Structural basis for decreased compressibility of constricted tracheae and bronchi.

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Abstract
To learn how muscular constriction increases the ability of tracheae and bronchi to withstand compressive narrowing, we examined the structure of 12 isolated tracheae and 11 bronchial segments of cats and, for comparison, 5 isolated tracheae of humans. With the muscle either relaxed or contracted, we rapidly froze the airways at distending pressures of 2.5 cm H2O and compressive pressures of 15, 50, or 100 cm H2O. When relaxed tracheae and bronchi were compressed, their soft parts herniated into the lumen. However, in cat and human tracheae, one edge of cartilage also curled inward to preserve a small lumen. Constricted tracheae and bronchi tended to retain a cylindrical shape during compression. Contraction of muscle pulled cartilages into overlapping apposition in both tracheae and bronchi; in bronchi, it also appeared to extend collagen fibers between cartilages. Muscular constriction of these airways may increase their ability to withstand compressive narrowing by covering their soft parts with a sleeve of f ibroc artilage.