Respiration: control of ventilation

Rhythmic ventilation is an automatic process controlled by the central nervous system. Groups of cells in the brainstem, predominantly the ventral and dorsal respiratory groups, are responsible for generating basic respiratory rhythm. This basic rhythm is subject to modulation by both conscious and reflex actions. In normal individuals the respiratory minute volume is set to closely regulate arterial carbon dioxide tension (PaCO2) at approximately 5.3 kPa, predominantly via a negative feedback reflex involving the central chemoreceptors. A separate group of chemoreceptors, the arterial chemoreceptors, are responsible for initiating the increased ventilatory response to counter arterial hypoxia, but a brisk response is not seen until PaO2 levels fall to approximately 8.0 kPa from the normal 13.3 kPa. Combined hypercarbia and hypoxia (asphyxia) is a very powerful stimulus to breathe as the two inputs interact in a synergistic manner. The chemoreceptor reflexes can be modified when the need arises (e.g. blockade of the respiratory part of the arterial chemoreflex by the trigeminal reflex as part of the diving response). Other reflexes such as the Hering-Breuer reflex contribute to setting the balance between tidal volume and respiratory rate to attain a given minute volume, although this reflex does not appear to play a major role in humans at resting tidal volumes. Superimposed on this 'tonic' control, additional protective reflexes (e.g. from receptors in the upper airways) are recruited to protect the lungs and airways with responses such as coughs and sneezes when required.