The lungs of the finch: three-dimensional pulmonary anatomy of the zebra finch (Taeniopygia castanotis)

Unidirectional airflow, air sacs or the horizontal septum: what does it take to make a bird lung?

In this review, we evaluate the differences between the pulmonary anatomy of birds and other sauropsids, specifically those traits that make the avian respiratory system distinct: a fully decoupled and immobilized, isovolumetric gas-exchanging lung separated from compliant ventilatory air sacs by a horizontal septum. Imaging data, three-dimensional digital anatomical models and dissection images from a red-tailed hawk (Buteo jamaicensis), common ostrich (Struthio camelus), barred owl (Strix varia), African grey parrot (Psittacus erithacus) and zebra finch (Taeniopygia castanotis) are used to demonstrate the anatomical variation seen in the pulmonary air sacs, diverticula and the horizontal septum. We address the current state of knowledge regarding the avian respiratory system and the myriad areas that require further study, including the comparative and quantitative ecomorphology of the bronchial tree and air sacs, the non-ventilatory functions of the sacs and diverticula, the fluid dynamics and anatomical mechanisms underlying unidirectional airflow, post-cranial skeletal pneumaticity, and how all of these factors impact reconstructions of respiratory tissues in extinct archosaurs, particularly ornithodirans (i.e. pterosaurs + non-avian dinosaurs). Specifically, we argue that without evidence for the horizontal septum, a fully avian lung should not be reconstructed in non-avian ornithodirans, despite the presence of post-cranial skeletal pneumaticity.This article is part of the theme issue 'The biology of the avian respiratory system'.

Biology of the avian respiratory system: development, evolutionary morphology, function and clinical considerations

The respiratory biology of birds has been of interest to researchers for centuries, particularly owing to its dramatically heterogeneous structure, unusual ability for non-ventilatory structures to invade nearly all parts of the body (including the skeleton) in many taxa, and its exceptional efficiency under high-altitude hypoxia. Advances in imaging, experimental and developmental techniques, as well as recent palaeontological specimens have facilitated new discoveries, analyses and progress in the field. Comprehensively, this theme issue shows the origin of the modern avian respiratory system, current controversies and how the evolution of respiratory structures in birds has impacted their biology from the molecular, to the cellular, to the phylogenetic level. This collection of articles addresses progress the field has made, gaps in our knowledge and where the field needs to go, with a primary focus on adult and embryonic form and function but also touching on vocalization and clinical aspects of avian respiratory biology.This article is part of the theme issue 'The biology of the avian respiratory system'.

Avian air sacs and neopulmo: their evolution, form and function

The avian respiratory system is composed of an exchange structure (parabronchi) and a pump (air sacs) to perform gas exchange. While there are many studies dealing with the morphology and function of the palaeopulmonic parabronchi, the air sacs and the neopulmo have been somewhat neglected from a comparative and functional point of view, not always receiving a closer examination that they deserve. While a decent amount of data are available regarding air sac and neopulmo morphology on a family level or for domestic species, several orders of birds have yet to be investigated. Owing to the lack of detailed specific data, we did not perform a comparative phylogenetic analysis but compiled data regarding air sac and neopulmo morphology and analysed them from the viewpoint of current phylogenetic relations while also discussing aspects of these structures regarding avian physiology.This article is part of the theme issue 'The biology of the avian respiratory system'.