Pulmonary transport of water and solutes: functional and structural correlations

Assessment of endothelial barrier parameters is based on formulations derived from the thermodynamics of irreversible processes. These formulations contain simplifying assumptions that, with uncertainties concerning the surface area involved in the transport, preclude definitive modeling with respect to the possible pathways. In the development of edema in isolated lung preparations perfused at 37 C, changes of structural features such as increases of vesicle volume density can be associated with changes of barrier parameters such as increased filtration coefficients and increased permeability coefficients to sodium ion. However, that edema can develop at 15 C without increased vesiculation and that cuffing of extraalveolar vessels also occurs in vivo with circulatory overload without increased vesiculation do not suggest that vesicles play a determining role in the development of edema. Available evidence indicates that the exchange of water across the endothelium is at a rate approximating the flow rate of blood and therefore must involve the entire endothelial surface. Separation or noncommunality of pathways for water and solutes makes modeling of the pathways from current formations even less certain.