Fig. 4.
The stress-bearing elements of acinar airspaces. In a previous study (Knudsen et al. 2018), healthy rat lungs were fixed in vivo at airway opening pressure (Pao) of 1 (a) and 10 cm H2O (b). At low pressure, the alveolar ducts are narrow and the inter-alveolar septal walls are characterized by foldings and pleats. The septal walls protrude into the alveolar duct and are connected to the duct via the alveolar entrance. By drawing a straight line between the edges of the septal walls, alveolar and ductal airspaces were separated from each other (fine dashed lines). The axial network of elastic and collagen fibers is concentrated at the edges of alveolar septa and coils the alveolar duct. Here, this system is illustrated as springs spanning the alveolar duct (red springs). At low Pao (or lung volume), the elastic fibers are only slightly stretched (a, b). The fibers exert pulling forces on the alveolar edges/entrance rings in the direction of the ductal lumen (red arrows in a, c) and counteract the surface tension forces (green arrows in a, c) which would pull the septal wall away from the duct and result in a piling up and finally collapse of airspaces. At Pao = 10 cm H2O the alveolar duct is widened, the axial fiber system stretched (red springs in c and d). The forces which are responsible for inflation of the lung are related to the pressure gradient between the pleural space (PPl) and the alveolar space (Palv). The outward forces (FO) are transmitted to the fiber system in the septal walls and correspond here to the inward forces (Fi). Depiction is based on models of Wilson and Bachofen (1982) and Mead et al. (1970). Scale bar 100 µm