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. 2021 Mar 11;274:119341. doi: 10.1016/j.lfs.2021.119341

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Fig. 1 — Collagen and elastin fiber network in the lung. I. A. Fluorescence microscopy of thin lung tissue sections stained with Sirius red collagen and green elastin staining concentrated at alveolar septal edges (arrows) (bar = 100 μm) (used with permission from [13]). II. Depiction of forces in the axial, septal and peripheral fiber systems, acting as the stress-bearing elements of the acinar airspaces, (a). Healthy rat lungs fixed at low pressure show narrow alveolar ducts with folded and pleated septal walls of alveoli. Fine dashed lines show the border between the duct and alveolar airspaces, while red and green arrows depict the direction of surface tension force (green) and the counteracting pull of axial system fibers at entrance rings (red). The axial network of collagen and elastin fibers is concentrated at the edges of alveolar septa, coil the ductal airspaces, and form the alveolar entrance rings (represented by the connecting thick gray lines), (c). Healthy rat lungs fixed at higher pressure, showing widening of alveolar ducts and stretching of alveolar septa, with the surface tension force counteracted by the tensile force of axial fiber system in the direction of the ductal lumen, (b, d). Schematic representation of the axial fiber network of collagen and elastin coiling the alveolar duct (depicted by red springs) and stabilizing the alveolar walls with changes in pressure. The axial fiber network is connected to the peripheral fiber system of the pleura (thick black line) through the alveolar septal fibers (green lines), which are found between the basal laminae of the alveolar epithelium and endothelium. Without these three fiber systems, the surface tension would cause the alveoli to collapse. During inflation, pressure gradient due to differences in the pleural pressure (P_Pl) and alveolar pressure (P_alv) induce outward forces (F_O) in the peripheral fiber system, which are transmitted into the fiber system of the septal wall (F_i) and eventually are counteracted by the tension in the axial fiber system of the ductal airspace (used from [11], Creative Commons Attribution 4.0 International License, http://creativecommons.org/licenses/by/4.0/). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)