The term retraction force refers primarily to the lungs or thorax and means their tendency to contract when stretched, creating the intrathoracic negative pressure. Lungs get their retraction force from elastic fibers and surface tension of alveoli. The retraction force of the lungs is crucial for respiration, especially in the sense of expiration.
What is the retraction force?
The term retraction force refers primarily to the lungs or thorax and means their tendency to contract when stretched, creating the intrathoracic negative pressure. Retraction corresponds to a contracting movement. Also, the term retraction force thus refers to the ability to and strength of contracting movements. In the human body, movements of this type occur primarily in the lungs. The retraction force of the lung corresponds to the tendency that the human lung follows in the stretched state: it tries to contract. As a consequence of its retraction force, intrathoracic or interpleural negative pressure is created. This pressure in the pleural space, together with fluid-mediated adhesion forces, ensures that the sheets of the lung do not adhere to each other and that collapse of the lung does not occur. Not only the lungs, but also the thorax possesses retraction force. In the so-called respiratory rest position, a balance is reached between the two passive retraction forces. This equilibrium occurs during normal breathing after expiration, once the lungs grasp only their residual capacity.
Function and task
The lungs obtain their retraction force from their elastic fibers and the surface tension of their alveoli. Surface tension is based on the interface of water and air that occurs in the moist alveolar cells. The surface tension of the alveoli in particular depends on external influences and can be lowered by substances such as surfactant. Since the retraction force of the lungs is directly related to their expansion, the force is smaller the less the lungs are expanded. The retraction force of the respiratory organ is sometimes the most relevant force for expiration. As such, it is called the phase of respiration in which air is transported out of the lungs and airways. Under resting conditions, expiration occurs based on lung elasticity and the retraction forces of the thorax and lungs. The assistance of respiratory muscles is not required for this purpose. When only the end-expiratory lung volume remains in the lungs after normal expiration, the term functional residual capacity is used. As soon as only the functional residual capacity is in the lungs, physicians speak of the resting breathing position. In this resting position, there is a balance between the passive retraction forces of the lungs and thorax. In the resting position, the lungs are satisfied with a small volume. The thorax, however, tries to expand. Ultimately, the retraction force corresponds to an elastic restoring force, as is mandatory for breathing. Interstitial elastic fibers are present in the lung. Thus, it achieves an ideal elasticity and can contract immediately after the stretch of inspiration and regain its original size in terms of expiratory position. Thus, the expiratory musculature is not required for resting breathing, but is used only to ventilate the retained reserve volume.
Diseases and complaints
Several medical conditions can limit the retraction power of the lungs. Other conditions are interrelated with retraction force. Pleural effusion, for example, is not insignificantly affected by retraction force. This effusion corresponds to a pathological accumulation of fluid between the individual pleural sheets. The distribution of a pleural effusion within the pleural space depends significantly on the retraction force of the lung, in addition to gravity and capillary force. At the beginning of the effusion, the fluid collects between the diaphragm and the underside of the lung. As soon as the amount of effusion increases due to the inflow of lymph, blood or pus, the capillary forces create an upward pointing crescent of fluid in the plaural fissure. The effusion continues to rise laterally, as laterally stronger restoring forces of the lung tissue are present.The retraction force of the lung similarly affects fluid accumulation and its medical appearance. Another clinical picture directly related to retraction force is pneumothorax. This term represents the entry of air into the pleural cavity. When the intrathoracic space opens, the lung follows its retraction force and contracts completely. For this reason, the intrathoracic space fills with air and a pneumothorax develops. The adherence of the visceral pleura and parietal pleura is no longer secured. Thus, the lung can no longer follow the movements of the thorax, so it no longer opens and suffers either partial or complete collapse. In most cases, pneumothorax has a traumatic cause and arises in this case as a result of a direct or indirect injury to the thorax or its organs. Typical causes include injuries to the lungs that occur as a result of rib fractures spearing inward. Equally common causes are stab or gunshot wounds that open the chest cavity as described above. Traumatic pneumothorax may also be favored following high-level crush injuries to the thorax, entrapment, or rollover, as the lung tissue is weakened by these events. Somewhat less common causes are barotrauma, which is associated with an extreme and sudden change in pressure within the lungs and can thus occur during flying, diving, or positive pressure ventilation. Sometimes pneumothorax is also a result of medical intervention, such as a malpuncture on the subclavian vein that injured the chest or lungs.
