Inhalation is the active phase of respiration in which the diaphragm and intercostal muscles contract, expanding the thoracic cavity and drawing air through the nostrils, pharynx, larynx, trachea, and bronchi into the alveoli. In horses, a single breath at rest moves approximately four to five litres of air; at peak exertion this rises to over 300 litres per minute.
Because horses are obligate nasal breathers, every substance they inhale passes through the nasal passages first. Inhaled allergens — dust, mould spores, and endotoxin from poor-quality hay — are the primary trigger for equine asthma, formerly called heaves or RAO. Inhalation is also the route by which respiratory pathogens such as Streptococcus equi (strangles) spread between horses in close contact, making isolating infected horses of infected animals a critical biosecurity step. Veterinarians exploit this route therapeutically: nebulised bronchodilators and corticosteroids reach the lower airway directly through inhalation, bypassing first-pass metabolism in the intestine. Environmental management — well-ventilated stabling, soaked or steamed hay, and clean bedding — reduces the inhalation load of harmful particles.
Further Reading
The respiratory mechanics of inhalation are described in the inhalation article on Wikipedia. Equine respiratory diseases that affect airway function are covered in the Merck Veterinary Manual section on Lung and Airway Disorders of Horses.