Tracheae

Tracheae are the plural form of trachea in biological terminology. The term is used for two distinct respiratory structures in different animal groups. In insects and other arthropods, tracheae refer to an extensive internal network of air-filled tubes that delivers oxygen directly to tissues. In vertebrates, the trachea is a single midline airway; its plural form tracheae is rarely used except in comparative or anatomical contexts.

Insects possess a tracheal system that begins at external openings called spiracles. Air enters through spiracles and travels through progressively smaller tubes, the tracheae, eventually reaching the finest tubes called tracheoles that extend close to individual cells. The tracheae are reinforced with taenidia, ring-like bands of cuticle that keep the tubes open. Gas exchange occurs primarily across the membranes of tracheoles, where oxygen diffuses into tissues and carbon dioxide diffuses out. Some insects have air sacs connected to the tracheal system to increase ventilation. Ventilation is aided by body movements and, in winged insects, by changes in thoracic volume during wing flapping.

In vertebrates such as mammals, the trachea is the airway that conducts air from the larynx to the bronchi. It is supported by C-shaped rings of hyaline cartilage that prevent collapse and remain open during breathing. The tracheal lining is a pseudostratified ciliated epithelium with goblet cells that produce mucus; cilia sweep mucus toward the pharynx to remove particles. The trachea branches into the right and left main bronchi, continuing the conduction pathway into the lungs. Developmentally, the trachea forms as part of the foregut; congenital abnormalities can involve stenosis or fistula with the esophagus.