= Solution
\b[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.<Anatomy and Physiology 2e by J. Gordon Betts and others/22.3 The Process of Breathing>[Mechanisms of Breathing; Pulmonary Ventilation: pressure changes and inspiratory versus expiratory activity]{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.
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