The concept of Retraction force Mainly refers to the lungs or thorax and means their tendency to contract when stretched, thus creating the intrathoracic negative pressure. The lungs get their retraction force from elastic fibers and the surface tension of the alveoli. The retraction force of the lungs is crucial for breathing, especially in terms of expiration.
What is the retraction force?
The retraction corresponds to a contracting movement. The term retraction force also refers to the ability and strength of contracting movements. In the human body, movements of this type mainly take place in the lungs.
The retraction force of the lungs corresponds to the tendency that the human lung follows when it is stretched: it tries to contract. As a result of their retraction force, intrathoracic or interpleural negative pressure arises. This pressure in the pleural space, together with fluid-mediated adhesive forces, ensures that the leaves of the lungs do not adhere to one another and that the lungs do not collapse.
Not only the lungs but also the thorax have retraction power. In the so-called resting breathing position, an equilibrium between the two passive retraction forces is achieved. With normal breathing, this equilibrium arises after expiration as soon as the lungs only hold their residual capacity.
Function & task
The lungs get their retraction force from their elastic fibers and the surface tension of their alveoli. The surface tension is based on the interface between water and air, which is created in the moist alveolar cells. The surface tension of the alveoli in particular depends on external influences and can be reduced by substances such as surfactant, for example.
Since the retraction force of the lungs is directly related to their expansion, the less the lungs are stretched, the smaller the force. The retraction force of the respiratory organ is sometimes the most relevant force for expiration. This is the phase of breathing in which air is transported out of the lungs and airways. Under resting conditions, expiration takes place on the basis of the lung elasticity and the retraction forces of the thorax and lungs. It is not necessary to use the breathing muscles for this. If, after normal expiration, only the end-expiratory lung volume remains in the lungs, we are talking about functional residual capacity.
As soon as only the functional residual capacity is in the lungs, the doctor speaks of the breathing rest. In this rest position there is a balance between the passive retraction forces of the lungs and thorax. When breathing is resting, the lungs are satisfied with a small volume. However, the thorax tries to expand.
Ultimately, the retraction force corresponds to an elastic restoring force that is absolutely necessary for breathing. There are interstitial elastic fibers in the lungs. In this way it achieves ideal elasticity and can contract immediately after the inspiration has been stretched and regain its original size in the sense of the expiratory position. The expiratory muscles are not required for resting breathing, but are only used to ventilate the remaining reserve volume.
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Various clinical pictures can limit the retraction force of the lungs.Other diseases are interrelated with the retraction force.
The pleural effusion, for example, is not insignificantly influenced by the retraction force. This effusion corresponds to a pathological accumulation of fluid between the individual pleural leaves. The distribution of a pleural effusion within the pleural space depends not only on gravity and capillary force, but also on the retraction force of the lungs. At the beginning of the effusion, the fluid collects between the diaphragm and the underside of the lungs. As soon as the amount of effusion increases due to the inflow of lymph, blood or pus, the capillary forces create an upward-pointing sickle of liquid in the Plaura cleft. The effusion continues to rise laterally, as the lung tissue has stronger restoring forces laterally. The retraction force of the lungs has a similar effect on fluid retention and its medical appearance.
Another clinical picture directly related to the retraction force is the pneumothorax. This term stands for the entry of air into the pleural space. When the intrathoracic space is opened, the lungs follow their retraction force and completely contract. For this reason, the intrathoracic space fills with air and a pneumothorax develops. The sticking together of the visceral pleura and parietal pleura is no longer certain. This means that the lungs can no longer follow the movements of the thorax, so it no longer opens and suffers either a partial or complete collapse. The pneumothorax usually has a traumatic cause and in this case arises as a result of a direct or indirect injury to the chest or its organs.
Typical causes are, for example, injuries to the lungs that occur as a result of inwardly speared rib fractures. Stab or gunshot wounds that open the chest cavity as described above are equally common causes. A traumatic pneumothorax can also be favored after severe bruising of the thorax, trapping or being rolled over, as the lung tissue is weakened by these processes. Somewhat rarer causes are barotraumas, which are associated with an extreme and sudden change in pressure within the lungs and can thus arise when flying, diving or through positive pressure ventilation. Sometimes the pneumothorax is also the result of medical measures, for example a faulty puncture on the subclavian vein that has injured the chest or lungs.
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