- During the first inspiration, the air travels through the nostrils, also called nares, of a bird, which are located at the junction between the top of the upper beak and the head. The fleshy tissue that surrounds them, in some birds, is called the cere. As in mammals, air moves through the nostrils into the nasal cavity. From there it passes through the larynx and into the trachea. Air moves through the trachea to the syrinx, which is located at the point just before the trachea divides in two. It passes through the syrinx and then the air stream is divided in two as the trachea divides. The air does not go directly to the lung, but instead travels to the caudal (posterior) air sacs. A small amount of air will pass through the caudal air sacs to the lung.
- During the first expiration, the air is moved from the posterior air sacs through the ventrobronchi and dorsobronchi into the lungs. The bronchi continue to divide into smaller diameter air capillaries. Blood capillaries flow through the air capillaries and this is where the oxygen and carbon dioxide are exchanged.
- When the bird inspires the second time, the air moves to the cranial air sacs.
- On the second expiration, the air moves out of the cranial air sacs, through the syrinx into the trachea, through the larynx, and finally through the nasal cavity and out of the nostrils.
Respiratory Evolution Facilitated the Origin of Pterosaur Flight and Aerial Gigantism (2009)
In this report we present various lines of skeletal evidence that indicate that pterosaurs had a highly effective flow-through respiratory system, capable of sustaining powered flight, predating the appearance of an analogous breathing system in birds by approximately seventy million years.
The skeletal breathing pump of pterosaurs, including the vertebral and sternal ribs, sternum, gastralia and prepubes, likely formed a highly integrated functional complex. The persistence of the basic components of this system in all pterosaur clades suggests that our inferences related to ventilatory mechanics, and primarily based upon Rhamphorhynchus and Pteranodon, can be safely assumed to have generally applied to the group.
"The aspiration pump of pterosaurs maximised trunk expansion in the ventrocaudal region, while at the same time limiting the degrees of freedom of movement of the trunk in other directions. This provided greater control over the location, amount and timing of trunk expansion, thereby enabling precisely-timed localized generation of pressure gradients within the pulmonary system, a trait that is also present in living birds where it is of paramount importance for the generation of air flow patterns in the lungs , .
The following references confirm that birds have an "avian aspiration pump" but dinosaurs did not.
Evidence for Avian Intrathoracic Air Sacs in a New Predatory Dinosaur from Argentina
Paul C. Sereno et al
Research on the gastral cuirass in archosaurs led to the suggestion that it may have functioned as an accessory aspiration pump in nonavian dinosaurs –. Although Claessens drew attention to the relationship between the gastral cuirass and abdominal air sacs, he concluded that “it appears impossible to ascertain exactly when lung diverticula stretching throughout the whole body cavity or unidirectional airflow originated” [83:102]. Later a “caudal origin model” for air sacs and flow-through lung ventilation (either uni- or bidirectional) was proposed [33: 255] based on (1) the presence of abdominal air sacs (inferred from posterior dorsal and sacral pneumaticity), (2) a dynamic gastral cuirass, and (3) vertebrocostal articulations in the posterior ribcage that allow greater excursion during aspiration (inferred from the more horizontal arrangement of posterior rib articulations). An independent study of rib morphology, in contrast, concluded that nonavian dinosaurs were characterized by an “anteriorly ventilated bellows lung”[84: 47].