Elasticity of the Pulmonary Alveolar Microvascular Sheet in the Cat

The pulmonary interalveolar capillary bed of the cat has previously been shown to be consistent with a sheet-like endothelium-lined space bridged by avascular endothelium-covered posts. This was termed the sheet-flow model, and blood flow through that space is termed sheet flow. The initial formulation of the sheet-flow theory and model required that sheet thickness be the independent variable. Sheet thickness ( h ) was measured in vertical and horizontal silicone-elastomer-perfused cat lungs over the transmural (alveolar-capillary) pressure (Δ) range of 6.3 to 27 cm H 2 O. A plot of h vs. Δ for 2753 individual measurements is linear over the measured pressure range. The regression line is described by h = 4.28 + 0.2191 Δ. Sheet thickness of 4.28 µ at zero pressure is an extrapolated value. The value 0.2191 µ/cm H 2 O is the compliance coefficient (α) of the pulmonary capillary bed. Electron microscopic studies demonstrated that the capillary posts have a highly organized internal structure with abundant collagen and an elastin or elastinlike core. Collagen fibers originate from the alveolar-capillary basement membrane, emerge in a herringbone pattern, and sweep toward the center of the post in a helical array around the elastinlike amorphous and fibrillar core. The unusual compliance of the microvascular blood vessels in the lung can be correlated with the architectural organization within the posts.