Structure of avian lungs.

Abstract Injection casts of the lung air sac system of 155 species of 47 families of birds were studied, together with preparations of fixed specimens of 45 species. Besides measurements of the fractional distribution of the volumes of the lung and the air sacs, the exchange surface of the air capillaries was determined in 8 species. The avian lung is firmly attached to the thoracic wall and remains constant in volume during both phases of respiration. The primary bronchus enters the lung, giving off at the hilus 4 ventrobronchi and, posteriorly, 7 to 10 dorsobronchi besides some laterobronchi, before it enters the abdominal air sac. The posterior thoracic air sac is connected to one large laterobronchus. These posterior air sacs act as bellows ventilating the lung, much more than the anterior air sacs which are connected to single ventrobronchi. The special manner of the origin of the ventro-, dorso- and latero-bronchi from the primary bronchus is responsible for the ventilation of the parabronchi which connect the total internal surfaces of the dorsobronchi and ventrobronchi. This structure is the “paleopuimo”, found in all birds. Systematically higher birds possess an additional connecting network of parabronchi between the primary bronchus and the posterior air sacs. This “neopuimo ” is best developed in fowllike birds and song birds. The tissue mantle of the parabronchi is made up of a meshwork of blood and air capillaries within which the gas exchange takes place. Development of small air capillaries requires a constant lung volume. The effective exchange surface per volume unit is ten times larger than that of comparable mammalian lungs.