Residual volume is the amount of air that remains as residual air in the lungs and airways even during deep exhalation. It maintains the internal pressure of the alveoli and prevents them from collapsing and becoming irreversibly stuck together. In addition, to a small extent, residual air allows continuity of gas exchange during the pause in breathing between exhalation and inhalation.
What is residual volume?
Residual volume is the amount of air that remains as residual air in the lungs and airways even during deep exhalation. The residual volume of the lungs corresponds to the amount of air that remains in the lungs and airways despite maximal voluntary exhalation. Maximum expiration means that the expiratory reserve volume, which normally remains in the lungs after expiration in addition to the residual volume, is also expired. In healthy and average-sized people, the residual volume is about 1.3 liters and is independent of athletic fitness. The total capacity of the lungs is the sum of vital capacity and residual volume. Vital capacity, in turn, is the sum of the respiratory volume and the inspiratory and expiratory reserve volumes. Except for the residual volume, all other lung volumes can be measured directly by spirometry using “small” pulmonary function testing. Residual volume can be determined exclusively by body or whole-body plethysmography. The plethysmograph consists of an enclosed glazed booth somewhat reminiscent of a telephone booth. The booth represents a closed gas-tight system. Volume expansion of the patient’s chest (during inspiration via a spirometer in communication with the air outside the booth) results in a minimal increase in pressure inside the booth, which is recorded and used for evaluation.
Function and task
The residual air that remains in the lungs even after maximum exhalation serves two important functions. The tiny alveoli (alveoli), with a variable diameter of 50 to 250 µm depending on the degree of unfolding or filling, are lined with a very fine epithelium and have a total surface area of about 50 to 100 square meters. If all air escapes from the alveoli, there is a risk that the epithelia of the respective opposing alveolar walls will irreversibly stick to each other due to adhesion forces. Even repeated inhalation would not be able to reverse this condition. Thus, the air of the residual volume is important for survival, as it protects the alveoli from sticking together after exhalation. The residual volume, in conjunction with the expiratory reserve volume, performs another important task: the two volumes of residual air, which together are referred to as the functional residual volume, provide a buffering of the oxygen and carbon dioxide partial pressures. This means that gas exchange through the membranes of the alveoli, which is controlled by the partial pressure gradient between the air in the alveoli and that of the capillaries at the alveoli, is almost continuous. The functional residual air volume ensures that the partial pressures remain constant as far as possible. This function is of particular importance because respiratory and pulse rates are not synchronized. If no residual air remained in the lungs after exhalation, this would be equivalent to discontinuous oxygen and carbon dioxide partial pressures, with the consequence that the mass transfer between blood and alveoli would also be discontinuous and would even be reversed at times. A mismatched heart and respiratory rate would exacerbate the problem, since in the worst case the blood would not come into contact with the freshly inhaled air in the alveolar capillaries over several breaths. The resulting fluctuating concentration of gases dissolved in the blood would make control of respiration via the carbon dioxide concentration in the blood as the main control parameter obsolete. The physiological size of the lungs is independent of athletic training. It is a genetically fixed quantity that determines the maximum achievable respiratory volumes when fully utilized. The variables that can be influenced by athletic training are all volumes that count toward vital capacity and can increase the effectiveness of the physiologically fixed lung size through good breathing technique.
Diseases and ailments
Various diseases may involve the restrictive or obstructive ventilatory disorders or functional failure of lung areas, influence the size of the residual volume, and be used as indicators for diagnoses or differential diagnoses. Ventilatory disorders are expressions of the underlying precipitating disease. In particular, chronic obstructive pulmonary disease (COPD), which can be caused by a variety of factors, is relatively common and is one of the top 10 causes of death worldwide. COPD, regardless of its causation, leads to an increase in residual volume and also functional residual capacity. Some lung diseases ultimately result in emphysema, a usually irreversible, functional failure of parts of the lung. Reversible disruption of gas exchange in the lungs can be caused by pulmonary edema, i.e., deposits of tissue fluid in the alveoli. The development of pulmonary emphysema in particular can be based on very different causes, but is usually associated with the long-term inhalation of pollutants in the form of dust particles or aerosols. The lung’s own protective system in the form of macrophages, which take up dust particles and remove them, may be overtaxed by excessive exposure. Another cause for the development of emphysema may be a genetic defect that manifests itself in an alpha-1 antitrypsin deficiency. The enzyme normally prevents the body’s own proteases from attacking alveolar membrane proteins. When the protease is deficient, the membranes can become holey, allowing many alveoli to coalesce into emphysema bubbles with loss of function. Common to all emphysema is a characteristic increase in residual volume.
