The functional anatomy of the pulmonary circulation of the domestic fowl.

The architecture of the pulmonary circulation in the domestic fowl was studied in 30 birds of different breeds by injection techniques and serial sections. The branches of the cranial, caudomedial and caudolateral rami of the pulmonary artery form interparabronchial arteries which are essentially transverse to the long axis of the parabronchi. At the periphery of the parabronchus they give rise to intraparabronchial arterioles which form capillaries in the exchange tissue. Most of the capillaries drain into atrial veins via intraparabronchial venules. The atrial veins lie near the parabronchial lumen below the interatrial septa. Some of the capillaries empty into septal venules which form a network underneath the atrial muscles and empty into the atrial veins. The atrial veins draining any particular region converge into a single intraparabronchial vein which penetrates the exchange tissue radially to join an interparabronchial vein. The interparabronchial veins contribute to the cranial and caudal radices of the pulmonary vein. The anatomical pathways of blood and gas indicate a cross-current relationship between pulmonary arterial blood and bulk parabronchial gas flow, and a counter-current relationship between capillary blood and gas in the air capillaries.