Respiration in Exercising Fowl. III. Ventilation

1. Minute volume (V), respiratory frequency (f) and tidal volume (VT) were continuously measured in domestic fowl running on a treadmill at speeds of 1.24–4.3 km h−1 in air temperatures of 18±2 °C and 35±2 °C. Oxygen extraction (E) was estimated using previously measured values of oxygen consumption. 2. At 18±2 °C V, f and VT rose abruptly at the start of exercise and V and f continued to rise except at the slower running speeds. V and f continued to rise throughout exercise at all speeds in the heat-stressed birds. In both groups VT reached a maximum at the start of exercise then fell to a new steady-state level. Steady-state V, f and VT increased with exercise speed at 18±2 °C. At 35–37 °C steady-state V and VT increased with speed but f changed relatively little. 3. E remained close to resting (0.21) at low speeds in room temperatures but decreased to 0.17 at 4.3 km h−1. In contrast E rose from 0.05 at rest in heat-stressed birds to 0.14 at 3.6 km h−1. The changes in oxygen extraction and respiratory pattern in the two groups during exercise are discussed in connexion with the control of ventilation by thermal and non-thermal factors. It is suggested that the latter play a predominant role during heavy exercise and that this accounts for the convergence of respiratory patterns in both heat-stressed and non heat-stressed birds at maximum work rates.