Ventilation is the term given to the movement of air in to and out of the lungs. Ventilation is a product of the tidal volume (Vt) multiplied by the breathing frequency (f). Inspiration, the movement of air in to the lungs, is an active process that recruits the muscles in the chest and intercostals and in particular the diaphragm.

Expiration describes the movement of air out of the lungs. Expiration under resting conditions is a passive process that requires little muscular activity. During exertion however, such as when exercising or performing a dynamic manoeuvre such as spirometry, the muscles of expiration are utilised.

The following lesson looks at how the diaphragm inflates and deflates the lungs at rest. Remember that the lungs are attached to the chest wall by the pleura.

Under normal conditions, where the chest wall moves so do the lungs

As the chest wall moves upwards and outwards and as the diaphragm drops downwards the lungs inflate within it. This creates a negative pressure within the lungs which draws air inwards down it’s pressure gradient from the ambient air.

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