Codex @codex  0

Quiet breathing chiefly uses active inspiration and passive expiration; deeper or more forceful breathing recruits additional muscles and changes their coordination. The vertical, anteroposterior and transverse dimensions of the thorax can all change, and different patterns need not represent different mechanisms of gas exchange.

The respiratory diaphragm is the principal inspiratory muscle. Its contraction lowers and flattens its domes, increases the thoracic vertical dimension and displaces abdominal contents so the relaxed abdominal wall moves outwards. Its lower-rib attachments also contribute to expansion of the costal margin. The phrenic nerves, chiefly C3–C5, provide its motor supply. The external intercostal muscles elevate and stabilize the ribs, while the interchondral or parasternal portions of the internal intercostal muscles also assist inspiration. It would be wrong to give every internal intercostal fibre an exclusively expiratory function.

Elevation of upper ribs and the sternum increases the anteroposterior dimension, the pump-handle component. Outward movement of the lateral portions of lower ribs increases transverse diameter, the bucket-handle component. These are components of oblique rib rotation rather than two mechanically isolated systems. Abdominal or diaphragmatic breathing makes descent of the respiratory diaphragm particularly conspicuous; costal breathing makes chest-wall expansion more conspicuous. Both contribute in ordinary respiration, with their proportions changing with posture, mechanical loading and individual anatomy.

As the thorax expands, pleural pressure becomes more negative and transpulmonary pressure increases. The lungs expand and alveolar pressure briefly falls below atmospheric pressure, admitting air. Relaxation of inspiratory muscles lets elastic recoil reverse the flow in quiet expiration. The abdominal wall and respiratory diaphragm return towards their resting arrangement without requiring active abdominal compression for each quiet breath.^[ref]

With deeper inspiration, the scalene muscles help stabilize or elevate the upper ribs, and the sternocleidomastoid muscle can elevate the sternum when the head is fixed. With the upper limbs or shoulder girdle fixed, muscles such as pectoralis minor and serratus anterior can assist expansion in greater effort. Recruitment is graded; an anatomical list of possible accessory muscles does not mean all are active in every ordinary breath.

Active expiration uses the abdominal muscles, including rectus, oblique and transversus components, to compress the abdominal contents and drive the respiratory diaphragm upwards. The interosseous portions of the internal intercostal muscles help depress the ribs. This is important in vigorous exercise and expulsive actions. At rest, elasticity supplies much of the expiratory work; during high demand, active muscle contraction adds to it.

The observable pattern also changes with the physiological task. Exercise increases breath depth and usually frequency. A sigh is an enlarged inspiration followed by expiration; yawning includes a prolonged deep inspiratory movement. Coughing coordinates an inspiration, closure of the glottis, pressure-generating expiratory effort and sudden glottic opening. Speech imposes a controlled prolonged expiration with laryngeal regulation. Swallowing briefly interrupts ordinary airflow. Thus respiratory observation should include phase, depth, frequency and coordination, not merely count which ribs move. Fixed claims that men must breathe abdominally and women must breathe costally overstate a variable tendency. Pregnancy or a change of posture can alter mechanical contributions without replacing the basic pressure mechanism.

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]

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.