Page 390 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

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However, measurements with osmometers as H 2O) –1 = mOsm · (L H 2O) . If two solutions of
well as the biophysical application of osmotic different osmolality (∆c osm) are separated by a
concentration refer to the number of osmoles water-permeable selective membrane , ∆c osm
per unit volume of solvent as opposed to the will exert an osmotic pressure difference (∆π)
total volume of the solution. This and the fact across the membrane in steady state if the
that volume is temperature-dependent are the membrane is less permeable to the solutes
reasons why osmolality (Osm/kg H 2O) is gen- than to water. In this case, the selectivity of the
erally more suitable. membrane, or its relative impermeability to
Ideal osmolality is derived from the molality the solutes, is described by the reflection
of the substances in question. If, for example, coefficient (σ), which is assigned a value be-
1 mmol (180 mg) of glucose is dissolved in 1 kg tween 1 (impermeable) and 0 (as permeable as
of water (1 L at 4!C), the molality equals water). The reflection coefficient of a semi-
1 mmol/kg H 2O and the ideal osmolality permeable membrane is σ = 1. By combining
equals 1 mOsm/kg H 2O. This relationship van’t Hoff’s and Staverman’s equations, the
changes when electrolytes that dissociate are osmotic pressure difference (∆π) can be calcu-
+
–
used, e.g., NaCl Na + Cl . Both of these ions lated as follows: and Units
are osmotically active. When a substance that ∆π = σ · R · T · ∆c osm. [13.3]
dissociates is dissolved in 1 kg of water, the
ideal osmolality equals the molality times the Equation 13.3 shows that a solution with the
number of dissociation products, e.g., 1 mmol same osmolality as plasma will exert the same
NaCl/kg H 2O = 2 mOsm/kg H 2O. osmotic pressure on a membrane in steady Dimensions
Electrolytes weaker than NaCl do not disso- state (i.e., that the solution and plasma will be
ciate completely. Therefore, their degree of isotonic) only if σ = 1. In other words, the mem-
electrolytic dissociation must be considered. brane must be strictly semipermeable.
These rules apply only to ideal solutions, i.e., Isotonicity, or equality of osmotic pressure,
those that are extremely dilute. As mentioned exists between plasma and the cytosol of red
above, bodily fluids are nonideal (or real) solu- blood cells (and other cells of the body) in
tions because their real osmolality is lower steady state. When the red cells are mixed in a
than the ideal osmolality. The real osmolality urea solution with an osmolality of
is calculated by multiplying the ideal osmolal- 290 mOsm/kg H 2O, isotonicity does not pre-
ity by the osmotic coefficient (g). The osmotic vail after urea (σ " 1) starts to diffuse into the
coefficient is concentration-dependent and red cells. The interior of the red blood cells
amounts to, for example, approximately 0.926 therefore becomes hypertonic, and water is
for NaCl with an (ideal) osmolality of drawn inside the cell due to osmosis (! p. 24).
300 mOsm/kg H 2O. The real osmolality of this As a result, the erythrocytes continuously
NaCl solution thus amounts to 0.926 · 300 = swell until they burst.
278 mOsm/kg H 2O. An osmotic gradient resulting in the sub-
Solutions with a real osmolality equal to sequent flow of water therefore occurs in all
that of plasma (! 290 mOsm/kg H2 O) are said parts of the body in which dissolved particles
to be isosmolal. Those whose osmolality is pass through water-permeable cell mem-
higher or lower than that of plasma are hyper- branes or cell layers. This occurs, for example,
osmolal or hyposmolal. when Na and Cl pass through the epithelium
+
–
of the small intestine or proximal renal tubule.
Osmolality and Tonicity The extent of this water flow or volume flow Jv
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3
Each osmotically active particle in solution (cf. (m · s ) is dependent on the hydraulic conduc-
–1
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real osmolality) exerts an osmotic pressure (π) tivity k (m · s · Pa ) of the membrane (i.e., its
as described by van’t Hoff’s equation: permeability to water), the area A of passage
(m ), and the pressure difference, which, in
2
this case, is equivalent to the osmotic pressure
π = R · T · c osm [13.2]
where R is the universal gas constant (8.314 J · difference ∆π (Pa): 377
–1
–1
K · Osm ), T is the absolute temperature in K, Jv = k · A · ∆π [m · s ]. [13.4]
–1
3
and c osm is the real osmolality in Osm · (m 3
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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