Lung parenchymal mechanics in health and disease

Abstract

The mechanical properties of lung tissue are important determinants of lung physiological functions. The connective tissue is composed mainly of cells and extracellular matrix, where collagen and elastic fibers are the main determinants of lung tissue mechanical properties. These fibers have essentially different elastic properties, form a continuous network along the lungs, and are responsible for passive expiration. In the last decade, many studies analyzed the relationship between tissue composition, microstructure, and macrophysiology, showing that the lung physiological behavior reflects both the mechanical properties of tissue individual components and its complex structural organization. Different lung pathologies such as acute respiratory distress syndrome, fibrosis, inflammation, and emphysema can affect the extracellular matrix. This review focuses on the mechanical properties of lung tissue and how the stress-bearing elements of lung parenchyma can influence its behavior.

FIG. 1.

Linear unicompartmental model (resistive-elastic model). A: anatomical representation: tube with resistance (R) and ballon with elastance (E). B: mechanical representation characterized by a dashpot with Newtonian resistance (R) in parallel with a spring with elastance (E) submitted to the same deformation volume (V).

FIG. 2.

Viscoelastic materials, such as soft biological tissues, when held at a constant deformation (strain) show a progressive decrease in stress, called stress relaxation (A), and when held at a constant stress they show a progressive increase in deformation (strain), i.e., creep (B).

FIG. 3.

Viscoelastic model. A: anatomical representation with two resistive components (R1 and R2) and two elastic components (E1 and E2). B: mechanical representation characterized by a resistive component (R1) in parallel with a Kelvin body, which consists of an elastic component (E1), representing the static elastance, in parallel with a Maxwell body, a dashpot assembled in series with a spring, representing the viscoelastic behavior. The distance between the two horizontal bars is analog to lung volume (V), and the tension between them represents the airway opening pressure (P).

FIG. 4.

Schematic representation of the shear stress mechanism. A thin-walled elastic tube (airway or blood vessel) is embedded in spring-network material, whereas pressures P₀, P₁, and P₂ act on the walls of the tube. P₁ is uniform throughout the spring network. In the initial pretensed state, P₁ and P₂ are assumed to be equal and greater than P₀, thus rendering uniform the initial state of the material. If the pressures in the three regions change slightly, a nonuniform deformation can occur, and the mechanics of the thin-walled tube are affected by the surrounding material. [Adapted from Wilson (168).]