Respiratory gas exchange and control in the chicken.

Gas exchange in parabronchial lungs can be described by: 1) representing the entire lung as one parabronchus, 2) solving iteratively a series of equations relating CO2 and O2 levels to the partial pressures of the gases, blood pH, and flows of gas and blood at thin sections of the parabronchus to give changes of gas pressures, and 3) integrating the changes over the parabronchus. At rest, diffusion resistance to exchange between gas and blood is a relatively small effect while ventilation/perfusion inequalities between parabronchi are responsible for most of the disequilibrium between gas and blood. Ventilation is altered to control body temperature and intrapulmonary and arterial PCO2. Chickens pant at a relatively lower respiratory frequency than mammals of comparable body weight. The respiratory time constant of hyperthermic chickens predicts maximal parabronchial ventilation at usual panting frequencies. Above this frequency, CO2 would increase from the minimum in hyperthermia, decreasing frequency of respiration. Inhaling CO2-enriched air in hyperthermia slows frequency but increases ventilation, protecting against lethally hot climates. Intrapulmonary chemoreceptors reduce ventilation if intrapulmonary PCO2 falls. Left ventricular CO2-sensitive mechanoreceptors also inhibit ventilation if PaCO2 falls or blood pressure increases. Both sensory systems may buffer respiratory gas and acid-base changes at the tissue. No known system can explain ventilation during exercise.