Summary

Delivery of water and hydrophilic nutrients to tissues may be regulated at the level of the microvasculature, primarily in capillaries and postcapillary venules. Hydraulic conductivity (Lp) is a coefficient that reflects the ease of passage of water across microvascular walls. Data derived from single-perfused microvessels demonstrate that control values of Lp have a broad distribution, with evidence of spatial differences within microvascular networks. Further, Lp is not a fixed coefficient of individual microvessels, but instead is subject to active regulation by both physiologic and pathologic stimuli. The cellular molecular basis for active changes in the endothelial pathways that regulate microvascular Lp is an area of ongoing research.

Glossary

Microvascular hydraulic conductivity (Lp): The ease of passage of water across microvascular walls, reflecting net volume flux (cm3 sec-1) per unit surface area (cm2) per unit pressure (cmH2O).

Protein effect: An effect of plasma proteins that reduces Lp, presumably at the level of the endothelial surface layer. Removal of proteins from microvessel consistently induces significant elevations in Lp.

Reflection coefficient (a): An index of the selectivity of the microvascular barrier to an osmotically active solute; it is a dimensionless coefficient, theoretically ranging from 0 to 1.

References

1. Sill, H. W., Chang, Y. S., Artman, J. R., Frangos, J. A., Hollis, T. M., and Tarbell, J. M. (1995). Shear stress increases hydraulic conductivity of cultured endothelial monolayers. Am. J. Physiol. 268, H535-H543.

A description of a cultured endothelial cell monolayer model to measure endothelial hydraulic conductivity in vitro. The control values of hydraulic conductivity of the endothelial cell monolayers in this model are comparable to those of single-perfused microvessels depicted in Figure. 3.

2. Michel, C. C., and Curry, F. E. (1999). Microvascular permeability Physiol. Rev. 79, 703-761. A comprehensive review of microvascular permeability under physiologic conditions as well as under conditions of hyperpermeability. Includes extensive modeling of the pathways responsible for volume and solute flux across microvessels and a discussion of signal transduction of changes in permeability.

3. Weinbaum, S., Zhang, X., Han, Y., Vink, H., and Cowin, S. C. (2003). Mechanotransduction and flow across the endothelial glycocalyx. Proc. Natl. Acad. Sci. USA 100, 7988-7995. An intriguing model of the role of the endothelial cell glycocalyx as a primary barrier to microvascular volume flux, based on current electron microscopic observations of a complex structure on the endothelial surface.

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Further Reading

Levick, J. R. (1995). Circulation of fluid between plasma, interstitium and lymph. In An Introduction to Cardiovascular Physiology, 2nd ed., pp. 158-187. Butterworth-Heinemann, Oxford. A practical overview of transvascular fluid filtration with an emphasis on fluid across the interstitium and the mechanisms responsible for formation of edema. Includes descriptions of capillary pressure and capillary filtration measurements in humans, as well as a discussion on lymph flow relative to transvascular water movement.

Capsule Biography

Dr. Rumbaut earned an M.D. degree from the Instituto Tecnológico y de Estudios Superiores de Monterrey and a Ph.D. in physiology from the University of Missouri-Columbia. He has been an Assistant Professor of Medicine and Pediatrics at Baylor College of Medicine in Houston, Texas, since 2000. His laboratory focuses on microvascular permeability and microvascular thrombosis; his research is funded by the National Institutes of Health.

Dr. Huxley earned a Ph.D. in biophysics from the University of Virginia and was a postdoctoral fellow at the University of California at Davis. She has been a Professor of Medical Pharmacology and Physiology at the University of Missouri-Columbia since 1994. Her laboratory focuses on regulation of microvascular permeability and gender influences on microvascular physiology; her research is funded by the National Institutes of Health.

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