Ventilation and Acid-Base Status during Thermal Panting in Pigeons (Columba livia)

Abstract

During heat exposure, the respiratory system of a panting bird must fulfill the potentially conflicting requirements of dissipating heat and maintaining arterial carbon dioxide tension ( ). To investigate these functions in panting pigeons (Columba livia, mass 0.34 kg), we simultaneously measured respiratory volumes, , and arterial blood acidity (pHa). At body temperatures ( ) of 41 and 42 C, we observed no panting. At of 43, 44, and 45 C, a compound pattern of ventilation containing two components replaced resting breathing. One component had approximately the same frequency (f), 28-39 min⁻¹, and tidal volume ( ), 2.5-3.0 cm³ body temperature and pressure, saturated with water vapor (BTPS), as at lower ; the other was rapid, about 500 mi⁻¹, and shallow, 1.1 cm³ BTPS or less, resulting in a ventilation of 550 cm³ air BTPS·min⁻¹. Since this approximates tracheal dead space, the rapid component of ventilation, although five times greater than the slow component, was confined to the non-gas-exchanging, evaporative surfaces of the anterior respiratory tract. Over this entire range of , 41-45 C, neither nor pHa varied significantly, averaging 25.8 torr and 7.50, respectively. Compound panting in moderate heat stress thus elevates respiratory evaporation while sustaining normal pulmonary ventilation and blood acid-base status. At of 46 C, the rapid component increased well beyond the dead space, and the slow component was abandoned, with a consequent hypocapnic alkalosis ( 18 torr, pHa 7.63) due to excessive CO₂ washout.