inspiration /in′spirā″shən/ [L, inspirare, to breathe in] , the act of drawing air into the lungs. The major muscle of inspiration is the diaphragm, the contraction of which creates a reduced pressure in the chest, causing the lungs to expand and air to flow inward. Accessory inspiratory muscles include the external intercostals, scaleni, scapular elevators, and sternocleidomastoids. Since expiration is usually a passive process, these muscles of inspiration alone produce normal respiration. Lungs at maximal inspiration have an average total capacity of 5.5 to 6 L of air. Also called inhalation. Compare expiration. See also inspiratory reserve volume. −inspiratory, adj.
