Past exam of the medicine course of the University of Oxford / 1865 / First examination / Anatomy and physiology / 5 / Solution

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.