Page 392 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

H 2 O influx +40 Actual
Changes in oncotic pressure of plasma proteins mmHg No
+30
deviation
+20
from
mL/100mL plasma +10 van’t Hoff ’s law
60 40 20
20 40 60
mL/100mL plasma
–10 and Units
H 2 O loss
–20
Dimensions

A. Physiological signification of deviations on oncotic pressure of plasma from van’t
Hoff’s equation. A loss of water from plasma leads to a disproportionate rise in oncotic
pressure, which counteracts the water loss. Conversely, the dilution of plasma due to the
influx of water leads to a disproportionate drop in oncotic pressure, though less pro-
nounced. Both of these are important mechanisms for maintaining a constant blood
volume and preventing edema. (Adapted from Landis EM u. Pappenheimer JR. Handbook
of Physiology. Section 2: Circulation, Vol. II. American Physiological Society: Washington
D.C. 1963, S. 975.)

– + Equation 13.9 is the general form of the
K a = [A ] · [H ] · f H [13.6]
[AH] Henderson–Hasselbalch equation (! p.
138ff.), which describes the relationship be-
Converted into logarithmic form (and insert- tween the pH of a solution and the concentra-
ing H activity for [H ]), the equation is trans- tion ratio of a dissociated to an undissociated
+
+
formed into: –
–
[A ] form of a solute. If [A ] = [AH], then the concen-
+
log K a = log + log ([H ] · f H) [13.7] tration ratio is 1/1 = 1, which corresponds to
[AH] pH = pK a since the log of 1 = 0.
or A weak acid (AH) and its dissociated salt
[A ]
–
–
+
–
+
$log ([H ] · f H) = $ log K a + log [13.8] (A ) form a buffer system for H and OH ions:
[AH] + – +
Based on the above definitions for pH and pK a, Addition of H yields A + H ! AH –
–
Addition of OH yields AH + OH ! A + H 2O.
–
it can also be converted into The buffering power of a buffer system is
–
[A ]
–
pH = pK a + log [13.9] greatest when [AH] = [A ], i.e., when the pH of
[AH]
the solution equals the pK a of the buffer.
Because the concentration and not the activity of A – –
and AH is used here, pK a is concentration-dependent Example: Both [A ] and [AH] = 10 mmol/L and pK a =
+
–
in nonideal solutions. 7.0. After addition of 2 mmol/L of H ions, the [A ]/
[AH] ratio changes from 10/10 to 8/12 since 2 mmol/
L of A are consequently converted into 2 mmol/L of 379
–
AH. Since the log of 8/12 !-0.18, the pH decreases
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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