Bird, lung (Gross, Low)

Structure List Cranial air sacs Caudal air sacs Trachea Lungs

The lungs of birds are very different from the lungs of mammals, for they do not change volume during inhalation and exhalation. It is the air sacs that change volume. The duct system of the lung is also very different from that of mammals. The avian respiratory system shows many other peculiarities. The lungs are small and very vascular, and expand very little. The lungs are firmly attached to the ribs and thoracic vertebrae. The lungs themselves are only half the size of those of a mammal of comparable size. However, the lungs connect to a system of air sacs that pass among the viscera and even extend into many of the bones. The air sacs can be grouped into an anterior and posterior set. Together, lungs and air sacs have a volume two to three times that of the lungs of a comparable sized mammal. Gas exchange does not occur in the air sacs, but instead occurs in the air capillaries which branch off the parabronchi in the lung. The air capillaries are surrounded by dense capillary beds, whereas the air sacs are poorly vascularized. Air sacs function to provide complete filling of the lungs. It takes two inspirations and expirations to move a unit of air through the respiratory system.