Pulmonary endothelium: a dynamic interface.

Appreciation of the cell biology of endothelial cells has contributed greatly to understanding of endothelium as a tissue. Endothelium can no longer be considered simply as an inert barrier with fixed permeabilities, nor as an unreactive expanse of non-thrombogenic surface. Pulmonary endothelium is now recognized to be a tissue composed of metabolically active, functionally responsive cells, that interact with circulating substrates and formed elements in ways that regulate the composition of systemic arterial blood, affect target organ functions, and contribute to thrombosis, hemostasis and immune reactions. Much impetus for the new appreciation of endothelial cell structures and functions has been derived from the ability to isolate and culture pulmonary endothelial cells. Improvements in culture conditions and the growing awareness that biochemical, ultrastructural, as well as physical (fluid mechanical), considerations must be taken into account should pave the way for greater understanding of the role of the endothelium in health and disease.