The closing capacity (CC) is the volume in the

lung The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of t ...

s at which its smallest airways, the respiratory bronchioles, collapse. It is defined mathematically as the sum of the closing volume and the residual volume. The alveoli lack supporting

cartilage Cartilage is a resilient and smooth type of connective tissue. In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage, and is a structural component of many body parts including the rib cage, the neck an ...

and so depend on other factors to keep them open. The closing capacity is less than the Functional Residual Capacity, 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 Inhalation (or Inspiration) happens when air or other gases enter the lungs. Inhalation of air Inhalation of air, as part of the cycle of breathing, is a vital process for all human life. The process is autonomic (though there are exceptions ...

and exhalation. As the lungs age, there is a gradual increase in the closing capacity (i.e. The small airways begin to collapse at a higher volume/before expiration is complete). This also occurs with certain disease processes, such as asthma, chronic obstructive pulmonary disease, and pulmonary edema. Any process that increases the CC by increasing the closing volume (CV) can increase an individual's risk of hypoxemia, as the small airways may collapse during exhalation, leading to

air trapping Air trapping, also called gas trapping, is an abnormal retention of air in the lungs where it is difficult to exhale completely. It is observed in obstructive lung diseases such as asthma, bronchiolitis obliterans syndrome and chronic obstructive ...

and atelectasis. A mnemonic for factors increasing closing capacity is ACLS-S: Age, Chronic bronchitis, LV failure, Smoking, Surgery. Alternatively, CLOSE: Cigarettes, LV failure, Old age, Surgery, Emphysema (& asthma). Often supine positioning will decrease functional residual capacity (FRC) but has no effect on closing capacity.

See also

Further reading