Tertiary bronchi in the lung of a chicken (HE)
The biggest part of the lung parenchyma of a bird consists of tertiary bronchi (parabronchi). The parabronchi are separated from each other by poorly elastic interparabronchial septa (septa interparabronchialia). These septa contain arterioles, venules and nerve fibers. The arterioles branch from the periphery into the parabronchial wall to form blood capillaries. Blood capillaries drain into small venules at the base of the atria who in term pass outwards again to drain into the venules within the septa.
From the central lumen of the parabronchi, polygonal out pouches (atria) bulge into the parabronchial wall and branch through funnel-shaped ducts (infundibula) into the air capillaries (pneumocapillares). The parabronchi and surrounding tissues can be considered as the lung unit. Gas exchange occurs between the blood capillaries and air capillaries in the wall of the parabronchi.