L total volume. The volume fraction (F) is
1 Eq/L = (1/valency) · mol/L. commonly used in respiratory physiology.
In highly diluted solutions, the only difference Fractions are expressed in units of g/g, mol/
between the molar and molal concentrations mol, and L/L respectively, i.e. in “units” of 1,
–3
is that the equation “1 L H 2O = 1 kg H 2O” holds 10 , 10 , etc. The unabbreviated unit (e.g.,
–6
at only one particular temperature (4!C). Bio- g/g) should be used whenever possible be-
logical fluids are not highly diluted solutions. cause it identifies the type of fraction in ques-
The volume of solute particles often makes up tion. The fractions %, ‰, ppm (parts per mil-
a significant fraction of the overall volume of lion), and ppb (parts per billion) are used for all
the solution. One liter of plasma, for example, types of fractions.
contains 70 mL of proteins and salts and only Conversion:
0.93 L of water. In this case, there is a 7% differ- 1% = 0.01
ence between molarity and molality. Differ- 1‰ = 1 · 10 –3
ences higher than 30% can occur in intracellu- 1 vol% = 0.01 L/L
lar fluid. Although molarity is more commonly 1 ppm = 1 · 10 –6
measured (volumetric measurement), molal- 1 ppb = 1 · 10 –9
ity plays a more important role in biophysical
and biological processes and chemical reac- Osmolality, Osmotic / Oncotic Pressure
tions. Osmolarity (Osm/L), a unit derived from
The activity (a) of a solution is a thermodynamic molarity, is the concentration of all osmotically
376 measure of its physicochemical efficacy. In physi- active particles in a solution, regardless of
ology, the activity of ions is measured by ion-sensi- which compounds or mixtures are involved.
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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