The microvasculature as a dynamic regulator of volume and solute exchange.

1. It has long been held that the microvessels are passive pipes through which the formed elements of blood and plasma course and from which exchange occurs. Physiological and structural data from vascular cells in culture to whole organs strongly support the notion that microvessels, whether in their arrangement as networks, in their microvascular phenotypic subtypes or with regard to their permeability properties, are anything but static. 2. Examples from different models, organs and species are presented to illustrate the spatial and dynamic nature of the microvasculature. 3. Given that the ultimate goal of this cardiovascular research is to apply what is learned from component parts of the microvasculature to understand integrated tissue function, examples of the danger of generalizing results from limited studies are presented. 4. Finally, accurate interpretation of most of the research in this area is predicated on the assumption that probe molecules, rendered visible by the binding of dyes, are biologically inert. 5. We present data demonstrating that some dyes used widely in microvascular studies can alter the barrier under study and/or alter the physicochemical characteristics of the molecule to which they were bound.