Functional implications of the pulmonary microcirculation. An update.

The microscopic anatomy of the pulmonary circulation was reviewed, comparing the evidence for two competing models, the sheet-and-post paradigm and the tubular paradigm. Implications of the two paradigms were analyzed for function, including flow, recruitment, distension, and diffusion. We conclude that the pulmonary microcirculation is not essentially different from the systemic microcirculation except that two layers of tubular capillaries are arranged on a central layer of connective tissue, the alveolar wall. We find no morphologic basis or theoretic advantage for the sheet-and-post concept.