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 great er than that on the lumen side. The pressure required for a given leve l of folding depends on the elastic properties of the tissue and on th e geometry of the folding. To quantify the elastic properties, uniaxia l tensile stiffness of the tracheal inner wall of nine sheep was measu red in two directions: parallel to the tracheal axis and circumferenti ally. The tissue showed anisotropic behavior, being approximately thre e times stiffer longitudinally than circumferentially. Histological ex amination showed that collagen in the lamina propria was randomly arra nged, whereas there were straight elastin fibers aligned with the trac heal axis. This observation could explain the observed elastic anisotr opy. Mechanical removal of the epithelium had no effect on tensile sti ffness. It was also found that the tissue was under tension in situ. W hen a strip was excised, its length decreased by greater than or equal to 30%. After allowing for the systematic errors inherent in this exp eriment, the in situ circumferential tensile stiffness is estimated to be greater than or equal to 20 kPa. If the equivalent tissue in the b ronchioles has the same tensile stiffness as that in the trachea, the forces required to fold the membrane are significant at small transbro nchial pressure differences and increase in the presence of membrane t hickening such as that seen in asthma.