Regulation of Respiration

Breathing proceeds automatically and rhythmically throughout life, entirely without conscious attention, yet it can also be brought briefly under voluntary control -- as when a person deliberately holds their breath or deliberately breathes faster or more deeply. This dual character reflects how respiration is actually regulated: a neural respiratory centre in the brainstem sets and continuously adjusts the basic, automatic rhythm, while higher brain centres can briefly override it, and a further set of chemoreceptors continuously monitors the chemical composition of the blood (and of the cerebrospinal fluid bathing the brainstem itself) to keep the automatic rhythm precisely matched to the body's actual, moment-to-moment metabolic demand.

The principal, and dominant, controlling structure is the respiratory rhythm centre, located in the medulla oblongata, the lowest part of the brainstem, immediately above the spinal cord. This centre generates the basic rhythm of breathing entirely on its own, through a network of neurons that fire in a regularly repeating pattern, and it sends the resulting rhythmic signals down through the spinal cord and peripheral nerves to the diaphragm and intercostal muscles described earlier in the chapter, directly setting both the rate (breaths per minute) and the depth of every breath under resting, quiet conditions.

Immediately above the medulla, in the pons, lies a second, smaller region called the pneumotaxic centre, whose role is not to generate the basic rhythm itself but to moderate the output of the medullary rhythm centre -- chiefly by reducing the duration of inspiration, and so indirectly increasing the overall breathing rate, when its own activity increases; in this sense, the pneumotaxic centre acts as a fine-tuning influence layered on top of the medulla's own fundamentally self-generated rhythm, rather than as an independent pacemaker of its own.

What actually drives the automatic, moment-to-moment adjustment of this basic rhythm to changing metabolic demand -- for instance, the marked increase in both the rate and depth of breathing during physical exercise -- is a further, separate region within the medulla itself, called the chemosensitive area, together with a smaller set of chemoreceptors located outside the central nervous system altogether. The chemosensitive area is highly sensitive to even quite small increases in the carbon dioxide concentration (and the accompanying fall in pH) of the blood and cerebrospinal fluid bathing it; when tissue metabolism rises and generates more carbon dioxide than usual, this rising CO2 level is quickly detected by the chemosensitive area, which in turn signals the respiratory rhythm centre to increase both the rate and the depth of breathing, so as to expel the extra carbon dioxide -- and, indirectly, take in more oxygen -- more rapidly. Carbon dioxide, and the associated change in pH, is therefore the primary chemical signal governing the automatic regulation of respiration under most everyday conditions, considerably more influential, moment to moment, than blood oxygen level itself. …