Major differences in the pulmonary circulations between birds and mammals

The two great classes of vertebrates capable of sustained high oxygen consumptions, the mammals and birds, have radically different lungs. Mammals have reciprocating ventilation with large terminal air spaces (alveoli) while birds have a flow‐through system with small air capillaries. Here we report major differences between the pulmonary circulations of chickens and dogs. The lungs of domestic chickens ( Gallus gallus ) were perfused with blood or dextran/saline and the pulmonary artery pressure (P a ) and venous pressure (P v ) were varied in relation to air capillary pressure (P A ). In zone 1 conditions the blood capillaries were open even when P A exceeded P a by over 30 cmH 2 O, which is very different behavior from that of mammalian lung. In zone 2 conditions blood flow was essentially independent of P v as in the mammalian lung but all the capillaries appeared to be open which seems incompatible with a Starling resistor mechanism. In zone 3 pulmonary vascular resistance (PVR) was virtually unchanged with increases in either P a or P v . This is very different from mammals where the same interventions greatly reduce PVR. We conclude that the air capillaries that surround the blood capillaries provide rigid support in both compression and expansion of the vessels. The work suggests a pathogenesis for Pulmonary Hypertension Syndrome in chickens, which costs the broiler industry $1 billion per year. Supported by NIH 5 R01 HL60968