The horse does not breathe through its mouth and nose like we do. The horse only breathes through its nostrils. ... When horses inhale during exercise, around 90% of the resistance (obstruction) to air movement is in the airways that are in the head, namely, the nostrils, the nasal passages, and the larynx.
The respiratory system consists of the large and small airways and the lungs. When a horse inhales, the air travels down the trachea, which divides into the tubes known as the right and left bronchi, then into the smaller airways called bronchioles in the lungs.