Note: Highlighted words, generated by AWL tag cloud, are from the AWL (Academic Word List).
Breathing is a highly coordinated abdominal and thoracic process. The diaphragm is the major muscle of inspiration , responsible for approximately two-thirds of quiet breathing in healthy humans. The external intercostal muscles are most active in inspiration, and the internal intercostals, which are not as strong, are most active in expiration. Increasing the vertical, transverse and anteroposterior dimensions of the chest increases the volume of the pleural space, and the resulting decrease in intrapleural pressure draws air into the lungs. During expiration, the diaphragm relaxes and moves superiorly. Air is expelled from the lungs and the elastic recoil of the lung creates a subatmospheric pressure that returns the lateral and anteroposterior dimensions of the thorax to normal (De Troyer & Estenne 1988, Celli 1998).
During inspiration, the lowest ribs are fixed, and contraction of the diaphragm draws the central tendon downwards. In this movement, the curvature of the diaphragm is scarcely altered. The cupolae move downwards and a little forwards almost parallel to their original positions. The associated downward displacement of the abdominal viscera is permitted by the extensibility of the abdominal wall, but the limit of this extensibility is soon reached. The central tendon, its motion arrested by the abdominal viscera, then becomes a fixed point from which the fibers of the diaphragm continue to contract. This causes the second to tenth ribs to be elevated and the inferior portions of the ribs are turned outwards as a result of direct transmission of pressure through the zone of apposition (Fig. 1). The medial aspect of the rib is elevated and this increases the transverse dimension of the chest in the same manner as a bucket handle swinging outwards (Fig. 2A). This effect is most evident in the lower ribs (seventh to tenth ribs). Movements at the costovertebral joints cause elevation of the anterior ends of the ribs that push the body of the sternum and the upper ribs forwards. This 'pump handle' movement is most evident in the superior ribs (second to sixth ribs) and increases the anteroposterior dimension of the thorax (Fig. 2B). The right cupola of the diaphragm, which lies on the liver , has a greater resistance to overcome than the left, which lies over the stomach, and so the right crus and the fibers of the right side are more substantial than those of the left. The balance between descent of the diaphragm, protrusion of the abdominal wall ('abdominal' breathing), and elevation of the ribs ('thoracic' breathing) varies in different individuals and with the depth of ventilation . The thoracic element is usually more marked in females, but increases in both sexes during deep inspiration.
Fig.1Inspiratory movements: pressure changes during inspiration. (Adapted from Drake, Vogl and Mitchell 2005.)
Fig.2 Movements of the ribs during inspiration (A) increase the transverse diameter of the chest by the 'bucket handle' movement, and (B) increase the anteroposterior dimension of the thorax by the 'pump handle' movement.
Diaphragmatic excursion is typically 1.5 cm in quiet breathing. During deep ventilation, the maximum movement ranges from 6 to 10 cm. After a forced inspiration, e.g. when breathing is partially obstructed, the right cupola of the diaphragm can descend to about the level of the eleventh thoracic vertebra, while the left cupola may reach the level of the body of the twelfth thoracic vertebra. After a forced expiration, the right cupola of the diaphragm is level anteriorly with the fourth costal cartilage, laterally with the fifth, sixth and seventh ribs, and posteriorly with the eighth rib, and the left cupola is a little lower.
The level of the diaphragm is affected by the phase and depth of ventilation, and by the degree of distension of the stomach and intestines and the size of the liver. Radiographs show that the height of the diaphragm within the thorax also varies considerably with posture. It is highest when the body is supine , and in this position it performs the greatest ventilatory excursions with normal breathing. When the body is erect, the diaphragm is lower, and its ventilatory movements become smaller. The diaphragmatic profile is still lower in the sitting posture, and ventilatory excursions are smallest under these conditions. When the body is horizontal and on one side, the two halves of the diaphragm do not behave in the same way. The uppermost half sinks to a lower level than that seen when sitting, and moves little with ventilation. The lower half rises higher in the thorax than it does even in the supine position, and its ventilatory excursions are considerably greater. Changes in the level of the diaphragm with alterations in posture explain why patients with severe dyspnea are most comfortable, and least short of breath, when sitting up.
The primary role of the intercostal muscles is to stiffen the chest wall, preventing paradoxical motion during descent of the diaphragm in inspiration. This becomes most obvious immediately after high spinal injury, when there is flaccid paralysis of the entire trunk and only the diaphragm is left functioning. In a healthy adult with a vital capacity of 4.5 L , some 3 L is accounted for by diaphragmatic excursion. Immediately after high spinal injury, the vital capacity decreases to about 300 mL, even though the diaphragm is moving maximally, because some 2.7 L is lost by paradoxical incursion of the flaccid chest wall as the diaphragm descends. With time (usually several weeks), the paralysis becomes spastic, stiffening the chest wall, and the vital capacityincreases towards its phrenic limit of about 3 L.
injury reveals the role of the abdomen in inspiration and expiration. The abdominal musculature plays a majorrole in active expiration in man. During the flaccid stage of high spinal paralysis, the only mechanisms available for returning the relaxed diaphragm into the thorax on expiration are passive recoil of the lungs and chest wall, and the weight of the abdominal viscera. The latter is the most important, and operates only when patients are lying down. If they are sat up or raised upright, they are unable to breathe out. Trussing the abdomen with an elastic binder can help such patients. Conversely, when paralysis becomes spastic, the stiff abdominal wall opposes inspiration.
The role of the abdomen in breathing is often underestimated. If, for example, the anterolateral wall were made of steel and linked the pelvic rim rigidly to the costal margins , inspiration would be impossible. The diaphragm could not descend (because the abdominal contents are incompressible), and the ribs could not rise (because the links to the pelvis would be inextensible). During normal breathing, the abdomen relaxes as the diaphragm contracts. It is possible to oppose this motion by tensing the abdomen, as in the ' beach posture ' adopted to exaggerate the size of the chest. In this case, the abdominal contents fix the central tendon of the diaphragm, so that it raises the ribcage as it contracts, but it is a condition of that maneuver that the gap between the ribs and the pelvic rim widens.
The ventilatory muscles must also work during sleep, when the pharyngeal muscles relax and upper airway resistance increases. It is now appreciated that in some people, particularly the obese, this relaxation can lead to periodic apnea and marked hypoxia during sleep, implying that the pharyngeal muscles play an important ventilatory role in waking life. It is also clear that although ventilatory muscles rarely tire in normal life, they do fatigue when placed under abnormal loads, e.g. in chronic obstructive pulmonary disease.
The different pulmonary regions do not all move equally in ventilation. In quiet ventilation, the juxtahilar part of the lung scarcely moves and the middle region moves only slightly. The superficial parts of the lung expand the most, and the mediastinal surface, posterior border and apex move less, because they are related to less movable structures. The diaphragmatic and costomediastinal regions expand most of all. Most of the volumetric change during ventilation occurs in the alveoli.
Word lists
( Level 1-10 corresponds to Sublist1-10,
more details: http://www.englishvocabularyexercises.com/academic-word-list/index.html)
Sorted by level
Level 1 available contract contracts creates evident functioning individuals major occurs periodic process role structures underestimated varies
Level 2 abnormal affected aspect element injury normal primary ranges region regions
Level 3 considerably coordinated implying linked links maximum sexes volume
Level 4 approximately dimension dimensions internal mechanisms obvious parallel phase
Level 5 alterations altered capacity expand external margins
Level 6 reveals
Level 7 adapted adult transmission
Level 8 appreciated displacement
Level 9 conversely passive portions relax relaxation relaxed relaxes rigidly
Level 10 none
