Parabronchi

Parabronchi are tubular airways that form the principal gas-exchange pathways in the lungs of birds. They are part of the avian respiratory system, which uses air sacs to generate a unidirectional flow of air through a branching network of parabronchi.

Structure and arrangement: Parabronchi are long, narrow tubes that run through the lung parenchyma. Their walls are thin and lined with a ciliated epithelium. From the walls, numerous infundibula extend to create a dense network of air capillaries; surrounding these are the blood capillaries that constitute the respiratory bed. Gas exchange occurs across the thin walls where the air-filled capillaries interface with the blood.

Ventilation and gas exchange: Birds ventilate the parabronchial system with a pair of nested air sacs that move air during both inspiration and expiration, producing a continuous, one-way flow through the lungs. The arrangement of air flow relative to the surrounding capillaries supports a cross-current form of gas exchange, in which blood capillaries intersect the airflow to extract oxygen efficiently and remove carbon dioxide, enabling high metabolic performance.

Comparative notes: In birds, the parabronchial system replaces the mammalian alveolar sacs as the primary site of gas exchange. The architecture—air moving through parabronchi with surrounding air and blood capillaries—contributes to the unusually high oxygen extraction efficiency of avian lungs, which supports flight and endurance.

Evolution and significance: Parabronchi are a defining feature of modern bird lungs and their closest extinct relatives. Their efficient design for unidirectional ventilation and cross-current gas exchange is closely linked to the ecological and physiological demands of powered flight and high-altitude activity.