Basic rhythm - control centers in medulla oblongata, spirograph and normal tidal cycle

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• Normal ventilation is rhythmic and involves continuous cycles of inspiration and expiration. • Various regions of the brain closely regulate this rhythmic pattern of ventilation. • The rhythmicity area in the medulla regulates the basic rhythm of ventilation. • The medullary rhythmicity area contains: • Inspiratory area. • Expiratory area. • During normal, quiet ventilation only the inspiratory area is active but during forced ventilation both areas are active. • Air flow within the respiratory system can be monitored by a spirogram. • Inspiration is typically depicted as an upward deflection of the graph • Expiration is typically depicted as a downward deflection of the graph. • During normal breathing, nerve impulses are spontaneously generated from the inspiratory area in the medulla. • Impulses stimulate inspiratory muscles (diaphragm and external intercostals). • Inspiratory muscles contract and air moves into the lungs. • After about two seconds, impulses from the inspiratory area cease and inspiratory muscles relax. • For the next three seconds, inspiratory muscles are not stimulated so passive elastic recoil produces expiration. Air moves out of the lungs. • After three seconds of relaxation, the inspiratory area again stimulates the inspiratory muscles to contract. • A new ventilation cycle begins. • The inspiratory area controls normal ventilation while expiration is passive.

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