Summary 1. The avian respiratory system comprises two separate struc- tures: the air sacs, mainly for ventilation, and the parabronchial lung, for gas exchange. The parabronchi are long tubes, open at both ends. 2. The direction of air flow through the parabronchi is the same during inspiration and expiration. Aerodynamic mechanisms are partly responsible for this unidirectional flow. 3. The ... [Show full abstract] cross-current system appears to be a suitable model for a quantitative evaluation of gas exchange between the air capillaries and blood capillaries in the periparabronchial tissue. Unidirectional air flow appears to be of only little advantage to the effectiveness of gas exchange in this model. 4. Single-unit recordings from vagal afferents have shown the existence of intrapulmonary receptors which increase their discharge rate when lung gas PC02 is lowered, but are insensitive to stretch. The ventilatory response to C02, particularly when carbonic anhydrase is blocked, suggests an important role of in- trapulmonary CO2 receptors (IPC) in the control of breathing. Considering the location of IPC in the parabronchial lung, un- directional air flow may provide optimal conditions for their operation during normal breathing.