As exercise increases, both tidal volume and respiratory frequency increase. In racing animals, tidal volume may be up to 12 L and frequency up to 90. At the gallop, respiratory rate is coupled to the stride frequency. Air in the lungs is not uniformly distributed. Dorsal portions of the lungs receive more air than ventral portions.
Perfusion is matched closely in the horse.
That means that dorsal portions of the lung receive more blood than the ventral portion. Efficient matching leads to a value of between 0.8 and 1.0. Horses are very able to match perfusion with ventilation so even under intense exercise can be matched. Gas exchange at the lungs occurs through diffusion. The partial pressure of O2 in the environment is a function of the percentage of the gas and the total pressures of gases.