Microvascular Permeability and Tumor Metastasis - Bioanalysis Advanced Materials Methods and Devices - page 52

Fenestrae . Fenestrae usually exist in fenestrated microvessels (in the kidney)
instead of a continuous microvessel endothelium. In some fenestrated microvessels,
there exists a very thin membrane of ~25 nm thick, diaphragm, which covers the
fenestra. Fenestrated endothelia have higher hydraulic conductivities and are more
permeable to small ions and molecules than are continuous endothelia. However,
their permeability to plasma proteins is about the same [ 57 ].

4.2.1 Transport Coefficients

The abovementioned ultrastructural study using electron microscopy and other
methods shows that the microvessel wall behaves as a passive membrane for
water and hydrophilic solute transport [ 57 ]. The membrane transport properties
are often described by the Kedem-Katchalsky equations derived from the theory of
irreversible thermodynamics,

J s ¼ PRT D C þð 1 s f Þ CJ v (4.1)

J v ¼ L p ðD p s d RT D C Þ (4.2)

where J s and J v are the solute and total volumetric fluxes; p are the

concentration and pressure difference across the membrane: L p, the hydraulic
conductivity, describes the membrane permeability to water, P , the diffusive
permeability, describes the permeability to solutes, s f is the solvent drag or ultrafil-
tration coefficient that describes the retardation of solutes due to membrane restric-
tion, and s d, the reflection coefficient, describes the selectivity of membrane to
solutes. In many transport processes, s f is equal to s d and thus we often use s ,
the reflection coefficient, to represent both of them [ 57 ]. R is the universal gas
constant and T is the absolute temperature.

D C and D

4.2.2 Determination of Microvascular Transport Coefficients

All of the permeability measurements have been interpreted in terms of L p, P and s ,
which are measured experimentally on intact whole organisms (including human
subjects), on perfused tissues and organs, on single perfused microvessels, and on
monolayers of cultured microvascular endothelial cells. Different experimental
preparations have their advantages and disadvantages. Although measurements
made on the intact regional circulation of an animal subject (usually using radioac-
tive isotope labeled tracers) suffer from uncertainties surrounding the exchange
surface area of the microvessel wall and the values of the transvascular differences