Ventilation refers to the movement of gas into and out of the lung. In the normal spontaneously breathing horse, inspiration and expiration are active processes utilizing metabolic energy.

       Alveolar ventilation is regulated by the CNS through chemoreceptors that sense CO2 and O2 partial pressures in the blood, and through pulmonary reflexes and non‐pulmonary neural input.

       Intrapleural pressure is approximately −5 cm H2O at rest, resulting in a transpulmonary pressure of 5 cm H2O.

       Inspiration is characterized by expansion of the chest, (due to contraction of the diaphragm and the external intercostal muscles) which results in a decrease in intrapleural pressure and an increase in the transpulmonary pressure gradient. As a result, gas moves from the atmosphere into the respiratory passages.

        Gas flow ceases at the end of inspiration, as there is no longer a gradient between the atmospheric and alveolar pressure.

       Expiration is characterized by relaxation of the inspiratory muscles, elastic recoil of the lung, and contraction of the internal intercostal and abdominal muscles. These processes result in an increase in the transpulmonary pressure, and movement of gas from the lung to the ambient atmosphere.

       In horses, both inspiration and expiration have a biphasic pattern. The first phase of inspiration is passive, from a forced and active contraction of abdominal muscles at the end of expiration that forces the respiratory system below its equilibrium. The second part of inspiration is active through contraction of the diaphragm and other inspiratory muscles. The first part of expiration is passive through recoil of the respiratory system down to its equilibrium position, but then is active so that the next inspiration begins with relaxation of the abdomen.

       Contraction of muscles in the nares, pharynx and larynx is necessary to prevent collapse of structures into the air passages in the presence of the negative pressures generated in the respiratory tract during inspiration,For example, muscle relaxation secondary to the effects of sedatives/anesthetics or as a consequence of nerve dysfunction can impair airflow.

       The work or energy expended in ventilation is due to the forces required to overcome the elasticity of the lung and frictional resistance to air flow.

       The relative amount of energy spent on these two forces in addition to the total amount of energy spent to achieve ventilation can be altered by the pattern of breathing.

       In the normal horse at rest, gas flow rate within the airways is slow, and the majority of the work of breathing is due to the elastic resistance of the lung.

       As flow rates increase during more rapid respirations, a greater amount of energy is spent overcoming the frictional resistance within the airways.

       Elastic resistance is a result of surface tension forces at the alveolar air‐liquid interface in addition to the elastic properties of the lung tissue matrix.

       Frictional resistance is primarily influenced by airway radius and length.The upper airways (nasal cavity, pharynx, and larynx) provide approximately 60% of the total resistance to breathing.The lower airway resistance primarily resides in the trachea and bronchi.The bronchioles provide only a small fraction of the total resistance due, in large part, to the low airflow in a high cross‐sectional area.

       Airway radius or diameter can be altered due to changes in the smooth muscle tone within the walls of airways.In the horse, smooth muscle extends from the trachea to the alveolar ducts.In general, parasympathetic mediated smooth muscle contraction results in airway narrowing and an increase in airway resistance.β‐adrenergic and nonadrenergic noncholinergic activation results in bronchodilation and a decrease in airway resistance.