The respiratory rest position exists when the opposing retractive forces of the thorax and lungs reach equilibrium and the compliance or distensibility of the lungs is at its highest. In the respiratory rest position, the lungs contain only their functional residual volume. When the lungs are overinflated, the respiratory rest position changes in a pathologic manner.
What is the respiratory rest position?
The respiratory rest position exists when the opposing retraction forces of the thorax and lungs reach equilibrium and the distensibility of the lungs is at its highest. Retraction force is the elastic restoring force of the lungs. There are interstitial elastic fibers in the organ. In addition, the alveoli of the lungs have a certain surface tension. Each of the individual, water-lined alveoli strives to shrink because water molecules at the interfaces between air and water exert a certain force of attraction on each other. For this reason, the lungs are ideally elastic. After expansion during inspiration (inhalation), the lungs retract to their original size on their own, thus returning to the so-called expiratory position. The muscles for expiration (breathing out) remain unused during resting breathing and are only called upon when the reserve volume is forced to ventilate. The retraction of the lungs is slowed down by surfactant, which reduces the surface tension of the alveoli by a factor of ten and prevents the lungs from collapsing. During inhalation, the inspiratory muscles actively overcome the resistances of the lung and thoracic retraction forces. The retraction forces of the lungs and thorax are released again only during expiration in the sense of relaxation of the respiratory muscles, so that expiration from the respiratory rest position takes place as a passive process. In this context, the respiratory rest position corresponds to the equilibrium between passive retraction forces of the thorax and lungs, which occurs automatically at the end of expiration during normal breathing.
Function and task
In the respiratory rest position, the lungs seek to regain a smaller volume because of the surface tension of the alveoli and the elasticity of their fibers. The retraction forces of the thorax counteract this. They try to expand the thorax. Lung expandability or lung compliance reaches a maximum in the state of respiratory rest. Lung distensibility is a physical quantity that summarizes the elastic properties of the lung. Extensibility is essentially the ratio of volume change to corresponding pressure change. Elastic bodies, such as inflated balloons, are a suitable illustrative example. Such a balloon has a defined volume and a pressure based on it. As soon as more air is added to the balloon, it changes volume and an increase in pressure occurs. Thus, the greater the distensibility, the smaller the pressure increase for a given filling volume. In the respiratory tract, the volume change corresponds to the so-called respiratory volume. Lung distensibility is indirectly proportional to elastic lung retraction pressure. Thus, high compliance requires only low pressure to keep the lungs filling. Low compliance, on the other hand, requires more pressure to fill the lungs. In the resting breathing position, the highest compliance is present. This means that the least pressure is required to fill the lungs. In the resting position, the lungs contain only their functional residual capacity. This functional residual capacity corresponds to the volume of gas that remains within the lungs after normal expiration in the resting phase. The capacity is the sum of the residual volume and the expiratory reserve volume. Thus, functional residual capacity is equal to end-expiratory lung volume. The thorax’s efforts to expand are exactly equal to the lungs’ efforts to contract in the resting breathing position. For this reason, neither passive expiration nor active inspiration occurs at the moment of respiratory rest.
Diseases and ailments
In chronic hyperinflation of the lungs, the resting breathing position is pathologically altered. Hyperinflation can lead to chronic airway obstruction in late stages and is usually caused by chronic endobronchial or exobronchial flow obstruction during expiration. With incomplete expiration, the respiratory resting position of the inspiratory reserve volume shifts to higher volumes.The respiratory rest position shifts to the inspiratory reserve volume of the lung as soon as expiration is no longer fully performed. These processes cause the vital capacity of the lungs to decrease, while the functional residual volume increases. By vital capacity, the pulmonologist means the lung volume between maximum inspiration in terms of maximum inspiration and maximum expiration in terms of expiration. The parenchyma of the lung loses elasticity during overinflation and the alveoli have only reduced retraction force. This results in the permanent increase in size of the lungs, which causes a substantial loss of efficiency, is associated with dyspnea, and often weakens the respiratory muscles. In all obstructive airway diseases, there is severe impairment of expiratory airflow, whereas inspiratory airflow is less impaired. Therefore, in these diseases, increased air automatically remains in the lungs at the end of expiration, so that acute pulmonary hyperinflation may develop, especially at the bottom of such diseases. Since chronic pulmonary hyperinflation is associated with the structural changes described further above, irreversible emphysema may develop from chronic hyperinflation. Pulmonology distinguishes between two different forms of pulmonary hyperinflation. Absolute hyperinflation is present in “static” or anatomically fixed hyperinflation and causes the total capacity of the lungs to increase. Relative hyperinflation is a “dynamic” hyperinflation, also known as “air trapping”. In this form, the residual volume increases at the expense of vital capacity, as described above. Affected patients suffer from an increased respiratory center after physical exertion.
