Functional morphology of the avian respiratory system, the lung-air sac system: Efficiency built on complexity

The avian respiratory system is separated into a lung (the gas exchanger) and the air sacs (the mechanical ventilators). The lung is intercalated between two functionally distinct sets of air sacs, a cranial and a caudal group. Avascular in nature, the air sacs are delicate, transparent, compliant and capacious. The lung is ventilated continuously and unidirectionally in a craniocaudal direction by synchronised bellows-like movements of the air sacs. Affixed to the ribs, the lung is virtually rigid. This has allowed for intense subdivision of the exchange tissue into very small respiratory units, the air capillaries, optimising the respiratory surface area. A thin blood-gas barrier is formed by restriction of cellular and connective tissue elements such as collagen and elastic tissue to the atria and the infundibulae. Complex vascular and bronchial systems are formed through elaborate morphogenetic processes. The airways comprise a primary bronchus, various secondary bronchi and numerous tertiary bronchi (parabronchi) that form a continuous loop. The directions of the air flow in the parabronchial lumen and that of the venous blood form a cross-current system, whereas the relationship between the air capillaries and blood capillaries is counter-current. A multicapillary serial arterialisation system is formed by the arrangement between the blood capillaries and air capillaries along the lengths of the parabronchi. A large volume of blood is exposed to air over an extensive surface area across a thin blood-gas barrier. Together with other physiological specialisations such as large tidal volume and cardiac output, these properties confer a high pulmonary diffusing capacity for oxygen. The exceptionally efficient gas exchange efficiency supports the high metabolic capacity and energetic lifestyle of birds. It should, however, be emphasised that since bats with a bronchioalveolar lung and insects with a tracheal system fly as well, if not better, than birds, the lung-air sac system is not a prerequisite for flight. It was one of the possible solutions to birds attaining life in the fast lane.