The Avian Respiratory System
The avian respiratory system is different from other animals because they have smaller lungs and nine air sacs, which help in the role of respiration for birds. These changes are important because there are some birds like the alpine swift (Tachymarptis melba) can stay in flight for over 200 days. This means that the birds are breathing in less oxygenated air for that entire flight.
Air Sacs
Air sacs in the respiratory system allow for unidirectional flow of air through the lungs of birds. Therefore, the air that is moving through the bird’s lungs is mostly “fresh” air that has a higher amount of oxygen compared to an animal without unidirectional flow because the “new” air coming into the bird’s lungs does not mix with any “old” air. A bird then has more oxygen available to diffuse into their blood. The unidirectional flow also allows for birds to take in oxygen during exhalation. This system is a more efficient lung structure for the birds and makes it possible to breathe at higher elevations for longer periods of time.
Another difference in birds is their upper respiratory tract. Birds have a nasal cavity, larynx, and a tracheal lumen like mammals. However, the length of a trachea for a bird is approximately 2.7 times larger than comparably sized mammals.
Evolution of the Avian Respiratry Sysytem
A theory on how birds have this unique respiratory system is thought to lead back to theropod dinosaurs. A research article by Patrick M. O’Connor and Leon P. A. M. Claessens explains this. The air sacs that are fixed to a non-expanding parabronchial lung that are critical to birds getting enough oxygen have also been reportedly found in certain dinosaurs. Including non-avian theropod dinosaurs.
Other researchers also have the same theory. Peter Ward, a paleontologist, presented one theory on how bird’s lungs developed. His theory is that 175-275 million years ago there were oxygen levels at about half of the percentage today. He believes that these oxygen levels favored in Saurischian dinosaurs. The Saurischians had lungs attached to a series of thin-walled air sacs. This breathing system is similar to what is found in birds today. At the time of this dinosaur the oxygen levels were low which translates to the same low oxygen levels for birds when they are flying at high elevations.
