ABSTRACT. The resin casts of the respiratory and vascular systems in pigeon lung were examined using a scanning electron microscope. The primary bronchi branched to form many secondary bronchi that anastomosed with each other via the parabronchi. Numerous infundibula protruded from the parabronchi via the atria and ramified into the air capillaries. The pulmonary artery entered into the lung and branched into three vessels that coursed the interparabronchial parts. The intraparabronchial arterioles penetrated the gas-exchange tissue to form the anastomosing networks of blood capillaries. The observation of the double casts of the respiratory and vascular systems revealed three-dimensional complicated networks of air capillaries and blood capillaries. KEY WORDS: lung, pigeon, vascular system. J. Vet. Med. Sci. 67(10): 1071-1074, 2005 The respiratory organs of birds are structurally different from those of mammals. The avian lung is devoid of lobulation, alveoli, and tree-like branching of the air ducts that terminate in alveoli [8]. Hence, it can be assumed that the mechanics of breathing of birds are quite different from those of mammals [4].The respiratory system of the avian lung is composed of ramifying and anastomotic parabronchi (tertiary bronchi) [5]. These parabronchi that arise from the secondary bronchi occupy more than half of the avian lung volume in the form of dense bundles of interanastomosing tubes [8]. Some narrow columnar chambers (infundibula) protrude from the parabronchi via the dilated chamber or depression of parabronchial wall known as atria [15], and a number of air capillaries arise from the end of the infundibula [3,5,8,16].Based on the relationship between the direction of air flow and the direction of capillary blood flow within the alveolar walls, two systems for gas exchange in the avian lung have been theorized. These theories, namely, the counter (parallel) current system and the cross current system, are based on the anatomical and physical models as well as gas analysis [3,14,16].The anatomical relationship between the air capillaries and blood capillaries in the chicken lung is studied by the examination of an injected cast by using scanning electron microscope (SEM) [3,9,10] and transmission electron microscope [7]. However, the detailed morphological relationships between the terminal parts of these capillaries have not been verified. The cast of the blood capillary system in the pigeon lung is studied by the SEM in detail [15] but the respiratory system is not described. In this study, the fine structure of the double resin casts of both the respiratory tract and blood vessels in the pigeon lung was observed using SEM in order to elucidate the structural relationship between them at the level of the terminal parts of air capillaries and blood capillaries.All procedures were done in accordance with the "Guiding principles for the care and use of animals in the field of physiological sciences" of the Physiological Society of Japan. A total of 15 adult male pigeons were used in...