design of the avian respiratory system: development, morphology and function

• Published in: Journal of ornithology Vol. 156; no. Suppl 1; pp. 41 - 63
• Main Author: Maina, John N
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01.12.2015
• Subjects: air; air flow; air sacs; Biomedical and Life Sciences; birds; blood; blood capillaries; blood flow; blood volume; bronchi; Ecology; Fish & Wildlife Biology & Management; gas exchange; Life Sciences; lifestyle; oxygen; Review; ribs; surface area; surface tension; ventilators; vertebrae; Zoology; Oxygen; Lung; Flight; Development; Birds; Respiration; Air sacs
• ISSN: 2193-7192; 2193-7206
• DOI: 10.1007/s10336-015-1263-9

The avian respiratory apparatus is separated into a gas exchanger (the lung) and ventilators (the air sacs). Synchronized bellows-like movements of the cranial and caudal air sacs ventilate the lung continuously and unidirectionally in a caudocranial direction. With the lungs practically rigid, after their insertion into the ribs and the vertebrae and on attaching to the membranous horizontal septum, surface tension is not a constraining factor to the intensity that the gas exchange tissue can subdivide. Delicate, transparent, capacious and avascular, the air sacs are not directly involved in gas exchange. The airway system comprises of a three-tiered system of passageways, namely a primary bronchus, the secondary bronchi and the tertiary bronchi (parabronchi). The crosscurrent system is formed by the perpendicular arrangement between the mass (convective) air flow in the parabronchial lumen and the centripetal (inward) flow of the venous blood in the exchange tissue; the countercurrent system consists of the centrifugal (outward) flow of air from the parabronchial lumen into the air capillaries and the centripetal (inward) flow of blood in the blood capillaries, and; the multicapillary serial arterialization system is formed by the blood capillaries and the air capillaries where venous blood is oxygenated in succession at the infinite number of points where the respiratory units contact exchange tissue. Together with the aforementioned systems, features like large capillary blood volume, extensive respiratory surface area and thin blood-gas barrier accord high pulmonary diffusing capacity of O₂ that supports the high metabolic capacities and energetic lifestyles of birds.