Biomedical Engineering Reference
In-Depth Information
RT
ρ
S
π
=
RTC
=
S
MW
S
where
ρ
S
is the solute density in the solvent (ration of solute mass to solvent volume
and MWS is the molecular weight of the solute. Equation (2.17) is rearranged to
calculate the molecular weight of the solute. A number of osmometers are avail-
able to measure the osmotic pressures. A simple technique is using membrane
osmometers shown in Figure 2.10. The pure solvent is separated from the solute-
containing solution by a semipermeable membrane, which is tightly held between
the two chambers. At equilibrium, hydrostatic pressure, which corresponds to os-
motic pressure, is calculated using the rise in the liquid level (
h
);
is the solution
density. Sensitivity of the membrane osmometer measurements are affected by the
membrane asymmetry observed when both cells are filled with the same solvent
but have a pressure difference between the two cells caused by membrane leakage,
compression, solute contamination, or temperature gradients. Ballooning caused
by pressure differentials is another problem and is detected by measuring the pres-
sure changes as the solvent is added or removed from the solvent cell. Alternatively,
vapor pressure osmometers are used either alone or in combination with membrane
osmometers. However, a more robust method for determining the osmolarity of
biological fluids is by measuring the depression in the freezing point between the
sample solution and pure water.
ρ
2.5
Transport Across Membranes
Interestitial fluid regulates the shape of the vessel by maintain a constant pressure,
while providing a suitable transport medium for nutrients and waste products be-
tween cells and capillary blood. Porous tissue membranes selectively allow various
molecules to pass through them. Small molecules carried in the plasma pass into the
interstitial fluid to supply nutrients to the cells, but blood cells and platelets are too
large to pass through capillary walls. Similarly, substances (i.e., waste) pass through
Solute-solvent
mixture
h
Solvent
Semipermeable
membrane
Figure 2.10
Membrane osmometer.
