Major differences in the pulmonary circulation between birds and mammals

Area of Science:

• Comparative physiology
• Avian respiratory system
• Pulmonary circulation

Background:

• Mammalian lung physiology is well-understood, but avian lung mechanics differ.
• Pulmonary hypertension syndrome (PHS) significantly impacts the broiler industry, causing substantial economic losses.
• The structural and functional characteristics of avian pulmonary vasculature remain incompletely understood.

Purpose of the Study:

• To investigate the pressure-volume relationships and vascular resistance in domestic chicken lungs.
• To compare the behavior of chicken pulmonary vasculature under varying pressures with that of mammalian lungs.
• To explore the structural basis for observed pressure-flow dynamics in avian lungs and its relation to PHS.

Main Methods:

• Domestic chickens' lungs were perfused with blood or dextran/saline.
• Pulmonary artery pressure (P(a)) and venous pressure (P(v)) were systematically varied relative to air capillary pressure (P(A)).
• Pulmonary vascular resistance (PVR) and blood flow were measured under different pressure conditions (Zone 1, 2, and 3).

Main Results:

• In Zone 3, chicken pulmonary vascular resistance (PVR) remained stable despite increased P(a) or P(v), unlike in mammals.
• In Zone 2, blood flow was independent of P(v), but capillaries appeared fully open, challenging the Starling resistor model.
• Capillaries in Zone 1 remained open even when P(A) exceeded P(a) by >30 cm H(2)O, a stark contrast to mammalian lungs.

Conclusions:

• Air capillaries surrounding blood capillaries in chicken lungs provide rigid structural support, influencing vascular mechanics.
• This unique structural support explains the distinct pressure-flow dynamics observed in avian pulmonary circulation.
• The findings suggest a potential pathogenesis for pulmonary hypertension syndrome in chickens, offering insights for the broiler industry.