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The closing capacity (CC) is the volume in the lungs at which its smallest airways, the alveoli collapse. The alveoli lack supporting cartilage and so depend on other factors to keep them open. The closing capacity is usually less than the functional residual capacity (FRC), the amount of gas that normally remains in the lungs during respiration. This means that there is normally enough air within the lungs to keep these airways open throughout both inhalation and exhalation. As the lungs age, there is a gradual increase in the closing capacity. This also occurs with certain disease processes, such as asthma, chronic obstructive pulmonary disease, and pulmonary edema. Any process that increases the CC (or decreases the FRC) can increase an individual's risk of hypoxemia, as the small airways may collapse during exhalation, leading to air trapping and atelectasis.
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