Functional design of the mature avian respiratory system

The avian respiratory system is structurally exceptionally complex and functionally remarkably efficient. It comprises a lung that serves as the gas exchanger and air sacs that function as the ventilators. Topographically, the lung is located between two sets of air sacs, namely, a cranial and a caudal group. They continuously ventilate the lung in a craniocaudal direction by synchronized activities. The air sacs are delicate, transparent and compliant structures. Since they are avascular, they play no part in gas exchange. The lung is attached to the ribs and the vertebrae on the dorsolateral aspects and to the horizontal septum on the ventral one. It renders the lung practically rigid. This allows the exchange tissue (parenchyma) to be very intensely subdivided, resulting in minuscular terminal respiratory units, the air capillaries. That way, the respiratory surface area is increased. The bronchial system of the avian lung forms a continuous loop that consists of a three-tier system of airways. These are a primary bronchus, secondary bronchi and tertiary bronchi (parabronchi). The arrangement of the airways and the blood vessels in the lung determines where and how air and blood are distributed and the respiratory media exposed to each other for gas exchange. A cross-current system is formed by the essentially perpendicular disposition between the flow of air in the parabronchial lumen and that of venous blood in the exchange tissue; an auxiliary countercurrent arrangement is formed by the relationship between the direction of the flow of air in the air capillaries and that of the blood in the blood capillaries, and a multicapillary serial arterialization system consists of the sequential interaction between the blood in the blood capillaries and the air in the capillaries in the parabronchus. In addition to these features, the bird lung has large capillary blood volume, extensive surface respiratory surface area and particularly thin blood-gas (tissue) barrier. The morphological specializations and the adaptive physiological features such as large tidal volume and cardiac output all together explain the exceptional gas exchange efficiency of the avian respiratory system, supporting the high metabolic capacity and energetic lifestyles of birds.