ABSTRACT
Normal respiration is characterized by quiet, active inspiration followed by passive expiration. The breath is initiated by medullary neurons, however neural control of breathing also involves other brainstem centers, cervical vertebral segments, and the cerebral cortex. Abdominal and internal intercostal muscles also are used for expiration; their contraction decreases thoracic size, thereby increasing intrathoracic pressure and forcing air out of the lungs. The breathing rate is influenced by numerous factors such as exercise, ambient temperature, excitement, digestive tract filling, pregnancy and other causes of abdominal enlargement, and many diseases. Pulmonary, pleural space, and airway diseases can influence the pattern of breathing and associated breath sounds. The nasal cavity and upper airways offer the greatest resistance to normal inspiration. Animals with severe dyspnea are reluctant to eat, drink, or even swallow saliva. Ventilation normally is regulated reflexly in response to changes in the partial pressure of arterial carbon dioxide (PaCO2), sensed by central and peripheral chemoreceptors.
