SUMMARY

The ostrich lung, with its lack of interparabronchial septa, the presence of very shallow atria and exceptional morphometric refinement, structurally resembles those of small, energetic flying birds, whereas it also displays features characteristic of the flightless ratites in which the neopulmo is relatively poorly developed and a segmentum accelerans may be generally lacking. The large size of the bronchial system of the ostrich may help explain the unique shifts in the airflow pathways that must occur from resting to panting breathing, explaining its insensitivity to acid–base imbalance of the blood during sustained panting under thermal stress. The mass-specific volume of the lung is 39.1 cm³kg⁻¹ and the volume density of the exchange tissue is remarkably high (78.31%). The blood–gas (tissue) barrier is relatively thick (0.56μm) but the plasma layer is very thin (0.14μm). In this flightless ratite bird, the mass-specific surface area of the tissue barrier (30.1 cm²g⁻¹), the mass-specific anatomical diffusing capacity of the tissue barrier for oxygen (0.0022mlO₂s⁻¹Pa⁻¹kg⁻¹), the mass-specific volume of pulmonary capillary blood (6.25 cm³kg⁻¹) and the mass-specific total anatomical diffusing capacity for oxygen (0.00073mlO₂s⁻¹Pa⁻¹kg⁻¹) are equivalent to or exceed those of much smaller highly aerobic volant birds. The distinctive morphological and morphometric features that seem to occur in the ostrich lung may explain how it achieves and maintains high aerobic capacities and endures long thermal panting without experiencing respiratory alkalosis.