The structure and possible functions of respiratory secretions are reviewed. In the equine, goblet (mucus producing) cells are the main source but little information is available on the volume or composition of equine respiratory secretions. Airway mucus has complex and incompletely understood physical characteristics which can be partially assessed by a wide range of in vitro and biological techniques. The complex relationship between mucus structure and its propulsion by the airway cilia are discussed, both in health and with pulmonary disease. Mucokinesis in the horse has been assessed visually, by bronchoscopically observing intratracheal markers and also by the use of radiographic and radioactive markers. All techniques indicate a tracheal mucus velocity of approximately 20 mm/min. A large number of mucokinetic agents have been claimed therapeutically to increase mucokinesis, using a range of mechanisms. These include mucus diluents, surface acting agents, mucolytics, bronchomucotropic agents, ciliary augmentors and broncho-dilators. A critical review of the literature shows that the bronchodilator clenbuterol is the most effective mucokinetic agent assessed to date in the equine.
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