The animation below provides an overview of lung volumes and spirometric indices.
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One can display the volume change of the lung as a function of time (spirogram, to the right), or display flow as a function of volume change (shown for forced expiration only, middle). These are different presentations of the same phenomenon.
The intrathoracic airways pass through three phases during a forced expiration: initially there is no dynamic airway compression, but subsequently during the greatest part of the forced expiration (starting from PEF) flow is limited by a check valve; towards the end of the forced expiratory flow is most likely not limited by a flow-limiting segment any more, but by progressive closure of small airways (see animation), a phenomenon which becomes more prominent with advancing age.