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and sequence of events during inspiration and expiration. The sequence of events in the left column includes volume changes during inspiration (top) and expiration (bottom). The lateral views in the middle column show changes in the superiorinferior dimension (as the diaphragm alternately contracts and relaxes) and in the interiorposterior dimension (as the external intercostal muscles alternately contract and relax). The superior views of transverse thoracic sections in the right column show lateral dimension changes resulting from alternate contraction and relaxation of the external intercostal muscles.

Reproduced from Fig. 21.15d, p. 622 from Human Anatomy, 3rd edn, by E.N. Marieb and J. Mallatt.

© 2001 by Benjamin Cummings. Reprinted by permission of Pearson Education, Inc.

The accessory muscles

The accessory muscles of the neck (the scalenes and sternocleidomastoid muscles) and the chest (pectoralis minor) can increase the volume of the thoracic cavity and therefore increase the volume of breathing; this can occur during exercise or if the individual is in respiratory distress (Marieb and Hoehn 2010). The muscles of the abdomen and those that cause flexion of the spine are also accessory muscles but aid with expiration (Davies and Moores 2003). When these muscles contract they press the abdominal organs upwards, forcing the diaphragm up, reducing the thoracic volume and causing expiration (Davies and Moores 2003).

Control of respiration

The respiratory centres

Within the medulla are two clusters of neurones which coordinate the respiratory system; these are the dorsal respiratory group and the ventral respiratory group (Marieb and Hoehn 2010). The ventral respiratory group contains neurones that fire during inspiration, stimulating the diaphragm and the intercostal muscles to contract, via the phrenic and intercostal nerves, and other neurones which fire during expiration causing the stimulation to stop so the muscles relax. These neurones deliver our normal respiratory rate (Marieb and Hoehn 2010).

The dorsal respiratory group coordinates input from the chemoreceptors and stretch receptors and relays this to the ventral respiratory group to alter respiratory rate as required (Marieb and Hoehn 2010). Chemoreceptors monitor the arterial blood for changes in the partial pressure of carbon dioxide (PCO2) and pH, but also, to a lesser extent, the partial pressure of oxygen (PO2) (Patton and Thiobodeau 2009). For more information on the chemical factors which affect respiration please, see Chapter 10.

The pontine respiratory centres also relay impulses to the ventral respiratory group to modify breathing rhythms so that there is a smooth transition from inspiration to expiration and so that breathing can be modified to allow for speech, exercise and sleeping patterns (Marieb and Hoehn 2010). In the higher cortical centres respiration can be altered through factors such as strong emotions, pain and alteration of temperature (Marieb and Hoehn 2010, Patton and Thiobodeau 2009). The cerebral motor cortex yields a degree of voluntary control over breathing; however, this can be overridden by the other mechanisms (Patton and Thiobodeau 2009).

Carbon dioxide

Although oxygen is essential for every cell in the body, the body’s need to rid itself of carbon dioxide is the most vital stimulus to respiration in a healthy person (Marieb and Hoehn 2010). The arterial PCO2 is very closely monitored and balanced and when it increases slightly, it causes the chemoreceptors to be stimulated (Patton and Thiobodeau 2009). Carbon dioxide passes easily from the blood into the cerebrospinal fluid where it forms carbonic acid, releasing hydrogen ions (H+) (Marieb and Hoehn 2010). This increase of H+ causes the pH to drop, stimulating the central chemoreceptors, found in the brainstem, to increase the rate and depth of breathing and increasing the amount of carbon dioxide exhaled (Marieb and Hoehn 2010). Similarly, metabolic causes for low pH levels, such as a buildup of lactic acid, will also cause an alteration of respiration