Effect of surface tension on alveolar surface area

Buckling forces and the wavy folds between pleural epithelial cells

Cell shapes in tissues are affected by the biophysical interaction between cells. Tissue forces can influence specific cell features such as cell geometry and cell surface area. Here, we examined the 2-dimensional shape, size, and perimeter of pleural epithelial cells at various lung volumes. We demonstrated a 1.53-fold increase in 2-dimensional cell surface area and a 1.43-fold increase in cell perimeter at total lung capacity compared to residual lung volume. Consistent with previous results, close inspection of the pleura demonstrated wavy folds between pleural epithelial cells at all lung volumes. To investigate a potential explanation for the wavy folds, we developed a physical simulacrum suggested by D'Arcy Thompson in On Growth and Form. The simulacrum suggested that the wavy folds were the result of redundant cell membranes unable to contract. To test this hypothesis, we developed a numerical simulation to evaluate the impact of an increase in 2-dimensional cell surface area and cell perimeter on the shape of the cell-cell interface. Our simulation demonstrated that an increase in cell perimeter, rather than an increase in 2-dimensional cell surface area, had the most direct impact on the presence of wavy folds. We conclude that wavy folds between pleural epithelial cells reflects buckling forces arising from the excess cell perimeter necessary to accommodate visceral organ expansion.

Physiological fluid interfaces: Functional microenvironments, drug delivery targets, and first line of defense

Fluid interfaces, i.e. the boundary layer of two liquids or a liquid and a gas, play a vital role in physiological processes as diverse as visual perception, oral health and taste, lipid metabolism, and pulmonary breathing. These fluid interfaces exhibit a complex composition, structure, and rheology tailored to their individual physiological functions. Advances in interfacial thin film techniques have facilitated the analysis of such complex interfaces under physiologically relevant conditions. This allowed new insights on the origin of their physiological functionality, how deviations may cause disease, and has revealed new therapy strategies. Furthermore, the interactions of physiological fluid interfaces with exogenous substances is crucial for understanding certain disorders and exploiting drug delivery routes to or across fluid interfaces. Here, we provide an overview on fluid interfaces with physiological relevance, namely tear films, interfacial aspects of saliva, lipid droplet digestion and storage in the cell, and the functioning of lung surfactant. We elucidate their structure-function relationship, discuss diseases associated with interfacial composition, and describe therapies and drug delivery approaches targeted at fluid interfaces. STATEMENT OF SIGNIFICANCE: Fluid interfaces are inherent to all living organisms and play a vital role in various physiological processes. Examples are the eye tear film, saliva, lipid digestion & storage in cells, and pulmonary breathing. These fluid interfaces exhibit complex interfacial compositions and structures to meet their specific physiological function. We provide an overview on physiological fluid interfaces with a focus on interfacial phenomena. We elucidate their structure-function relationship, discuss diseases associated with interfacial composition, and describe novel therapies and drug delivery approaches targeted at fluid interfaces. This sets the scene for ocular, oral, or pulmonary surface engineering and drug delivery approaches.

What do we know about mechanical strain in lung alveoli?

The pulmonary alveolus, terminal gas-exchange unit of the lung, is composed of alveolar epithelial and endothelial cells separated by a thin basement membrane and interstitial space. These cells participate in the maintenance of a delicate system regulated not only by biological factors but also by the mechanical environment of the lung, which undergoes dynamic deformation during breathing. Clinical and animal studies as well as cell culture studies point toward a strong influence of mechanical forces on lung cells and tissues including effects on growth and repair, surfactant release, injury, and inflammation. However, despite substantial advances in our understanding of lung mechanics over the last half century, there are still many unanswered questions regarding the micromechanics of the alveolus and how it deforms during lung inflation. Therefore, the aims of this review are to draw a multidisciplinary account of the mechanics of the alveolus on the basis of its structure, biology, and chemistry and to compare estimates of alveolar deformation from previous studies.

The effect of tissue elastic properties and surfactant on alveolar stability

This paper presents a novel mathematical model of alveoli, which simulates the effects of tissue elasticity and surfactant on the stability of human alveoli. The model incorporates a spherical approximation to the alveolar geometry, the hysteretic behavior of pulmonary surfactant and tissue elasticity. The model shows that the alveolus without surfactant and the elastic properties of the lung tissue are always at an unstable equilibrium, with the capability both to collapse irreversibly and to open with infinite volume when the alveolus has small opening radii. During normal tidal breathing, the alveolus can becomes stable, if surfactant is added. Including the passive effect of tissue elasticity stabilizes the alveolus, further allowing the alveoli to be stable, even for lung volumes below residual volume. The model is the first to describe the combined effects of tissue elasticity and surfactant on alveolar stability. The model may be used as an integrated part of a more comprehensive model of the respiratory system, since it can predict opening pressures of alveoli.

Mechanics of airway and alveolar collapse in human breath-hold diving

A computational model of the human respiratory tract was developed to study airway and alveolar compression and re-expansion during deep breath-hold dives. The model incorporates the chest wall, supraglottic airway, trachea, branched airway tree, and elastic alveoli assigned time-dependent surfactant properties. Total lung collapse with degassing of all alveoli is predicted to occur around 235 m, much deeper than estimates for aquatic mammals. Hysteresis of the pressure-volume loop increases with maximum diving depth due to progressive alveolar collapse. Reopening of alveoli occurs stochastically as airway pressure overcomes adhesive and compressive forces on ascent. Surface area for gas exchange vanishes at collapse depth, implying that the risk of decompression sickness should reach a plateau beyond this depth. Pulmonary capillary transmural stresses cannot increase after local alveolar collapse. Consolidation of lung parenchyma might provide protection from capillary injury or leakage caused by vascular engorgement due to outward chest wall recoil at extreme depths.

Increased surface tension decreases pulmonary capillary volume and compliance

Increased surface tension is an important component of several respiratory diseases, but its effects on pulmonary capillary mechanics are incompletely understood. We measured capillary volume and specific compliance before and after increasing surface tension with nebulized siloxane in excised dog lungs. The change in surface tension was sufficient to increase lung recoil 5 cm H(2)O at 50% total lung capacity. Increased surface tension decreased both capillary volume and specific compliance. The changes in capillary volume and compliance were greatest at the lung volumes at which the surface tension change was greatest. Near functional residual capacity, capillary volume postsiloxane was approximately 30% of control. Presiloxane capillary specific compliance was approximately 7%/cm H(2)O near functional residual capacity and approximately 2.5%/cm H(2)O near total lung capacity. Postsiloxane capillary-specific compliance was 3%/cm H(2)O, and was independent of lung volume. We conclude that in addition to their well-known effects on lung mechanics, changes in surface tension also have important effects on capillary mechanics. We speculate that these changes may in turn affect ventilation and perfusion, worsen gas exchange, and alter leukocyte sequestration.