Parabronchi

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Parabronchi are millions of tiny passages in the lungs of birds that constitute the primary sites of gas exchange between air and blood for birds; delivering oxygen and removing carbon dioxide. It is not an air sac. This function in mammals is performed by the alveoli, which is an air sac. Bird air sacs are anatomical structures (typically nine) unique to the dinosaur [1] and bird respiratory system that allow unidirectional flow of air through the lungs. The nine flexible air sacs act like bellows to move air through the almost completely rigid lungs. "A portion of the air sac actually integrates with the skeleton, forming air pockets in otherwise dense bone." [2]

Air flows from the parabronchi into air vesicles, called atria, which project radially from the parabronchi. These atria give rise to air capillaries, where oxygen and carbon dioxide are traded with cross-flowing blood capillaries by diffusion. [3]

[edit] External links

• image "The avian airway divides into bronchi which then divide into smaller passages called parabronchi. Running between the parabronchi are tiny air capillaries that are penetrated by blood capillaries of roughly equal size. It is between these two capillary types that gas exchange occurs."