The relationship between pressure (P) and volume (V) in the human lung has been extensively studied. However, the combined effects of gravity and the mechanical properties of elastin and collagen on alveolar and lung P–V curves during breathing are not well understood. Here, we extended a previously established thick-walled spherical model of a single alveolus with wavy collagen fibers during positive pressure inflation. First, we updated the model for negative pressure-driven inflation that allowed incorporation of a gravity-induced pleural pressure gradient to predict how the static alveolar P–V relations vary spatially throughout an upright human lung. Second, by introducing dynamic surface tension and collagen viscoelasticity, we computed the hysteresis loop of the lung P–V curve. The model was tested by comparing its predicted regional ventilation to literature data, which offered insight into the effects of microgravity on ventilation. The model has also produced novel testable predictions for future experiments about the variation of mechanical stresses in the septal walls and the contribution of collagen and elastin fibers to the P–V curve and throughout the lung. The model may help us better understand how mechanical stresses arising from breathing and pleural pressure variations affect regional cellular mechanotransduction in the lung.

Real-time, cellular resolution imaging is essential for probing the highly dynamic functions of the lung at the interface of physics, biology, and immunology. CT and MRI modalities have low spatial resolution, and histological approaches provide only snapshots of fixed lungs with little temporal information. Existing intravital imaging approaches cannot include and manipulate the physical and spatiotemporal changes involved in respiratory function. Here, we describe the development of a platform, termed "LungEx", to visualize and mechanistically probe the dynamics of a functioning lung at optical resolutions. LungEx is equipped with a novel transparent ribcage, termed "crystal" ribcage, that provides physiological conditions for a functioning lung and allows high-resolution and real-time optical imaging of nearly the entire lung surface. This imaging capability is obtained while LungEx preserves the complex 3-D architecture, cellular diversity, and integrative function of the ex vivo ventilated and perfused lung at near in vivo conditions. Utilizing LungEx in health and key lung diseases such as metastasis, pneumonia, and fibrosis, we probed a wide range of lung dynamic functions and remodeling at multiple spatial scales including alveolar deformation and elasticity, circulation-respiration coupling at the capillary level, cellular deformation, immune cell motility, and vascular transport. By modulating the biophysical environment of LungEx, we discovered that intravascular and interstitial, but not intra-alveolar neutrophils are highly and reversibly responsive to vascular pressure. Thus, by allowing high spatiotemporal imaging coupled with controlled biophysical interventions, LungEx, provides a powerful platform to mechanistically probe dynamic and multiscale lung functions in health and disease.