Panting and blood carbon dioxide in birds.

CALDER, W+ A., AND KNUT SCHMIDT-NELSEN. Punting and blood carbon dioxide in birds. Am. J* Physiol, 215(2): 4771482. 1968.-The respiration of resting and panting birds was studied in nine species representing a wide range of size, morphology, and habits. All these birds had similar values for arterial blood PCU~ and pH during normal resting respiration (28 mm C02, pH = 7.5). These values are much lower than those for mammals, which suggests a more thorough ventiXatory air renewal in the avian lungs. Panting during heat stress resulted in hypocapnia and alkalosis in aI1 species, even in the pelican which makes extensive use of the nonrespiratory gular pouch for evaporative cooling. The findings suggest that the respiratory system of birds is a more effective gas exchange system than that of mammals, and also that the mesobronchial bypass route cannot prevent alkalosis during panting. the respiratory system which exceed the needs for gas exchange, Panting and gular movements and their thermoregulatory effects have been studied in several birds (e.g., 3, 7, 11, 16, 21-23) but information on the simultaneous gas exchange is available only for the pigeon (6) and the ostrich (26). Within the avian lung, the large mesobronchi and the smaller parabronchi are parallel routes for air passing to and/or from the air sacs. The mesobronchial route could serve as a bypass route or shunt, so that evaporative cooling could be augmented without hyperventilating the parabronchial gas exchange surfaces (30). However, considerable hypocapnia and alkalosis occur in pigeons that pant vigorously (6) which indicates that shunting, if it exists at all, is insufficient to prevent acid-base shifts. In contrast, the large, flightless ostrich does not become hypocapnic and alkalotic during panting (26). Are these phenomena, low Pcoz at rest and thermoregulatory hypocapnia, generally characteristic of the respiratory systems of birds capable of flight or peculiar to the pigeon? This paper reports a comparative study in which we sought to answer this question. hypocapnia; hyperventilation; alkalosis; temperature regulation; respiration; gular flutter