Normal respiratory function

The work of breathing

Respiration is the combined process of inspiration and expiration, driven by the body’s physiological need to absorb oxygen (O₂) and eliminate carbon dioxide (CO₂) from the blood, maintaining optimal acid-base balance (pH) to support cellular metabolism.

Under normal conditions, the work of breathing is minimal at rest and during low-intensity activity.

As metabolic demand increases, the work of breathing also increases to maintain adequate ventilation.

With extreme exertion, the work of breathing becomes unsustainable over time, eventually leading to fatigue or exhaustion—depending on individual cardiorespiratory fitness.

Central and autonomic nervous system roles

Respiration depends on the coordinated function of the central and autonomic nervous systems.

The central nervous system (CNS)—primarily the brainstem, including the medulla oblongata and pons—regulates the rhythmic pattern of breathing by generating and modulating respiratory drive. These brain centres integrate various inputs to adjust the rate and depth of breathing in response to changing metabolic demands.

The CNS—primarily the brain to spinal segment pathways—regulates the strength, tone and associated compliance of respiratory muscles. It is also the central pattern generator for respiratory motor function. Distinct neural patterns of muscle recruitment govern inspiration and passive expiration, as well as forced expiratory actions such as sighing, coughing and sneezing.

The autonomic nervous system (ANS)—particularly its sympathetic and parasympathetic branches—modulates pulmonary blood flow, airway reactivity, sputum production and further changes to respiratory rate, so that:

• sympathetic activation promotes bronchodilation and increases in respiratory rate
• parasympathetic input contributes to bronchoconstriction and associated sputum production.

Diaphragm roles

The diaphragm, is a large, domed muscle located beneath the lung bases. It separates the thoracic and abdominal cavities, and is the primary muscle recruited for respiratory actions.

The diaphragm generates dynamic transdiaphragmatic pressures through its selective contraction and relaxation. This means:

• during inspiration, the diaphragm contracts and descends, creating negative intrathoracic pressure and positive intra-abdominal pressure which generates at least 65% of inspiratory volume
• during passive expiration, the diaphragm relaxes, reversing these pressures
• during forced expiration, the diaphragm co-contracts with abdominal, pelvic floor, internal intercostal, and intrinsic laryngeal muscles to regulate intra-abdominal, intrathoracic, and subglottic pressures. This controls airflow for forced expiratory actions e.g. coughing, as well as more complex functions e.g. singing.

The diaphragm’s positioning (between the thoracic and abdominal cavities) and dome-shape, along with its capacity to generate varying transdiaphragmatic pressures, also:

• aids venous and lymphatic return
• supports lower oesophageal sphincter function to limit reflux
• contributes to dynamic postural stability.

Inspiration

Normal diaphragmatic contraction not only requires intact spinal nerve root innervation, but it also requires normal intercostal and abdominal muscle tone and compliance.

Normal intercostal muscle tone stabilises the chest wall and intercostal spaces against increasing negative intrathoracic pressures as the diaphragm contracts and descends. This prevents the collapse of the chest wall and development of intercostal recessions during inspiration. A stabilised chest wall also provides a stable base for the diaphragm to contract against during later inspiration, enhancing inspiratory volume further.

Normal abdominal muscle tone positions the abdominal contents against the diaphragm, creating its dome-shape at rest, while supporting its shape during contraction. This shaping is critical to permit the diaphragm to contract through a greater excursion range, from fully domed to flattened. By optimising the diaphragm’s length-tension relationship throughout inspiration, inspiratory volumes are generated more efficiently.

At the same time, normal intercostal and abdominal muscle compliance permits the expansion of the chest wall during inspiration. Similarly, it allows the abdominal contents to be displaced by the diaphragm’s descent, causing some protrusion of the abdominal wall during inspiration—especially in sitting. This normal compliance permits the diaphragm to complete its full excursion and generate maximal inspiratory volume when required.

Active contraction of the scalene and external intercostal muscles assists rib cage elevation, along with the final stage of diaphragm contraction to expand the chest wall further and increase inspiratory lung volume. Other accessory muscles such as sternocleidomastoid may also be recruited to increase inspiratory force, flow rates or the overall respiratory rate as needed.

Expiration

Expiration is primarily a passive process that occurs when the diaphragm relaxes and recoils back to its original dome-shape. In turn, the negative intrathoracic pressure reduces, allowing the elastic recoil of the lungs and chest wall too.

This recoil assists with regulating CO₂ levels, enhanced by periodic sighing—a gentle, forced expiration. However, secretion clearance typically requires more powerful, forced expiration via coughing, huffing, sneezing and/or nose-blowing. Other accessory muscles may be recruited to further increase expiratory volumes, force and flow rates as needed.

Normal breathing pattern

The normal breathing pattern is:

1. abdominal rise during inspiration – due to the descent and flattening of the diaphragm into the abdomen, with normal abdominal wall compliance
2. lateral expansion of the lower ribs – due to activity of the external intercostals, but also the final stages of diaphragm contraction from its insertion on the lower ribs
3. anterior-posterior elevation of the rib cage – due to the activity of the external intercostals and other accessory muscles
4. abdominal fall during expiration – due to the relaxation of the diaphragm, allowing the abdominal contents to track against the relaxing diaphragm and re-establish its dome-shape at rest.

The following video (up to 1min 44sec) illustrates the normal breathing pattern generated by the synergistic action of the diaphragm, intercostal and abdominal muscles.

Normal ventilation

Volume and flow rate

To achieve optimal gas exchange, ventilation ultimately depends on the degree of air volume and flow rate, entering and exiting the lungs.

Various aspects of lung volume and flow rate can be measured using spirometry, for comparison with normalised values determined by sex, age, height, weight and ethnicity. Measures of respiratory muscle strength can also be completed, which indicate capacity to generate lung volumes and flow rates.

Airway resistance

As discussed above, respiratory muscle recruitment produces the mechanical action of inspiration, followed by passive expiration. Effectively, they generate the pressure gradient needed to overcome innate airway resistance and drive a specific volume and flow rate of air in and out of the lungs.

Airway resistance is determined by a range of factors such as airway diameter. Smaller airways, like bronchioles, naturally have higher resistance compared to larger airways, like the trachea. As lung volume decreases during expiration, airways narrow and increase resistance. 

A combination of any, or all of the following, can further reduce the compliance of the lungs and chest wall, increasing airway resistance:

• bronchospasm (increased airway smooth muscle tone)
• atelectasis (alveoli collapse)
• secretions (sputum etc.)
• pulmonary oedema (swelling internal or external to the alveoli)
• pleural effusions (swelling in the pleural space)
• rigidity of the chest wall (rib cage and muscles).

Gas exchange

Ventilation is the gaseous exchange of O₂ and CO₂ which occurs via the lungs.

The degree of gas exchange can be measured using various methods, including the analysis of blood or exhaled air.

Overall, any adverse and sustained changes to these aspects of ventilation- lung volumes and flow rates, airway resistance and gas exchange- will increase the work of breathing to meet the physiological needs of respiration.

Normal airway clearance

The internal epithelial layer of the whole tracheobronchial tree produces sputum, which is a thick, viscous secretion. It consists of mucus, which mops up inflammatory cells and traps inhaled cellular particles, pathogens, and debris.

The mucociliary escalator is comprised of hair-like cilia which extrude from the epithelium and beat in a coordinated manner. This sweeps sputum, aided by expiratory airflow, toward the oropharynx for clearance via swallowing or expectoration (coughing).

Coughing—either reflexive or voluntary—is the body’s primary “host defence” mechanism for clearing the airway of secretions and debris, as well as inhaled and aspirated foreign matter. An effective cough consists of three phases, which all require unique transdiaphragmatic pressures:

1. inspiration – maximal inspiratory muscle contraction of the diaphragm and accessory muscles to achieve a full volume of inhaled air within the lungs
2. compression – glottic closure to temporarily close the airway
3. expulsion – forceful expiratory muscle contraction of the abdominal and internal intercostal muscles, combined with rapid glottic opening to suddenly open the airway; together this allows the sudden and forced exhalation of the full volume of air from the lungs.

Normal respiratory muscle strength, including the inspiratory volume prior to a cough, ultimately determines the cough expiratory volume and flow rate. In a healthy adult, producing an effective cough is estimated to require at least 50%—but typically 80-90%—of inspiratory lung volume and should generate a peak cough flow >360 L/min.

For more information, refer to Respiratory muscles.