Introduction
The exchange of respiratory gas is limited by the capacity of the respiratory muscles to circulate air in the lung. Ventilation fails when muscles do not have the capacity to cope with the load brought by the mechanics of the pulmonary system. It is crucial to comprehend the mechanics of respiration/breathing not only for purposes of diagnosing how the system has failed and to offer the correct prescription for treatment of the failure, but to be capable of providing the most effectual temporary respiratory support through the use of a mechanical ventilator, as well.
The first category explains the effects for pulmonary mechanical role of the anatomical structure of the lungs while the other category explains the alterations in the pulmonary mechanical factors which can result in respiratory failure. The effects of anatomical structure are that the lungs and the thoracic container behave like springs while the blood vessels and the airways of the lungs become like collapsible tubes. On the other hand, the mechanical factors in breathing failure include increased/decreased lung compliance and increased airway resistance.
The Respiratory Muscles
The air movement into the lungs is generated mainly by the diaphragm, which is a dome-shaped sheet of muscle that separates the abdominal cavities and the thoracic. The diaphragm, which is inserted into the lower ribs, contracts in the process of inspiration and pushes the abdominal contents forward and downward, which increases the thoracic cavity’s vertical size.
The phrenic motoneurons in spinal segments are responsible for controlling the diaphragm.
The axons of the phrenic motoneurons make up the phrenic nerve. In addition, afferent signals move in the phrenic nerve, although the diaphragm has few muscle spindles. While the diaphragm descends, it also flattens, and results in a decrease in its efficiency and effectiveness.
More contraction leads to the pulling of the rib cage instead of the expansion of the lung volume. Expiration takes place because of passive recoil of the system. However, in active expiration, the abdominal muscles are contracted, and this forces the contents of the abdomen upward and inward, which decreases the volume of the thorax. The external intercostals muscles assist the diaphragm in pulling the ribs upwards and forward. Consequently, this increases the volume of the thoracic cavity and prevents the pulling in of the rib cage as the diaphragm contracts. In babies with the undeveloped intercostals muscles, the rib cage is pulled inwards during inspiration.
A comparable movement can be seen in chest injuries in which a section of the rib cage is broken that is flail chest.
In heavy physical exercises, accessory muscles including the sternomastoids that raise the sternum and the scalene which elevate the first two ribs may assist inspiration, as well. On the other hand, expiration is passive and occurs with the recoil of the elastic elements of the system that empty the lungs to the functional residual capacity (FRC), the resting volume. Expiration, however, may be assisted in voluntary control or heavy exercise by the internal intercostals and the abdominal muscles that decrease the volume of the thoracic cavity. Even though respiratory muscles may have considerable effort, the flow of expiration is limited and is, thus, of little value in overcoming circumstances in which the system recoil is reduced.
Illustration of the Process
The Balloon in a Box Model
The lungs may be seen as balloons inside a box that has flexible walls. These walls represent the diaphragm and the rib cage that jointly make up the thoracic container. The respiratory muscles partly form and act on the thoracic container walls. The balloon (lung) is elastic and recoils to a small volume while at rest and expands when pressure is used. In addition, the box is elastic, although its resting volume is large.
Even though the model demonstrates the idea that exerting pressures on elastic components results to change of their volume, it is a significant simplification. Among the faults of the model is that the air inside the box and that which surrounds the balloon expands to a lower density as pressure is reduced. The variation in the balloon volume is thus less that the variation in the box volume. In the instance of the real lungs that nearly fill the thoracic container, both the outside of the lungs and the inside of the thoracic container are covered with a pleural membrane.
The thin space between the pleural membranes known as the intra-pleural space is filled with milliliters of liquid that serves as a lubricant to facilitate the sliding of lung lobes over each other as the expansion of the lungs take place. Since liquids are incompressible, their volume does not expand when subjected to reductions in pressure. Because of this reason, the lungs follow the motion of the thoracic container precisely, and the variations in lung volume match those of the thoracic container. FRC is the volume of air which is inside the lungs during rest. The volume results from a balance of forces between the diaphragm composing the thoracic container (the relaxed volume is higher than the FRC) and the chest wall and the lungs whose relaxed volume is quite small. Because of this balance of pressures or forces, the intra-pleural pressure while at rest is lower than the atmospheric pressure.
If air enters the intra-pleural space such as through a puncture wound, the lungs will then collapse, the diaphragm descends as the intra-pleural force becomes similar to that of the atmosphere, and the chest wall springs outward. The case is known as pneumothorax that is air inside the thorax. In this case, the lungs no longer follow the thoracic container’s movements but rather the expansion of the thoracic container in inspiration leads to air entering the intra-pleural space through the puncture wound. In expiration, the air goes out through the puncture wound and any amount of pressure created in the intra-pleural space leads to a further compression of the lungs. The instantaneous treatment for the situation should now be evident from a consideration of the mechanics of breathing learnt so far. A wet dressing is then applied to the puncture wound in order to act as a one-way valve and allow air to exit the intra-pleural space instead of entering. A positive inflation of pressure of the lungs through the mouth can then be used to re-establish gas exchange and expand the lungs to normal volume.
The Two-Spring Model
In this case, both the thoracic wall and the lungs will behave as if they were springs. The resting volume of the lungs will be low while the volume of the container will be high in that when intra-pleural fluid is added, the system will come to rest at a transitional volume, the FRC. Therefore, the lungs are partially filled while the thoracic walls are inwardly pulled. The case may be modeled by the use of two springs. The thoracic wall spring may be represented by a flat spring while the lung spring is a coiled and short spring with a small resting length. The FRC is the equilibrium position of volume in which the lung and thoracic wall springs pull in opposite directions. In this way, the lungs will be filled to the FRC without the expense of any muscular effort.
Since the springs are pulling in opposite directions, pressure in the intra-pleural fluid space will be lower than the atmospheric pressure. It is advantageous to maintain the FRC instead of allowing the lungs to empty completely in the expiration process. The FRC is large in regards to tidal volume and thus buffers the variations in alveolar gas extensions that occur during respiration. In addition, it is easier to inflate the lungs when they are partly filled than when they are empty.
This is because in the empty lungs, the surface tension of the fluid lining the alveoli makes them to remain closed and extra pressure must be used in order to inflate them. The balance of pressures between the thoracic wall and the lungs ensures that the FRC is maintained and this is an essential system parameter. When the FRC is too low, then perfusion/ventilation increases making the lungs to become harder to inflate. If it is too high, however, the diaphragm becomes flattened and useless/ineffective. Thus, it is essential to understand the factors that determine the strength of the springs in this model.
Opinion and Explanation in Detail
In my opinion, the information given is sufficient to facilitate the understanding of the mechanics of breathing. However, further explanation is essential for further comprehension of the learner. The movement of air in and out of the lungs (ventilation) is produced by variations in the size of the thoracic cavity.
When the expansion of the cavity occurs during inspiration, air enters the lungs since at that moment the atmospheric pressure is higher than the pressure in the lungs. The diaphragm is the main respiration muscle. It separates the abdominal cavity that consists of muscle fibers and the thoracic cavity. During normal breathing, the diaphragm contracts and moves downwards whereas the parietal pleura descends. The movement causes the pulling down of the visceral pleura so that the alveoli and the airways expand leading to sucking in of air.
During expiration, the diaphragm relaxes and elastic tissues recoil in the lung, which results in the expulsion of air from the airways and the alveoli. In addition, the ribcage movement contributes to breathing through increasing the chest’s diameter that increases the volume of the thoracic cavity. It also makes the negative force in the lungs more negative and allows air to be drawn in. The transverse processes of the vertebrae and the joints between the posterior ends make the lower ribs swivel outwards and upwards in order to increase the lateral distance of the chest. On the other hand, the anterior ends of the ribs move out and up in order to increase the anteroposterior distance. The movement of the ribcage adds about 25 percent in thoracic volume while the diaphragmatic movement contributes about 75 percent. In forced breathing when the demand for oxygen is increased such as in a disease or physical activity, the accessory muscles of breathing come into action. These refer to muscles that are not basically involved in breathing, although they enlarge the rib cage in any means possible in inspiration in order to increase the amount of air, and thus oxygen taken in.
The ribcage is moved upwards and outwards by the external intercostal muscles in order to increase the anteroposterior and lateral diameter of the thorax. The muscles of the neck pull the ribcage upwards. The ribcage is pulled downwards by the transverses, oblique, and the rectus abdominis muscles. The latissimus dorsi and pectoralis major muscles pull the ribcage outwards through fixing the shoulder girdle. The muscles may be seen especially with patients that have respiratory distress as is the case with asthma and at the end of a tough race in athletes. In addition, the contraction of the abdominal muscles elevates intra-abdominal force and the ribcage is pulled downwards in forced breathing.
Inspiration
As noted earlier, the diaphragm is the major muscle of breathing. It consists of a motor nerve supply. When stimulated, it moves downwards resulting in an increase in the volume of the thoracic cavity. Consequently, this allows for air to be sucked into the lungs. In normal cases (normal breathing), the diaphragm moves by 1.5 centimeters while in deep or forced breathing, it may show up to 7 centimeters excursion. In injuries involving the spinal code, the breaks that are above C3-C4 are the only ones that will affect the phrenic nerve and lead to lack of breathing and eventually death. The diaphragm position changes with the posture of the body. When the body is in an upright posture, the abdominal components sink under gravity just like the diaphragm itself. The diaphragm flattens, which increased the surface area. In an upright posture, therefore, the movement of the diaphragm needed to attain thoracic cavity is lower than that needed to achieve a similar increase in the supine body stance, when the diaphragm is more dome shaped. This describes why people suffering from respiratory illnesses, especially those that involve respiratory muscle fatigue have a preference for an upright posture.
Expiration
In normal respiration, expiration is normally passive and is a result of the elastic recoil of the chest wall and the lungs, as well as, the inward force of surface tension in the pleural gap/space. The intercostals and the diaphragm muscles relax and there is a decrease of the intrapulmonary and thoracic volumes. In addition, the intrapulmonary pressures rise above the atmospheric pressure. On the other hand, forced expiration, the intra-abdominal pressure is increased by the contraction of the abdominal muscles. The movement results in the depression of the lower ribs and the upward movement of the diaphragm. This then decreases the thoracic cavity volume and expels air.
Conclusion
The above information offers a clear understanding of the mechanics involved in respiration, which is essential for knowledge and enhancement of the related disciplines.
References
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