Tensile stiffness of ovine tracheal wall

The epithelial folding that occurs during bronchoconstriction requires that the pressure on the muscle side of the folding membrane be greater than that on the lumen side. The pressure required for a given level of folding depends on the elastic properties of the tissue and on the geometry of the folding. To quantify the elastic properties, uniaxial tensile stiffness of the tracheal inner wall of nine sheep was measured in two directions: parallel to the tracheal axis and circumferentially. The tissue showed anisotropic behavior, being approximately three times stiffer longitudinally than circumferentially. Histological examination showed that collagen in the lamina propria was randomly arranged, whereas there were straight elastin fibers aligned with the tracheal axis. This observation could explain the observed elastic anisotropy. Mechanical removal of the epithelium had no effect on tensile stiffness. It was also found that the tissue was under tension in situ. When a strip was excised, its length decreased by > or = 30%. After allowing for the systematic errors inherent in this experiment, the in situ circumferential tensile stiffness is estimated to be > or = 20 kPa. If the equivalent tissue in the bronchioles has the same tensile stiffness as that in the trachea, the forces required to fold the membrane are significant at small transbronchial pressure differences and increase in the presence of membrane thickening such as that seen in asthma.