Page 391 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Since it is normally not possible to separately plasma protein concentration and thus the on-
determine k and A of a biological membrane or cotic pressure difference π will rise (! pp.
cell layer, the product of the two (k · A) is often 152, 208). This rise is much higher than equa-
calculated as the ultrafiltration coefficient K f tion 13.3 leads one to expect (! A). The differ-
–1
3 –1
(m s Pa ) (cf. p. 152). ence is attributable to specific biophysical
The transport of osmotically active particles properties of plasma proteins. If there is a pres-
causes water flow. Inversely, flowing water sure-dependent efflux or influx of water out of
drags dissolved particles along with it. This or into the bloodstream, these relatively high
type of solvent drag (! p. 24) is a form of con- changes in oncotic pressure difference auto-
vective transport. matically exert a counterpressure that limits
Solvent drag does not occur if the cell wall is the flow of water.
impermeable to the substance in question (σ =
1). Instead, the water will be retained on the pH, pK, Buffers
side where the substance is located. In the case The pH indicates the hydrogen ion [H ] con-
+
of the aforementioned epithelia, this means centration of a solution. According to Sörensen,
that the substances that cannot be reabsorbed the pH is the negative common logarithm of
from the tubule or intestinal lumen lead to the molal H concentration in mol/kg H 2O.
+
osmotic diuresis (! p. 172) and diarrhea re-
Examples:
Appendix spectively. The latter is the mechanism of ac- 1 mol/kg H 2O = 10 mol/kg H 2O = pH 0,
0
tion of saline laxatives (! p. 262).
–1
0.1 mol/kg H 2O = 10 mol/kg H 2O = pH 1,
and so on up to 10
mol/kg H 2O = pH 14.
-14
13 Oncotic Pressure / Colloid Osmotic Pressure Since glass electrodes are normally used to measure
As all other particles dissolved in plasma, mac- the pH, the H activity of the solution is actually
+
romolecular proteins also exert an osmotic being determined. Thus, the following rule applies:
pressure referred to as oncotic pressure or col- pH = – log (f H · [H ]),
+
loid osmotic pressure. Considering its contribu- where f H is the activity coefficient of H . Considering
+
tion of only 3.5 kPa (25 mm Hg) relative to the its ionic concentration (see above), the f H of plasma is
total osmotic pressure of the small molecular ! 0.8.
components of plasma, the oncotic pressure The logarithmic nature of pH must be con-
on a strictly semipermeable membrane could sidered when observing pH changes. For ex-
be defined as negligible. However, within the ample, a rise in pH from 7.4 (40 nmol/kg H 2O)
body, oncotic pressure is so important because to pH 7.7 decreases the H activity by 20 nmol/
+
the endothelium that lines the blood vessels kg H 2O, whereas an equivalent decrease (e.g.,
allows small molecules to pass relatively easily from pH 7.4 to pH 7.1) increases the H activity
+
(σ ! 0). According to equation 13.3, their by 40 nmol/kg H 2O.
osmotic pressure difference ∆π at the en- The pK is fundamentally similar to the pH. It
dothelium is virtually zero. Consequently, only is the negative common logarithm of the disso-
the oncotic pressure difference of proteins is ciation constant of an acid (K a) or of a base (K b):
effective, as the endothelium is either partly or pK a = $log K a
completely impermeable to them, depending pK b = $log K b.
on the capillary segment in question. Because For an acid and its corresponding base,
the protein reflection coefficient σ # 0 and the pK a + pK b = 14, so that the value of pK a can be
protein content of the plasma (ca. 75 g/L) are derived from that of pK b and vice versa.
higher than that of the interstitium, these two The law of mass actions applies when a
factors counteract filtration, i.e., the blood weak acid (AH) dissociates:
pressure-driven outflow of plasma water from – +
the endothelial lumen, making the en- AH A + H [13.5]
dothelium an effective volume barrier be- It states that the product of the molal concen-
tween the plasma space and the interstitium. tration (indicated by square brackets) of the
If the blood pressure drives water out of the dissociation products divided by the concen-
378
blood into the interstitium (filtration), the tration of the nondissociated substance re-
mains constant:
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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