The act of taking a breath requires contraction of the diaphragm and expansion of the chest. This causes negative pressure within the chest, which pulls air through the nostrils, nasal passages, nasopharynx, larynx, trachea, bronchi, small airways and expands the lung tissue itself. Conditions that cause obvious difficulty breathing are usually a result of a problem somewhere within the respiratory tract, but severe abdominal distention (bloat) from intestinal obstruction can make it very difficult for horses to breathe.

A horse that is having difficulty breathing may make respiratory noise or appear to "work hard" to get a breath. You may notice that the horse's sides seem to pull or draw hard in order to draw air in or push it out. They may have an anxious expression. Their nostrils may flare. The respiratory rate will often be higher than normal. There may be sounds either on the inhalation or exhalation.