Compare inspiration and expiration in terms of pressure and volume changes producing airflow. When breathing
quietly, why is inspiration considered active while expiration is considered passive? What will be an ideal response?
ANSWER: Air moves into and out of the lungs due to pressure gradients between the lungs and
atmosphere. During inspiration the diaphragm and external intercostal muscles contract,
increasing the volume and decreasing the pressure within the lungs. Intra-alveolar pressure
drops below atmospheric pressure and air enters the lungs down the pressure gradient. The
opposite happens during expiration. These muscles relax and this combined with the elastic
recoil decreases the volume and increases the intra-alveolar pressure above atmospheric
pressure. This forces air out of the lungs as it moves down the pressure gradient. Inspiration
is considered active because it involves the contraction of skeletal muscles to produce the
pressure changes associated with inspiration. Expiration is considered passive under resting
conditions because the decrease in lung volume occurs when the muscles relax and the
naturally elastic lungs recoil to their preinspiratory size.
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