Ordinary inspiration is active enlargement of the thorax; ordinary expiration is chiefly passive recoil after the inspiratory muscles relax. The movements increase and decrease the vertical, anteroposterior and transverse dimensions together. Their relative prominence varies with posture and the individual; there is no exclusive division into abdominal and costal kinds of breathing.
The principal inspiratory muscle is the respiratory diaphragm. Its peripheral fibres shorten towards the central tendon, lowering and flattening its domes. This increases the vertical dimension of the thorax and displaces the abdominal contents downwards and forwards, making the relaxed abdominal wall project. Its costal attachments also act on the lower ribs: with abdominal support opposing unlimited descent, shortening can lift and expand the lower costal margin. Thus the respiratory diaphragm does more than pull down a piston. Its motor supply is through the phrenic nerves, principally from cervical segments C3–C5.
The external intercostal muscles elevate the ribs and help keep the intercostal spaces from being drawn inwards as pleural pressure falls. The interchondral, or parasternal, portions of the internal intercostal muscles also contribute to inspiration. It would therefore be inaccurate to label every part of the internal intercostal muscles expiratory. Coordinated activity braces the rib cage so that the shortening of one layer moves the ribs rather than simply deforming the spaces between them.
Rotation about the posterior articulations raises the anterior ends of the upper ribs and the sternum, increasing the anteroposterior diameter: the usual pump-handle description. The more laterally directed portions of the lower ribs rise and move outwards, increasing the transverse diameter: the bucket-handle description. These are descriptions of components of an oblique movement, not two independent sets of hinges. The scalene muscles can stabilize and elevate the uppermost ribs, with recruitment varying in quiet breathing. The sternocleidomastoid muscle, when the head is fixed, is an accessory elevator of the sternum in greater inspiratory effort; it is not required for an ordinary quiet breath.
The lungs follow the expanding chest wall because their surfaces remain coupled to it through the thin fluid-filled pleural cavity. More negative pleural pressure increases the distending transpulmonary pressure; enlargement of the alveoli briefly makes alveolar pressure lower than atmospheric pressure, so air enters. At the end of inspiration, airflow stops when these last two pressures equalize. During quiet expiration, relaxation of the respiratory diaphragm and inspiratory intercostal muscles allows the stretched lungs and chest-wall system to recoil towards their resting balance. The domes rise, the ribs and sternum descend, and alveolar pressure becomes slightly positive relative to atmospheric pressure while air leaves. Active abdominal muscles and the interosseous portions of the internal intercostal muscles become important in forced expiration, rather than supplying the main force in quiet expiration.[ref]
22.3 The Process of Breathing. Mechanisms of Breathing and Pulmonary Ventilation; Figures 22.16–22.17
A simple mechanical demonstration uses two elastic bags in an airtight transparent vessel, connected through its stopper to the air, with a movable rubber sheet forming the base. Pulling the sheet down expands the vessel and inflates the bags; letting it return allows deflation. A side-tube manometer can show the associated pressure change. This illustrates expansion by lowering the floor, but its rigid vessel cannot demonstrate the movements of the ribs, the action of the intercostal muscles, or the real geometry of the pleural cavity. Observing lower-chest widening and abdominal displacement during quiet breathing supplies those missing observations without confusing the model with the anatomy.
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