Ventilatory pump

During inspiration, a ramp-like increase in the discharge of inspiratory spinal motoneurons contracts the diaphragm and inspiratory intercostal muscles and expands the thorax. Laryngeal abductor muscles are also activated during inspiration and dilate the larynx. Exhalation is generally driven by the stored recoil forces of lung tissue released during postinspiration when inspiratory movements have come to an end, but inspiratory muscles relax slowly. Thus exhalation occurs passively during this initial phase of exhalation (PI phase) without any active innervation of expiratory muscles. Lung volume is held by an actively controlled slow relaxation of the diaphragm during this phase. Expiratory airflow is additionally retarded by contraction of laryngeal adductor muscles (e.g. thyrohyoidic muscle) which increases the resistance to expiratory air flow.

Expiratory airflow can be reinforced by active contraction of expiratory intercostal and abdominal muscles (E2 phase). The dimensions of the upper airways, such as the pharynx, the larynx, and the bronchial tree, determine the flow resistance to air. They can be actively adjusted through cranial motoneurons which are synaptically coupled to the central respiratory network.

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